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gessle
31-08-08, 16:43
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In curand ,voi incepe acest topic!:cool2:

gessle
05-11-08, 17:32
Pt un inceput:lexicon


A/B Switch:
A switch that selects one of two inputs (A or B) for routing to a common output while providing adequate isolating between the two signals.

Access Control System - ACS:
Access Control System/s, comprising all conditional access components such as S/1, IDAC, ISAC, minicons, etc...

ACS number:
This is the version number of the cards software.
There are several different software versions: 1.2, 1.4, 1.6 en 3.82, 3.83.
Versions 1.4 and 1.6 are almost identical.

Adaptation Header:
A block of data that forms an extension to a transport packet header. It may be of fixed format and/or of general data

Adjacent Channel:
An adjacent channel is immediately next to another channel in frequency. For example, PAL channels 5 and 6 as well as 8 and 9 are adjacent.

Alignment:
The process of fine tuning a dish or an electronic circuit to maximize its sensitivity and signal receiving capability.

Alphacrypt:
The Irdeto successor, decodes both Irdeto and Betacrypt.

AM:
An abbreviation for amplitude modulation.

Analog:
A system in which signals vary continuously in contrast to a digital system in which signals vary in discrete steps.

Analogue-to-Digital Converter:
A circuit that converts analogue signals to an equivalent digital form. The varying analogue signal is sampled at a series of points in time. The voltage at each of these points is then represented by a series of numbers, the digital value of the sample. The higher this sampling frequency, the finer are the gradations and the more accurately is the signal represented

Antenna:
A device that collects and focuses electromagnetic energy, i.e., contributes an energy gain. Satellite dishes, broadband antenna and cut-to-channel antennas are some types of antennas encountered. In the case of satellite dishes, gain is proportional to the surface area of the microwave reflector.

Antenna Efficiency:
The percentage of incoming satellite signal actually captured by an antenna system.

Aperture:
The collection area of a parabolic dish.

Aperture Blocking:
An obstruction such as the feed assembly which causes a blocking of the incoming signal.

Asciiserial:
The number that identifies the card. It is also printed on the card in bar-code.
Although it is accessible by software, to my knowledge it is never really used.
It only serves identification purposes.

Aspect Ratio:
The ratio of television screen width to height. The standard aspect ratio is 4 to 3.

Aston Seca:
Although the real name for the coding system is Mediaguard, it is often referred to as Seca or Aston Seca. Mediaguard is developed by Seca, so the also used name Seca Mediaguard is more suitable. Aston is a company that builds the CAM's (http://www.duwgati.com/uk/lexikon.php#cam) (among others) that are used to decode the Mediaguard system.
The Seca Mediaguard coding is used by the Canal + organization which is no wonder. Canal + is shareholder in the Seca organization and it also takes part in the development of the Mediaguard coding system. Because of the influence of Canal +, the Seca Mediaguard system is very popular in France.

Attenuation:
The decrease in signal power that occurs in a device or when a signal travels to reach a destination point (path loss).

Attenuator:
A passive device which reduces the power of a signal. Attenuators are rated according to the amount of signal attenuation.

ATR:
Answer To Reset, or ATR for short, is the string a smart card sends to the receiver upon every reset. The ATR of each smart card conforms to the ISO7816-3 specifications. The ATR contains information about the card, for instance information on how the receiver should communicate with the card: Voltage, Amp, Baudrate, Synchronous or Asynchronous communication etc.

Audio Subcarrier:
The carrier wave that transmits audio information within a video broadcast signal. Satellite transmissions can relay more than a single audio subcarrier in the frequency range between 5 and 8.5 MHz.

Auto Update:
The auto update (AU) technique makes sure the card is kept up to date in order to provide the correct keys to the CAM (http://www.duwgati.com/uk/lexikon.php#cam) when requested. Providers will regularly change their operational keys and unless you have a valid set of management keys, you will soon be left with a black screen. For different coding systems, the actual keys that are used for decoding, have different names. For instance, in Irdeto they are called Plainkeys and for Seca they are called Operational Keys.

Automatic Brightness Control:
A television circuit used to automatically adjust picture tube brightness in response to changes in background or ambient light.

Automatic Fine Tuning:
A circuit that automatically maintains the correct tuner oscillator frequency and compensates for drift and for moderate amounts of inaccurate tuning. Similar to AFC (http://www.duwgati.com/uk/lexikon.php#afc).

Automatic Frequency Control - AFC:
A circuit that locks an electronic component to a chosen frequency, so that the the tuning will not drift from that chosen frequency.

Automatic Gain Control - AGC:
A circuit that uses feedback to maintain the output of an electronic component at a constant level. This is achieved by locking the gain onto a fixed value and thus compensating for varying input signal levels keeping the output constant.

Azimuth-Elevation (Az-El) Mount:
A dish mount that tracks satellites by moving in two directions: the azimuth in the horizontal plane and elevation up from the horizon.

Azimuth:
A compass bearing expressed in degrees of rotation clockwise from true north. It is one of the two coordinates, azimuth and elevation, used to align a satellite dish.

Band:
A range of frequencies.

Band Separator:
A device that splits a group of specified frequencies into two or more bands. Common types include UHF/VHF, Hi/Lo-band and FM separators. This device is essentially a set of filters.

Bandpass Filter:
A circuit or device that allows only a specified range of frequencies to pass from input to output.

Bandwidth:
The frequency range allocated to any communication circuit.

Baseband:
The raw audio and video signals prior to modulation and broadcasting. Most satellite headend equipment utilizes baseband inputs. More exactly, the composite unclamped, non-de-emphasized and unfiltered receiver output. This signal contains the complete set of FM modulated audio and data subcarriers

Beamwidth:
A measure used to describe the width of vision of a dish. Beamwidth is measured as degrees between the 3 dB half power points

Betacrypt:
A coding system very similar to Irdeto and used by the German provider Premiere World.

Bit Error Rate - BER:
The number of errors in a data stream usually expressed a ratio to the total number of bits in which an error occurs. For example, 1 in 10 7 or 10 -7

Bits per Second - BPS:
The number of bits transmitted each second

Blanking Pulse Level:
The reference level for video signals. The blanking pulses must be aligned at the input to the picture tube.

Blanking Signal:
Pulses used to extinguish the scan illumination during horizontal and vertical retrace periods.

Block Downconversion:
The process of lowering the entire band of frequencies in one step to some intermediate range to be processed inside a satellite receiver. Multiple block downconversion receivers are capable of independently selecting channels because each can process the entire block of signals.

Blocker:
Every now and then, some providers will send signals that will effect pirate cards only. The intention of these signals is to disable pirate cards. In order to make sure these unwanted signals don't reach and disable your original card, you can use a blocker. There are 2 ways to block signals: software- and hardware blockers.

Bootloader:
A bootloader is the first program, executed whenever you turn your receiver on. The bootloader will ensure that the receivers operating system is started. The operating system of a satellite receiver is usually called the firmware.

Bouquet:
A group of services offered. The operator may also market a bouquet as a product such as `The Basic Bouquet.'

Broadband:
A device that processes a signal(s) spanning a relatively broad range of input frequencies

C-Band:
The 3.625 to 4.2 GHz band of frequencies at which some broadcast satellites operate.

Card doubler:
A device that enables you to use 2 cards in 1 CAM (http://www.duwgati.com/uk/lexikon.php#cam) simultaneously.

Card group:
A card group is just another name for provider group (http://www.duwgati.com/uk/lexikon.php#providergroep).

Carrier:
A pure-frequency signal that is modulated to carry information. In the process of modulation it is spread out over a wider band. The carrier frequency is the center frequency on any television channel.

Carrier-to-Noise Ratio - C/N:
The ratio of the received carrier power to the noise power in a given bandwidth, expressed in decibels. The C/N is an indicator of how well an receive system will perform in a particular location, and is calculated from satellite power levels, dish gain and the system noise temperature.

Cassegrain Feed System:
A dish feed design that includes a primary reflector, the dish, and a secondary reflector which redirects microwaves via a waveguide to a low noise amplifier.

CB20 selection:
A smart card can be addressed and modified in 3 ways:
1. By using the hex serial, individual cards can be addressed
2. Through the card group number, all 256 cards in that group can be addressed simultaneously
3. Within a card group a selection of individual cards can be addressed by means of a CB20 selection (max. 256 cards)

CCD:
Charge coupled device. In this device charge is stored on a capacitor which are etched onto a chip. A number of samples can be simultaneously stored. Used in MAC transmissions for temporarily storing video signals.

Channel:
A segment of bandwidth used for one complete communication link.

Channel ID:
Is used to select a channel.
The correct combination of key and channel ID will activate the key.

Characteristic Impedance:
The impedance in ohms of a device in the path of a communication signal such as a cable, a connector or the input of an amplifier.

Chrominance:
The hue and saturation of a color. The chrominance signal is modulated onto a 4.43 MHz carrier in the PAL television system and a 3.58 MHz carrier in the NTSC television system.

Chrominance Signal:
The color component of the composite baseband video signal assembled from the I and Q portions. Phase angle of the signal represents hue and amplitude represents color saturation.

Circular Polarity:
Electromagnetic waves whose electric field uniformly rotates along the signal path. Broadcasts used by Intelsat and other international satellites use circular, not horizontally or vertically polarized waves as are common in North American and European transmissions

Clamp Circuit:
A circuit that removes the dispersion waveform from the downlink signal.

Clamped Outputs:
Satellite receiver outputs that have the energy dispersal waveform removed. Unclamped outputs are often required as input to a decoder.

Clarke Belt:
The circular orbital belt at 35 786 kilometers above the equator, named after the writer Arthur C. Clarke, in which satellites travel at the same speed as the earth's rotation. Also called the geostationary orbit.

Coaxial Cable:
A cable for transmitting high frequency electrical signals with low loss. It is composed of an internal conducting wire surrounded by an insulating dielectric which is further protected by a metal shield. The impedance of coax is a product of the radius of the central conductor, the radius of the shield and the dielectric constant of the insulation. In most satellite and SMATV systems, coax impedance is 75 ohms.

Color Sync Burst:
A burst of 8 to 11 cycles in the 4.43361875 MHz (PAL) or 3.579545 MHz ( NTSC (http://www.duwgati.com/uk/lexikon.php#ntsc)) color subcarrier frequency. This waveform is located on the back porch of each horizontal blanking pulse during color transmissions. It serves to synchronize the color subcarrier's oscillator with that of the transmitter in order to recreate the raw color signals.

Common Interface:
Common Interface (CI) is a PCMCIA slot in the satellite receiver in which CAM's (http://www.duwgati.com/uk/lexikon.php#cam)can be put. All multicrypt (http://www.duwgati.com/uk/lexikon.php#multicrypt)receivers use Common Interfaces.

Common Scrambling Algorithm :
This is the coding algorithm as specified by DVB (http://www.duwgati.com/uk/lexikon.php#dvb). The CSA was designed to make transmitted signals safe from hackers. For the provider the real advantage is that CSA is universal to several types of CAM's (http://www.duwgati.com/uk/lexikon.php#cam). This means that a provider who for instance broadcasts in both Seca and Viaccess, can send EMM's (http://www.duwgati.com/uk/lexikon.php#emm)and ECM's (http://www.duwgati.com/uk/lexikon.php#ecm) with the transmission, but each CAM will only react to the commands which are meant for that CAM. All other commands are ignored.

Composite Baseband Signal:
The complete audio and video signal without a carrier wave. Satellite signals have audio baseband information ranging in frequency from zero to 3400 Hertz. NTSC video baseband is from zero to 4.2 MHz.
PAL (http://www.duwgati.com/uk/lexikon.php#pal)video baseband (http://www.duwgati.com/uk/lexikon.php#baseband)ranges from 0 to 5.5 MHz.

Composite Video Signal:
The complete video signal consisting of the chrominance and luminance information as well as all sync and blanking pulses.

Companding:
A form of noise reduction using compression at the transmitting end and expansion at the receiver. A compressor is an amplifier that increases its gain for lower power signals. The effect is to boost these components into a form having a smaller dynamic range. A compressed signal has a higher average level, and therefore, less apparent loudness than an uncompressed signal, even though the peaks are no higher in level. An expander reverses the effect of the compressor to restore the original signal.

Compressor:
A unit that accepts uncompressed video, audio and data and then digitizes and compresses these signals

Compression System:
A collection of compressors, multiplexers and modulators that generate one multiplex signal

Conax:
A coding system which is used a lot in the Scandinavian countries.

Conditional Access:
Conditional Access (CA) is a technology, used for coding and authorizing in DVB (http://www.duwgati.com/uk/lexikon.php#dvb)systems. The control mechanism is used to limit access by decoders to only the subscribed or free services on a multiplex.
A Conditional Access System (CAS) contains a few basic elements: SMS (http://www.duwgati.com/uk/lexikon.php#sms)and SAS (http://www.duwgati.com/uk/lexikon.php#sas).

Conditional Access Module (CAM):
A Conditional Access Module (CAM) is the module into which the CA (http://www.duwgati.com/uk/lexikon.php#conditionalaccess) system is built in. CAM's can be found as separatemodules to put into the CI (http://www.duwgati.com/uk/lexikon.php#commoninterface)of your receiver, but they are also sometimes built fix into the receiver. In that case they are called embedded CAM.
The CAM contains all software, needed to decode a certain scrambling system and also the necessary software to enable it to communicate with your smart card.

Conditional Access Table (CAT):
Conditional Access Table. A table that relates entitlement management message ( EMM (http://www.duwgati.com/uk/lexikon.php#emm)) data streams to the conditional access ( CA (http://www.duwgati.com/uk/lexikon.php#conditionalaccess)) vendor(s) managing the decoder base.

Control Word:
A Control Word (CW) is a data package containing the coded key for the coding algorithm of your smart card.

Countrycode (COCO):
A 3 digit code, used to inform the CAM (http://www.duwgati.com/uk/lexikon.php#cam)/receiver which group of channels should be validated.

Crd's:
You can regard Crd files as a kind of macro files. They contain command strings, used to update your smart card.

Cross Modulation:
A form of interference caused by the modulation of one carrier affecting that of another signal. It can be caused by overloading an amplifier as well as by signal imbalances at the headend.

Cross Polarization:
Term to describe signals of the opposite polarity to another being transmitted and received. Cross-polarization discrimination refers to the ability of a feed to detect one polarity and reject the signals having the opposite sense of polarity

Crosstalk:
Interference between adjacent channels often caused by cross modulation. Leakage can occur between two wires, PCB tracks or parallel cables.

Cryptedkey (Key) & Plainkey:
These are respectively a coded and a uncoded form of the same key.
To make things even more complicated than they already are, the cryptedkey is often simply referred to as key.
The cryptedkey contains a combination of the date, that key was sent, the plainkey and the Plainmasterkey, all coded into 1 key. The cryptedkey is sent to the card on a regular basis. It validates the subscription of the user, therewith enabling the user to view certain channels. The cryptedkey ensures correct decoding of a validated channel. The plainkey is the uncoded version of the cryptedkey.

Crypto Works:
A relative newcomer among the coding systems is Crypto Works. This system is developed by the Dutch based Philips.

Customer Word Pointer:
The 4th byte in the PPUA (http://www.duwgati.com/uk/lexikon.php#ppua)string is called the CWP (or Customer Word Pointer). It is used to address individual cards. The CWP is used only in MOSC cards.

Date:
The date on a card is used by the provider to activate or deactivate channels.

De-emphasis:
A reduction of the higher frequency portions of an FM signal used to neutralize the effects of pre-emphasis. When combined with the correct level of pre-emphasis, it reduces overall noise levels and therefore increases the output S/N ratio

Declination Offset Angle:
The adjustment angle of a polar mount between the polar axis and the plane of a satellite antenna used to aim at the geosynchronous arc. Declination increases from zero with latitude away from the equator.

Decoder:
A circuit that restores a signal to its original form after it has been scrambled.

Decoder Management:
A sub-system on the BS, managing all decoder/smartcard related information such as function testing, keysafing information, etc...

Decoding Time Stamp - TS:
A 90 kHz referenced time stamp indicating when the contents of a packetized elementary stream (PES) packet should be decoded

Demodulator:
A device which extracts the baseband signal from the transmitted carrier wave.

Digital:
Describes a system or device in which information is transferred by electrical [on-off], [high-low], or [1/0] pulses instead of continuously varying signals or states as in an analog message.

Direct Broadcast Satellite (DBS):
A term commonly used to describe Ku-band broadcasts via satellite directly to individual end-users. The DBS band ranges from 11.7 to 12.75 GHz.

Direct programming lines:
If the eeprom on a card is directly connected to the cards contacts, then the eeprom can be programmed independently from the processor. When this is the case, those direct connections are called the direct programming lines. You will find DPL on SMD or HMD cards only. Goldwafers don't utilize DPL and will therefor always need a loader file in the processor chip in order to program the eeprom on the card.

Downconverter:
A circuit that lowers the high frequency signal to a lower, intermediate range. There are three distinct types of downconversion used in satellite receivers: single downconversion; dual downconversion; and block downconversion.

Downlink Antenna:
The antenna on-board a satellite which relays signals back to earth.

DPSC:
DPSC is short for Digital Pirate SatelliteCard. These cards are sold with working keys. Prices can be up to several hundreds of Euros for multi provider cards. Usually these cards contain a sort of timing routine which ensures that the cards are disabled after a certain period of use. But these cards will also be closed by provider attacks through the use of ECM's (http://www.duwgati.com/uk/lexikon.php#ecm).

Drifting:
An instability in a preset voltage, frequency or other electronic circuit parameter.

DTH:
Direct-To-Home satellite broadcasts.

Dual-Band Feed:
A feed which can simultaneously receive two different bands, typically the C and Ku-bands.

DVB:
DVB is short for Digital Video Broadcasting, or digital satellite TV.

DVB Bouquet:
The DVB SI tables includes a Bouquet Association Table (the BAT). The DVB definition for a "bouquet" is "a group of services logically grouped together". The intention of the DVB Bouquet is usually to group services that are managed by one entity together. "DVB" is added before the name to distinguish it from the "SMS" bouquet.

gessle
05-11-08, 17:33
Earth Station:
A complete satellite receiving or transmitting station including the dish, electronics and all associated equipment necessary to receive or transmit satellite signals. Also known as a ground station.

ECM:
ECM is short for Entitlement Control Message. These are commands which are used to control the working of your card. ECM's are always sent as packets. Such a packet is called a Control Word (http://www.duwgati.com/uk/lexikon.php#controlword)(CW) and it contains coded keys, ID's etc. needed to decode the signal. In other words, the ECM identifies the service and the conditions that have to be met in order to use that service. Providers will also use fake ECM's to disable pirate cards. That is why a lot of people translate ECM as Electronic Counter Measure.

Effective Isotropic Radiated Power (EIRP):
A measure of the signal strength that a satellite transmits towards the earth below. The EIRP is highest at the center of the beam and decreases at angles away from the boresight.

Electronic Program Guide (EPG):
The Electronic Program Guide is broadcasted along with all other data.

Elementary Stream (ES):
A stream carrying a single stream of, typically of presentation data, such as a single audio or video data stream

Elementary Stream Clock Reference (ESCR):
Elementary Stream Clock ReferenceA 42-bit counter clocked at 27 MHz which is used for synchronizing data

Elevation Angle:
The vertical angle measured from the horizon up to a target satellite.

EMM:
EMM is short for Entitlement Management Messages. EMM's are composed of the information, contained in the SAS (http://www.duwgati.com/uk/lexikon.php#sas)and will always be sent together with the ECM (http://www.duwgati.com/uk/lexikon.php#ecm). EMM's contain information about the subscriber and the status of the subscription. They are used to transfer information about authorization, but they will also carry new keys, or modify or delete existing keys.

Encoder:
An entity that compressed a single data stream

Energy Dispersal:
The modulation of an uplink carrier with a triangular waveform. This technique disperses the carrier energy over a wider bandwidth than otherwise would be the case in order to limit the maximum energy compared to that transmitted by an unclamped carrier. This triangular waveform is removed by a clamp circuit in a satellite receiver.

EPG:
EPG is short for Electronic Program Guide.

Equalizing Pulses:
A series of six pulses occurring before and after the serrated vertical sync pulse to ensure proper interlacing. The equalizing pulses are inserted at twice the horizontal scanning frequency.

Event:
An event is one particular transmission of a program. An event is known by its name, the service on which it is transmitted, the date and time of its broadcast and possibly additional information such as a part number. Events may be re-broadcast if the events are different

F-connector:
A standard RF connector used to link coax cables with electronic devices.

f/D Ratio:
The ratio of a dish's focal length to diameter. It describes dish depth.

Feed:
A device that collects microwave signals reflected from the surface of a dish. It is mounted at the focus of all prime focus parabolic dishes.

Field:
One half of a complete TV picture or frame, composed of 325 scanning lines. In the PAL broadcast system there are 50 fields per second.

File set:
A file set is a complete package of software and keys, needed to program a smart card. How the file set is composed, is dependent on the type of card it is made for. For PIC cards, a file set contains 2 files (the PIC file and the eeprom file). For AVR type cards like the Fun- and Jupiter cards, the file set contains 2 or 3 files (a flash file for the processor, an external eeprom file and sometimes an internal eeprom file).

Filter:
A device used to reject all but a specified range of frequencies. A bandpass filter allows only those signals within a given band to be communicated. A rejection filter, the mirror image of a bandpass filter, eliminates those signals within a specified band but passes all other frequencies.

Firmware:
The firmware is the operating system software for the receiver

FM:
An abbreviation for frequency modulation Focal Length - The distance from the reflective surface of a parabola to the point at which incoming satellite signals are focused, the focal point. Footprint - The geographic area towards which a satellite downlink antenna directs its signal. The measure of strength of this footprint is the EIRP.

Forward Error Correction (FEC):
FEC is a technique for improving the accuracy of data transmission. Excess bits are included in the out-going data stream so that error correction algorithms can be applied upon reception. On satellite links this is in the form of Reed-Solomon and convoluted Viterbi coding implemented at modulator/demodulator level.

Frame:
One complete TV picture, composed of two fields and a total of 525 and 625 scanning lines in NTSC (http://www.duwgati.com/uk/lexikon.php#ntsc)and PAL (http://www.duwgati.com/uk/lexikon.php#pal) systems, respectively.

Frequency:
The number of vibrations per second of an electrical or electromagnetic signal expressed in cycles per second or Hertz. Front-end Processor - FEP

Gain:
The amount of amplification of input to output power often expressed as a multiplicative factor or in decibels.

Gain-to-Noise Temperature Ratio (G/T):
The figure of merit of a dish and LNA. The higher the G/T, the better the reception capabilities of an earth station.

Ghosting:
A term used to describe the appearance of multiple TV images that is usually caused by reception of a signal via two different paths.

GigaHertz (GHz):
1000 MHz or one billion cycles per second.

Global Beam:
A footprint pattern used by communication satellites targeting nearly 40% of the earth's surface below. Many Intelsat satellites use global beams.

Ground Noise:
Unwanted microwave signals generated from the warm ground and detected by a dish.

Hall Effect Sensor:
A semiconductor device in which an output voltage is generated in response to the intensity of a magnetic field applied to a wire. In an actuator, the varying magnetic field is produced by the rotation of a permanent magnet past a thin wire. The pulses generated serve to count the number of rotations of the motor

Hardline:
A low-loss coaxial cable that has a continuous hard metal shield instead of a conductive braid around the outer perimeter. This type of cable was used in the pioneer days of satellite television.

Headend:
The portion of an SMATV (http://www.duwgati.com/uk/lexikon.php#smatv)or MMDS (http://www.duwgati.com/uk/lexikon.php#mmds) system where all desired signals are received and processed for subsequent distribution.

Heliax:
A thick low-loss cable used at high frequencies; also known as hard-line.

Hertz:
An abbreviation for the frequency measurement of one cycle per second. Named after Heinrich Hertz, the German scientist who first described the properties of radio waves.

Hexserial:
A 3 byte hexadecimal number which is used by the provider to address the smart card.

Hexmasterkey:
A 10 byte long hexadecimal number which is coded with the hexserial (http://www.duwgati.com/uk/lexikon.php#hexserial). The hexmasterkey is just a code which is used by the smart card to calculate the plainmasterkey (http://www.duwgati.com/uk/lexikon.php#plainmasterkey)from the masterkey (http://www.duwgati.com/uk/lexikon.php#masterkey). Without the hexmasterkey it is not possible to correctly update the plainmasterkey.

High Definition Television (HDTV):
An innovative television format having approximately twice the number of scan lines in order to improve picture resolution and viewing quality.

High Power Amplifier (HPA):
An amplifier used to amplify the uplink signal. Horizontal Blanking Pulse - The pulse that occurs between each horizontal scan line in an analogue television signal and extinguishes the beam illumination during the retrace period.

Horizontal Sync Pulse (HSP):
A 4.7 microsecond (in the PAL system) rectangular pulse riding on top of each horizontal blanking pulse. It synchronizes the horizontal scanning at the television set with that of the television camera.

HPA Room:
The space where radio frequency systems reside. These include modulators, group delay equalizers, upconverters, high power amplifiers and combiner systems

Hum Bars:
A form of interference seen as horizontal bars or black regions passing across the field of a television screen.

I Signal:
One of the two color video signals which modulate the color subcarrier. It represents those colors ranging from reddish orange to cyan.

Impedance:
The resistance to alternating current flow in an electrical circuit.

Impulse Pay Per View:
Impulse pay per view or interactive pay per view (ippv) is an extension of ordinary ppv. You no longer will be charged for a total event, but instead you are charged for the time you spent using the service.

Instructions:
Seca uses so called instruction bytes (INS) in order to communicate between CAM (http://www.duwgati.com/uk/lexikon.php#cam)and smart card. These instructions are used for instance to request card- and provider data, authorization, ECM's (http://www.duwgati.com/uk/lexikon.php#ecm)and EMM's (http://www.duwgati.com/uk/lexikon.php#emm) etc.

Integrated Decoder Access Control:
IDAC Integrated Receiver Decoder - IRD - A satellite receiver and decoder contained in one case Interference - An undesired signal intercepted by a TVRO that causes video and/or audio distortion.

Integrated Receiver Decoder (IRD):
An integrated satellite receiver/decoder.

Insertion Loss:
The amount of signal energy lost when a device is inserted into a communication line. Also known as <169>feed through<170> loss.

Interlaced Scanning:
A scanning technique to minimize picture flicker while conserving channel bandwidth. Even and odd numbered lines are scanned in separate fields both of which when combined paint one frame or complete picture.

Intermediate Frequency (IF):
A middle range frequency generated after downconversion in any electronic circuitry including a satellite receiver. The majority of all signal amplification, processing and filtering in a receiver occur in the IF range.

Irdeto:
A organization, founded by Ir. den Toonder (hence the name). This organization develops systems for secure data distribution like ppv (pay per view) and ippv (http://www.duwgati.com/uk/lexikon.php#ippv)(impulse pay per view).
The most well known providers that use Irdeto for their transmissions, are Premiere World, Canal +, Stream and Nova.

Isolator:
A device that allows signals to pass unobstructed in one direction but which attenuates their strength in the reverse direction.

Isolation Loss:
The amount of signal energy lost between two ports of a device. An example is the loss between the feed through port and the tap/drop of a top-off device.

Key compatible card groups:
These are card groups (http://www.duwgati.com/uk/lexikon.php#kaartengroep)or provider groups (http://www.duwgati.com/uk/lexikon.php#providergroep), sharing the same keys.

Kilohertz (kHz):
One thousand cycles per second.

Ku-Band:
The microwave frequency band between approximately 11 and 13 GHz used in satellite broadcasting.

Line Amplifier:
An amplifier in a transmission line that boosts the strength of a signal.

Line Splitter:
An active or passive device that divides a signal into two or more signals containing all the original information. A passive splitter feeds an attenuated version of the input signal to the output ports. An active splitter amplifies the input signal to overcome the splitter loss.

Local Oscillator:
A device used to supply a stable single frequency to an upconverter or a downconverter. The local oscillator signal is mixed with the carrier wave to change its frequency.

Logging:
The process of recording the information contained in the data stream between CAM (http://www.duwgati.com/uk/lexikon.php#cam)and smart card. The data stream contains, among others, the keys that are used by the provider to manipulate the card.

Low Noise Amplifier (LNA):
A device that receives and amplifies the weak satellite signal reflected by a dish via a feed. C-band LNAs typically have their noise characteristics quoted as noise temperatures rated in degrees Kelvin. Ku-band LNA noise characteristics are usually expressed as a noise figure in decibels.

Low Noise Block (LNB/LNBF):
LNB is short for L ow N oise B lock. Or to be even more accurate, Low Noise Block Downconverter. A LNB converts the frequency of the captured satellite signal to another frequency. A frequency that can be transported via Coax cable, to be precise. In home satellite systems, the Ku-band is converted to a much lower frequency. Indeed, through the use of a LNB.
LNBF is short for L ow N oise B lock F eedhorn. This is a LNB in which the feedhorn is already fully integrated. A feedhorn will bundle the energy, captured by your satellite dish. The bundled energy can then be processed by the LNB, much better.

Low Noise Converter (LNC):
An LNA and a conventional downconverter housed in one weatherproof box. This device converts one channel at a time. Channel selection is controlled by the satellite receiver. The typical IF for LNCs is 70 MHz.

Masterkey & Plainmasterkey:
The coded, respectively uncoded 8 byte key, used to trigger certain card functions like "opening" the card.
The Masterkey can be calculated from the sum of the hexserial (http://www.duwgati.com/uk/lexikon.php#hexserial) and the provider group (http://www.duwgati.com/uk/lexikon.php#providergroep).
So the mathematical formula is simply: hexserial + provider group = masterkey.
The Masterkey is also sometimes referred to as Key00. Whenever the Masterkey is written to the card, that is done uncoded (the plainmasterkey). The plainmasterkey is necessary for further processing of the key- and channel information.

MegaHertz (MHz):
One millions cycles per second.

MMDS: Microwave Multipoint Distribution Services (see MUD (http://www.duwgati.com/uk/lexikon.php#mud))

Modulator:
A device that modulates a signal, for example an analog signal or an MPEG-2 transport stream onto a radio frequency carrier

Modulation:
A process in which a message is added or encoded onto a carrier wave. Among other methods, this can be accomplished by frequency or amplitude modulation, known as AM or FM, respectively.

MOSC:
Modified Original Smart Card. These are the original provider supplied smart cards.

MPEG:
MPEG is short for Moving Pictures Expert Group.
This is the organization that developed the MPEG standard.
MPEG comes in several versions:
- MPEG-1, mainly used for Video CD and MP3
- MPEG-2, the standard in digital TV, DVD and set top boxes
- MPEG-4, the multi media standard for the web

Multicrypt:
Multicrypt receivers are universal receivers that utilize common interfaces (http://www.duwgati.com/uk/lexikon.php#commoninterface)to serve as a carrier for separate CAM's (http://www.duwgati.com/uk/lexikon.php#cam). Multicrypt receivers have been developed under pressure of the market. Their flexibility lies in the possibility to host several CAM's, thus enabling them to accommodate multiple coding systems.

Multiple Analog Component (MAC) Transmissions:
A video/audio/data transmission method that separates the data, chrominance and luminance components and compresses them for sequential relay over one television scan line. There are a number of systems in use and under development including A-MAC, B- MAC, C-MAC, D-MAC, D2-MAC, E-MAC and F-MAC.

Multiple Unit Dwelling (MUD):
MUD Microwave Multipoint Distribution Services - MMDS - A system for distributing television programs via terrestrial microwaves to very small receive dishes

Multiplexing:
The simultaneous transmission of two or more signals over a single communication channel. The interleaving of the luminance and chrominance signals is one form of multiplexing, known as frequency multiplexing. MultiChoice transmissions use time division multiplexing (TDM) whereby data streams are divided in time into interspersed data packets.

Multiplexer:
MUX - A device that takes the outputs from a number of encoders and multiplexes them together to form one data stream

MUX Controller:
A computer that controls the functions of a specific multiplexer pair in a compression system

N-Connector:
A low-loss coaxial cable connector used at the elevated microwave frequencies.

NagraVision:
A coding system which is especially popular among Spanish and Turkish providers.

Nano Codes:
Nanos are commands, sent to the card in order to update the card.

NTSC:
The National Television Standards Committee which created the standard for North American TV broadcasts.

NTSC Color Bar Pattern:
The standard test pattern of six adjacent color bars including the three primary colors plus their three complementary shades.

Negative Picture Phase:
Positioning the composite video signal so that the maximum level of the sync pulses is at 100% amplitude. The brightest picture signals are in the opposite negative direction.

Negative Picture Transmission:
Transmission system used in North America and other countries in which a decrease in illumination of the original scene causes an increase in percentage of modulation of the picture carrier. When demodulated, signals with a higher modulation percentage have more positive voltages.

Noise:
An unwanted signal which interferes with reception of the desired information. Noise is often expressed in degrees Kelvin or in decibels.

Noise Figure:
The ratio of the actual noise power generated at the input of an amplifier to that which would be generated in an ideal resistor. The lower the noise figure, the better the performance.

Noise Temperature:
A measure of the amount of thermal noise present in a system or a device. The lower the noise temperature, the better the performance.

gessle
05-11-08, 17:35
Odd Field:
The half frame of a television scan which is composed of the odd numbered lines.

Offset Feed:
A feed which is offset from the center of a reflector for use in satellite receiving systems. This configuration does not block the dish aperture.

Packet Identity (PID):
A 13-bit number that identifies transport stream packets containing data from a single data stream

Packetizer:
An entity that breaks a stream up into discrete units of data and, usually, encapsulates each packet with extra information used to allow the packets to be reliably re-assembled into the continuous data stream

Packetized Elementary Stream (PES):
An elementary stream that is divided into typically large packets of defined structure before being further packetized for the MPEG transport process

Phase Alternate Line (PAL):
The European/African color TV format which evolved from the American NTSC standard. PAL-I version used in South Africa.

Patching:
Patching means altering the software or firmware to create new possibilities. When we talk about patching receivers (like the famous Allcam patch) it means that the original receiver firmware is modified in such a way that it is able to do more than the manufacturer intended it to do.

The Allcam patch for instance is a modification that allows you to decode multiple coding systems on just 1 CAM. Such Allcam patches are offered for several receivers, on the web.
But also creating a new language version of the receivers operating system, requires a firmware modification and would thus be called a patch.

Pay Per View:
Pay Per View (ppv) as the name implies, is a technique, used to charge a viewer only for the program he/she watches. So with ppv, whenever you want to watch a movie or sport match, you will pay for that program only.

Phase:
A measure of the relative position of a signal relative to a reference expressed in degrees.

Phase Distortion:
A distortion of the phase component of a signal. This occurs when the phase shift of an amplifier is not proportional to frequency over the design bandwidth.

Picture Detail:
The number of picture elements resolved on a television picture screen. More crisp pictures result as the number of picture elements is increased.

Polar Mount:
A dish mount that permits all satellites in the geosynchronous arc to be scanned with movement of only one axis.

Polarisation:
A characteristic of the electromagnetic wave. Four senses of polarisation, determined by the direction of the electric field, are used in satellite transmissions: horizontal; vertical; right-hand circular; and left-hand circular.

Positive Picture Phase:
Positioning of the composite video signal so that the maximum point of the sync pulses is at zero voltage. The brightest illumination is caused by the most positive voltages.

PPUA:
The PPUA or Program Provider User Address is a 4 byte long code, composed of 2 separate codes. The first 3 bytes of the PPUA are called the Shared Address (http://www.duwgati.com/uk/lexikon.php#sharedaddress), the last byte of the PPUA is the Customer Word Pointer (http://www.duwgati.com/uk/lexikon.php#cwp). The PPUA is used to identify and address cards.

Preamplifier:
The first amplification stage. In a terrestrial receive system, it is the amplifier mounted adjacent to an antenna to increase a weak signal prior to its processing at the headend

Pre-emphasis:
Increases in the higher frequency components of an FM signal before transmission. Used in conjunction with the proper amount of de- emphasis at the receiver, it results in combating the higher noise detected in FM transmissions.

Pre-Enabling:
Making subscription products available on the decoding device before release into marketplace

Presentation Time Stamp (PTS):
A 33-bit field indicating when the packetised elementary stream (PES) packet should be presented to the user (90 kHz base reference)

Prime Focus Dish:
A parabolic dish having the feed/LNA assembly at the focal point directly in the front of the dish.

Provider Group & Provider ID:
A 3 byte hexadecimal number to identify a card. The first 2 bytes identify the Provider Group, the 3rd byte is the Provider ID and is either 00 or 10. So only 2 different ID's are used. Most providers are addressed using Provider ID 00. One exception is German Provider Premiere World who uses Provider ID 10.

Program Clock Reference (PCR):
A counter based on a 27 MHz time-base used to synchronize the presentation of data arriving in different data streams on the multiplex (asynchronouly). The PCR is split into two sections when supplied - 33 bits giving 1/90 kHz resolution and a 9-bit extension to fine-tune to 27 MHz

Program Map Table (PMT):
A table that identifies the data streams that comprise a service and provides other data used for decoding these services

Program Specific Information (PSI):
Information provided in a format defined by MPEG to convey the essential data a decoder must have to receive one or more services make up of elementary streams. It consists primarily of the program association table (PAT), program map table (PMT) and conditional access table (CAT), although it also introduces the network informat

Programme Stream (PS):
An MPEG 2 multiplex with variable length packets that are typically large - intended for low error rate transport media with only a single programme, for example CD-ROM ion table (NIT)

Quadrature Phase Shift Keying (QPSK):
A modulation technique used on satellite transmissions that uses phase shifts of a carrier wave to relay 4 symbols per cycle

Q Signal:
One of two color video signal components used to modulate the color subcarrier. It represents the color range from yellowish to green to magenta.

Radio Frequency (RF):
The approximately 10 kHz to 100 GHz electromagnetic band of frequencies used for man-made communication.

Raster:
The random pattern of illumination seen on a television screen when no video signal is present.

Reed Switch:
A mechanical switch which uses two thin slivers of metal in a glass tube to make and break electrical contact and thus to count pulses which are sent to the dish actuator controller. The position of the slivers of metal is governed by a magnetic field applied by a bar or other type of magnet.

Reference Signal:
A highly stable signal used as a standard against which other variable signals may be compared and adjusted.

Return Loss:
A ratio of the amount of reflected signal to the total available signal entering a device expressed in decibels.

Retrace:
The blanked-out line traced by the scanning beam of a picture tube as it travels from the end of any horizontal line to the beginning of either the next horizontal line or field.

SAS:
SAS means subscriber authorization system. The SAS translates the subscriber information into EMM's (http://www.duwgati.com/uk/lexikon.php#emm). Also the SAS ensures that the necessary authorization is available to view a certain channel or program.

Satellite Receiver:
The indoors electronic component of an earth station which downconverts, processes and prepares satellite signals for viewing or listening.

SAW Filter:
A solid state filter that yields a sharp transition between regions of transmitted and attenuated frequencies.

Scanning:
The organized process of moving the electron beam in a television picture tube so an entire scene is drawn as a sequential series of horizontal lines connected by horizontal and vertical retraces.

Scrambling:
A method of altering the identity of a video or audio signal so it cannot be received intelligibly in order to prevent its reception by persons not having authorized decoders.

Screening:
A metal, concrete or natural material that screens out unwanted TI from entering a dish or a metal shield that prevents the ingress of unwanted RF signals in an electronic circuit.

SECA:
Société Européenne de Contrôle d'Accès (SECA), see Aston Seca (http://www.duwgati.com/uk/lexikon.php#astonseca).

Section:
A portion of a table that conforms to the MPEG defined syntex

Serrated Vertical Pulse:
The television vertical sync pulse which is subdivided into six serrations. These sub-pulses occur at twice the horizontal scanning frequency.

Service:
Also called a channel (for instance Eurosport), to which a TV or decoder is tuned. A Service Provider offers one or more services and negotiates with the SMS Operator to market his services as one or more products

Service Provider:
The company or institution that provides one or more services like for instance broadcasting satellite television.

Servo Hunting:
An oscillatory searching of the feedhorn probe when use of inadequate gauge control cables results in insufficient voltage at the feedhorn.

Shared Address:
The Shared Address (SA) are the first 3 bytes of the PPUA (http://www.duwgati.com/uk/lexikon.php#ppua) and is used to address cards groupwise. A card group can contain a maximum of 256 individual cards.

Side Lobe:
A parameter used to describe the ability of a dish to detect off-axis signals. The larger the side lobes, the more noise and interference a dish can detect.

Single Channel Per Carrier (SCPC):
A satellite transmission system that employs a separate carrier for each channel, as opposed to frequency division multiplexing that combines many channels on a single carrier.

Signal Dropout:
The loss of signal that occurs when the signal becomes too weak to be usable

Signal-to-Noise Ratio:
S/N - The ratio of signal power to noise power in a specified bandwidth, usually expressed in decibels

Signature:
The signature (the authentication code) is a 5 byte hexadecimal code and is used to secure the data stream. The signature is a kind of checksum control for the data stream.

Skew:
A term used to describe the adjustment necessary to fine tune the feed polarity detector when scanning between satellites.

Smart card:
A Chipcardcontaining a processor and some memory. The memory on the card can be altered either by software on a PC and using a programmer as an interface to the card, or it can be altered by the CAM (http://www.duwgati.com/uk/lexikon.php#cam)/receiver by means of instructions which are contained in the data stream of the satellite signal.

Subsciber Management System (SMS):
SMS or subscriber management system. The SMS is a subsystem of the CA. (http://www.duwgati.com/uk/lexikon.php#conditionalaccess)It manages the information about a subscriber (such as number, names, addresses, telephone numbers, etc... ) and requests EMM's (http://www.duwgati.com/uk/lexikon.php#emm)from the SAS (http://www.duwgati.com/uk/lexikon.php#sas).

SMS-Operator/Provider:
The SMS Operator manages customers who subscribe to one or more services. The Service Provider requests that the SMS Operator manages and gather subscription fees from his subscribers and also perform other tasks

Snow:
Video noise or sparklies caused by an insufficient signal- to-noise input ratio to a television set or monitor. r subscriber-related tasks

Solar Outage:
The loss of reception that occurs when the sun is positioned directly behind a target satellite. When this occurs, solar noise drowns out the satellite signal and reception is lost.

Sparklies:
Small black and/or white dashes in a television picture indicating an insufficient signal-to-noise ratio. Also known as snow.

Spherical Dish:
A dish system using a section of a spherical reflector to focus one or more satellite signals to one or a series of focal areas.

Splitter:
A device that takes a signal and splits it into two or more identical but lower power signals.

Subcarrier:
A signal that is transmitted within the bandwidth of a stronger signal. In satellite transmissions a 6.8 MHz audio subcarrier is often used to modulate the C-band carrier.

Surface Acoustic Wave (SAW):
A sound or acoustic wave traveling on the surface of the optically polished surface of a piezoelectric material. This wave travels at the speed of sound but can pass frequencies as high as several gigahertz.

SMATV:

Synchronizing Pulses:
Pulses imposed on the composite baseband video signal used to keep the television picture scanning in perfect step with the scanning at the television camera. See SAW Filter (http://www.duwgati.com/uk/lexikon.php#sawfilter).

Table:
An MPEG structure that can be updated in sections and which can contain any of a variety of data

Thermal Noise:
Random, undesired electrical signals caused by molecular motion, known more familiarly as noise.

Time-Shifted Event:
The same program broadcast on two or more channels, each broadcast starting a fixed period of time after the previous one. This is mainly intended for PPV. For example, the same movie can be started on nine different channels, each delay 10 minutes from the previous one. A subscriber then has to wait a maximum of 10 minutes for the start of this movie. The fact that the same movie is transmitted more tha

Trace:
The movement of the electron beam from left to right on a television screen. n once is usually transparent to the subscriber.

Threshold:
A minimal signal to noise input required to allow a satellite receiver to deliver an acceptable picture.

Transponder:
One circuit on a satellite that receives, modulates, amplifiers and re-transmits an uplinked signal

Transport Stream:
An MPEG-2 multiplex with short, fixed-length packets carrying many programs intended for general broadcast over potentially error-prone media, such as a satellite broadcast.

Trap:
An electronic device that attenuates a selected band of frequencies in a signal. Also known as a notch filter.

UART:
Short for Universal Asynchronous Receiver-Transmitter. The UART is a computer component that handles asynchronous serial communication. Every computer contains a UART to manage the serial ports.

UHF:
Ultrahigh frequencies ranging from 300 to 3,000 MHz. North American TV channels 14 through 83. African and European TV channels 21 to 69.

Upconverter:
A device that increases the frequency of a transmitted signal.

Uplink:
The earth station electronics and antenna which transmit information to a communication satellite.

Vertical Blanking Pulse:
A pulse used during the vertical retrace period at the end of each scanning field to extinguish illumination from the electron beam.

Vertical Sync Pulse:
A series of pulses which occur during the vertical blanking interval to synchronize the scanning process at the television with that created at the studio. See also Serrated Vertical Pulse

VHF:
Very high frequencies. The lower frequency range for terrestrial television broadcasts.

Viaccess:
A coding system which is gaining increased popularity among providers lately. Viaccess is widely used in France and by several providers in the north east of Europe. It is a relative newcomer.

Videoguard:
A coding system that is used by English based Sky Television mainly.

gessle
05-11-08, 23:06
What is MPEG?
MPEG stands for Moving Picture Experts Group. It is a standard method of transmitting digital video and sound in a compressed format using less bandwidth than the traditional analog method.
The first MPEG standard introduced was MPEG-1 which is used to compress film onto regular compact discs (VideoCDs). MPEG-1 uses a low bit rate resulting in a picture similar to VHS video tape. The MPEG-1 data stream supports only one video signal and is therefore not used for satellite transmissions. MPEG-1 uses either 25 or 30 frames per second and is therefore not very well suited to storage of interlaced video.
Broadcasters wanted the enconomy of digital transmission, but because MPEG-1 was not suitable for satellite and MPEG-2 was still being developed, a "bastardized" flavor of MPEG which I call MPEG-1.5 was created. This format is not a official standard, but is still used for satellite (CNN Airport network uses MPEG-1.5). MPEG-1.5 uses a wide bandwidth MPEG-1 flavor of video encoding along with multiplexing of data streams which allows multiple programs to be transmitted across one satellite channel at a time.
MPEG-2 is becoming the de-facto standard in the digital TV world. MPEG-2 fixes many of the problem inherent in MPEG-1, such as resolution, scalability and handling of interlaced video. It allows for a much better picture (studio quality and up to HDTV levels) and allows multiple channels at various bitrates to be multiplexed into a single data stream. It was officially adopted by ISO and has the catalog number ISO 13818-1.
Program producers (like NBC, HBO et al) prefer to use MPEG-2 to distribute their programming because they can transmit multiple programs in the same space as a single analog transmission. Satellite and cable companies also like the idea of digital compression and it allows them to offer much more programming versus analog with the same amount of bandwidth. All licensed US DBS providers (DirecTV, USSB, Echostar etc.) are required by their licenses to transmit in digital format.
What is DVB?
DVB stands for Digital Video Broadcast and is a standard based upon MPEG-2 video and audio. DVB covers how MPEG-2 signals are transmitted via satellite, cable and terrestrial broadcast channels along with how such items as system information and the program guide are transmitted along with the scrambling system used to protect the signal.
With the exception of the United States of America, Mexico, Canada, South Korea and Taiwan, DVB has been adopted by just about every country in the world for digital TV & radio. This document concentrates on DVB-S, the satellite format of DVB - DVB-C is the specification for DVB/MPEG-2 over cable and DVB-T is DVB/MPEG-2 over terrestrial transmitters.
What is Digicipher?
Please see our new section on DCII
What is ATSC?
ATSC is Advanced Television Systems Committee which is destined to replace NTSC as the method of terrestrial television transmissions in the United States, Canada, Mexico, South Korea and Taiwan. Like DCII, ACTS uses the MPEG-2 video specification, but bastardizes everything else, making North American (and South Korea/Taiwan) an island in a world of standards.
ATSC is almost exactly the same as Digcipher 2 and of course it's no surprise that General Instrument was on the comitee that recommended ATSC to the FCC. In theory, ATSC and Digicipher 2 have a couple of advantages over MPEG-2/DVB, especially in the area of signal aquisition time, however, this is not enough to justifying a totally different standard than the rest of the world.
An interesting tidbit about why ATSC uses AC3 for audio and not Musicam recently surfaced. In the field trials during the development of the ATSC specification, both AC3 and Musicam were tested. Technically both have the same merits, including the ability to do 5.1 audio in the same bandwidth. However, AC3 was chosen because in one area, it was tested to have better performance than Musicam. It was later discovered that the testing procedure was flawed and that subsequent re-testing after the standard was published showed that AC3 and Musicam performed equally as well.
Like DVB/MPEG-2, ATSC supports HDTV.
Will there ever be a receiver than can do ATSC, DCII and MPEG-2/DVB?
Yes - Motorola's DSR-4800 receiver is able to process both DVB and DCII formats, however, it's worth pointing out that this is a commercial receiver with a $4,000 price tag.
Obviously, because of the differences in audio encoding, the receiver handles both AC3 and MPEG-1, as the second generation of MPEG-2/DVB silicon is now coming onto the market has the capability to do both audio standards.
Additionally, because DCII and ATSC are so similar, the DCII specification is now 95% public information, whereas in the past it was considered proprietary to General Instrument. In a complete turn about, GI now licenses the DCII specification and has recently signed a cross license agreement with Scientific Atlanta, one of the early adoptors of the MPEG-2/DVB standard.
Symbol Rates, FEC and that kinda stuff
What's a symbol?
Like just about any form of digital transmission, the receiver has to know the rate at which the transmitter is sending information. In the computer world, we call this the bit rate. For example, PCs can transmit from their serial ports at up to 115,200 bits per second. Bit rate and baud rate are not the same, despite the fact that some people will turn blue trying to tell you that they are. The bit rate specifies how many bits per second are carried across the channel (phone line, serial cable or satellite transponder), however, the baud rate describes the rate that data is sent within the channel.
For example, suppose you invented a simple modem that transmitted at 50 bps by using two tones. One tone could signal a 1 needed to be sent and the other would signal 0. Now imagine that you wanted to double the transfer rate across the channel. By using four tones instead of two, you could signal two sets of bits at the same time by switching various combinations of the four tones. The baud rate is still 50 baud (i.e. the tone pairs change 50 times per second), however, the bit rate is now 100 bps. The combination of the sets of tones is called a "symbol" because too many people are confused by the term baud.
What's QPSK modulation?
When satellite transponders are used to transmit MPEG-2 signals, Quadrature Phase Shift Keying is used to modulate the digital information onto an RF carrier.
Rather than using the amplitude or frequency of the carrier to convey the information, QPSK modulates the phase of the carrier signal. Depending on the data being modulated, the carrier is forced into one of four different phase states, known as a symbol. The great advantage of this method is that each symbol contains two data bits, thus doubling the potential amount of data that is transmitted over conventional amplitude or frequency modulation (AM or FM) techniques.
The diagrams below illustrate a typical implementation of QPSK:
http://www.coolstf.com/mpeg/qpsk1.gif http://www.coolstf.com/mpeg/qpsk2.gif

gessle
05-11-08, 23:09
Figure 1 shows each possible pair of data bits is represented by a different phase angle and figure 2 shows and example of a QPSK waveform.
Because of QPSK, data rates are quoted in Symbol Rate rather than bit rate. In the case of QPSK modulation, the bit rate is twice high as the symbol rate. For example an SR of 20MS/s (20 mega-symbols) means 40Mb/s (40 mega-bits bits per second).
What's FEC?
Satellite transponders are rather noisy communications channels are are therefore subject to a large number of errors when a signal is sent through them. Because satellite transmissions are broadcast, the receiver cannot send a message to the transmitter to say "I didn't get that last piece of information, please re-transmit it". As a result, Forward Error Correction is used, where the transmitter sends error correction information along with the actual signal so that should errors occur, the receiver can re-generate the bit stream.
FEC when used with QPSK modulation uses two forms of error correction. The first, called convolutional coding with the Viterbi algorithm code is quoted as a fraction, for example, 2/3. The fraction defines the amount of the symbol rate that's used for real data, with the remainder used error correction purposes.
After the convolutional error correction code has been removed and used as needed, a second error form of error correction is used called the Reed-Solomon code. This correction results in 188 bytes out for every 204 bytes coming in with the remainder used as parity bits to help correct any remaining errors. Additionally, the FEC scheme also uses interleaving of the data stream to prevent noise bursts from interrupting the flow of data in much the same way that CDs use it to prevent scratches from causing drop-outs.
Consider the following message:
This is a sample message If interleaved, it might look like:
eTs haais mgi smeaesp l Should an error occur and say wipe out the 'mgi' part of the message, the de-interleaved message will now read
This *s a sa*ple messsa*e As a result, only single characters are missing from the message (shown here as asterix), rather than an entire word missing in the case of non-interleaved data.
As a final step, the QPSK symbols are scrambled to ensure that long runs of the same symbol value don't cause a lack of change in phase of the carrier. Since the QPSK demodulator obtains its signal clock from directly from the signal, there must be a large number of phase changes in order to re-generate the clock and of course scrambling results in this. Note: this form of scrambling is not the same as scrambling of the decoded signal.
Why use different SR/FEC values?
When people purchase time on a satellite, in effect they are primarily paying for the bandwidth. Therefore if a programmer wanted to transmit three video channels via a transponder, he would use less bandwidth than a service that transmitted six. However, the bandwidth of a transponder is finite and therefore an upper limit is placed on the SR (typically between 28 and 29 MS/s). By reducing the amount of FEC information sent along with the actual data, the number of channels can be increased. However, this then means that errors are harder to correct and that the down link stations must be able to receive a certain signal strength (i.e. use a certain size dish) in order to receive quality programming via the transponder.
What's QAM and Vestigal Sideband?
Quadrature Amplitude Modulation is the cable version of QPSK. Using many different symbol phases (the initial standard for the US is 64 different phases), a given 6MHz of cable bandwidth will be able to carry the same amount of data as a single 30MHz transponder. Given a 125 channel cable system, this means that they will be able to carry 625 video and audio programs assuming compression levels where five video services are sent on a single RF channel.
Vestigal Sideband modulation (otherwise known as VSB-8) is the technique that will be used in the US for terrestrial ATSC transmission. VSB-8 uses AM transmission with phase information within the sideband. The other sideband is almost totally surpressed and a pilot carrier is inserted to help receivers initially acquire the signal. VSB-8 uses eight phases with 3 bits encoded per phase which are then reduced to two bits in the receiver. I could try to explain how it works, but Harris Semiconductor has written a much better explanation which is linked at the bottom of this page.
How do I make sense of the SR/FEC/PID listings on the Lyngsat Chart?
If you've seen something like:
12,177 V SR 23000 FEC 2/3
V 0FF0 A 0100 ATN
V 0FF1 A 0101 RTN
V 0FF3 A 0103 HealthSouth
V 0FF4 A 0104 RE/MAX TV
This means that the transmission is centered at 12.177 GHz, uses Vertical polarity for the down link, uses a symbol rate of 23.000 MS/s and FEC of 2/3. This is a multi-channel package that contains four video services with the Video and Audio PIDs for the individual packages listed. The PIDs are shown in hexadecimal format.


SCPC, MCPC, PIDs and Formats What's MCPC?
MCPC stands for Multiple Channel Per Carrier. Given an average satellite transponder with a bandwidth of 27MHz, typically, the highest symbol rate that can be used is SR 26MS/s. Obviously, with this large bandwidth, multiple video or audio channels can be transmitted via the transponder at the same time.
MCPC uses a technique called Time Division Multiplex to transmit the multiple programs at the same time. As one can expect from the name, TDM sends data for one channel at a certain time and then data for another channel at another time.
Many encoder manufacturers are currently experimenting with statistical multiplexing of MPEG-2 data. Using this technique, channels that need high data rate bursts in order to prevent pixelization of the picture (such as live sports events), will obtain the bandwidth as they need it by reducing the data rate for other services that don't.
Statistical multiplexing should improve perceived picture quality, epecially on video that changes rapidly and has the advtange of requiring no changes in the receiver equipment.
What's SCPC?
SCPC stands for Single Channel Per Carrier. In the case of this type of transmission, only a part of the available transponder is used for the signal. The satellite operator can sell the remaining space on the transponder to other up linkers. SCPC is typically used for feeds rather than for direct programming. SCPC has the advantage over MCPC that the signals up linked to the same transponder can be transmitted up to the satellite from different locations (SNG trucks for example), but has the disadvantage of not being quite as efficient as MCPC because of "guard bands" which keep the SCPC signals on the same transponder separated from each other.
NBC uses SCPC MPEG-2 for its back haul feeds and is able to use up to four SCPC transmissions on a single satellite transponder (GE-1 Ku-Band). Microspace uses the same type of transponder on the same satellite, but in MCPC format and is able to transmit six video channels and a few audio channels in the same space.

gessle
05-11-08, 23:11
What are PIDs?
MPEG-2 transmits its data in packets of 188 bytes each. At the start of each packet is a package identifier (or PID) that tells the receiver what to do with the packet. Because the MPEG-2 data stream might be in MCPC mode, the receiver has to decide which packets are part of the current channel being watched and therefore pass them onto the video decoder for further processing. Those packets that aren't part of the current channel are simply discarded.
There are typically four types of PIDs used by satellite receivers. The VPID is the PID for the video stream and the APID is the audio stream. Occasionally, a PCR PID (program clock reference) is used to synchronize the video and audio packets, however, most of the time, this data is embedded into the video stream. The forth data PID is used for data such as the program guide, information about other frequencies that make up the total package etc. This data is called the System Information and uses a PID value of between 0000 and 0014 (hex).
The System Information stream
SI packets tell the receiver about the format of the transmission along with information such as multiple language selections, program guide information and other transponders that are related to the current transponder.
The primary reason that MPEG-2/DVB receivers cannot handle Digicipher 2 and ATSC signals is because the SI packets are totally different between the two standards. In theory, it should be possible to make an MPEG-2 receiver receive DCII/ATSC, however, this would either require access to the source code of the MPEG-2 receiver's firmware (and probably a license from General Instrument) or the DCII/ATSC signal being transmitted using both DCII/ATSC and MPEG-2/DVB SI packets. This is possible (see the ATSC technical documentation page for information on how this is done), however, the audio will either have to be sent twice or the receiver will need to handle both Musicam and Dolby AC3 as this is another big difference between the systems.
What's 4:2:2 and HHR MPEG-2?
When MPEG-2 encodes color and picture information, it samples the analog picture at certain resolution both as horizontal and vertical pixels, but seperately as color (chrominance/hue) and brighness (luminance). The DVB specification calls for 4:2:0 encoding which put simply means that the resolution of the color information is one quarter of the resolution of the video information.
Since studios need better quality than DVB offers, an extension to MPEG-2 has come about that isn't part of the DVB spec but has its own specialized defintion within the MPEG-2 standard. This is called 4:2:2 format or MP@4:2:2SP meaning "Main Profile 4:2:2 Studio Profile". In this system, double the amount of vertical color information is transmitted.
Another format exists that is in very common use today. Called HHR for half horizontal resolution, this part of the MPEG-2/DVB standard transmits only half of the normal 720 pixel horizontal resolution while maintaining normal vertical resolution of 480 pixels (although, since it's 4:2:0 format, the color information is only encoded at 240 pixels vertically and 176 pixels horizontally. A lot of the smaller DBS (like the ethnic packages on T5 etc) use HHR format since it dramatically reduces the bandwidth needed for channels - of course at the expense of picture quality. Special logic in the video decoder chip in the set top box, re-expands the picture to its normal horizontal size by interpolation prior to display.
4:2:2 video at Standard Definition looks just as good as the NBC analog feeds on GE-1 Ku. High bandwidth 4:2:0 video like the NBC digital feeds on GE-1 Ku come very close to studio quality and the low bandwidth stuff encoded in HHR format, looks a lot like VHS quality.
The following diagram shows the ratios of 4:2:0, 4:2:2 and HHR resolutions. I could explain why the ratio used for 4:2:0 is written as 4:2:0 but that gets mega-complex and is beyond the scope of this document.



http://www.coolstf.com/mpeg/mp2format.gif

gessle
05-11-08, 23:13
MPEG-2 Sample Shots When using DVB2000 software on a Nokia Mediamaster receiver and a PC equipped with a SCSI bus and DVBEdit, it's possible to capture the recontstructed video directly out of the MPEG-2 decoder's buffer memory. In the following screen shots from Dish Network, you can see how each of the individual components of the picture are transmitted and how pan/scan is used to interpolate both the base video and chroma information.


http://www.coolstf.com/mpeg/dn-regular.jpg What you see http://www.coolstf.com/mpeg/dn-no-pan-scan.jpg But as you can see, the horizontal resolution isn't full frame. Dish Network uses 480x480 resolution. http://www.coolstf.com/mpeg/dn-no-pan-scan-y.jpg The luminance is sent at the same 480x480 resolution. http://www.coolstf.com/mpeg/dn-no-pan-scan-cb.jpg But the resolution of the blue signal is much lower http://www.coolstf.com/mpeg/dn-no-pan-scan-cr.jpg As is the red component The MPEG-2 Transport Stream
As mentioned above, MPEG-2 transmissions are either transmitted as SCPC or MCPC feeds. However, at an individual channel level, both techniques use the same method for building a data stream containing the video, audio and timing information. In this section, I'll concentrate on MCPC because once this is understood, SCPC becomes obvious. This combination of compressed video and audio is called the PES or Packet Elementary Stream and is built as follows:
http://www.coolstf.com/mpeg/pes.gif

gessle
05-11-08, 23:16
The time field isn't the actual time that the encoding was done, but timing information to allow the audio and video to stay synchronized together. This part of the PES is called the PCR (Program Clock Reference) and may be sent either as part of the video stream or as a seperate stream (hence the reason that some MPEG-2 receivers like the d-box have a seperate field for the PCR).
Multiple PES streams get multiplexed together into a faster stream and the System Information or SI stream gets added, resulting in the final MPEG-2/DVB multiplex that gets uplinked to a transponder on the satellite:
http://www.coolstf.com/mpeg/ts.gif The SI is responsible for telling the receiver all kinds of useful information about the data stream, so that the receiver can write the appropriate data into its program guide. The first part of the SI is called the Program Association Table or PAT. The PAT is always transmitted on PID 0000 and contains a list of Program Map Tables or PMTs that are part of the data stream. For example:
PAT (PID 0000) = 0100, 0200, 0300, 0400
PMT 1 (PID 0100) = Video PID 0101, Audio PID 0102, Audio PID 0103, PCR 01FF
PMT 2 (PID 0200) = Video PID 0201, Audio PID 0202, PCR 01FF
PMT 3 (PID 0300) = Video PID 0301, Audio PID 0302, PCR 02FF
PMT 4 (PID 0400) = Video PID 0401, Audio PID 0402, PCR 0401
Given this information, the receiver knows that the DVB transport on the current frequency contains four programs. The first channel contains two audio services (perhaps for multiple languages) and all of them except for the fourth program contain seperate timing information - the fourth has the PCR timing embedded into its video stream.
The reason that the PCR might be transmitted seperately from the video stream is in the case of multiplexed channels which were encoded with a common clock reference. In this case, it would be redundant to send the PCR again, since the reciver would always use the same clock refernece for all the signals within the multiplex. In the above example, one might assume that the first three video channels came from a common encoder and the fourth stream was multiplexed in, perhaps after being received from an SCPC feed or line-line and not re-encoded prior to multiplexation.
Thanks to Scott Bidstrup at Vyvx for his contributions relating to the above.
Obviously, the PMT contains other information, such as pointers to the name of the channel in the SDT table and things like information about data services that might be mutliplexed in as part of the PES. But in addition to the PAT and PMT, there are a few more interesting ones. The Network Information Table (NIT) on PID 0010 contains a list of associated transponders that make up the package along with their SR and FEC values, which can be different.
When doing a search on a single channel on the Echostar DBS service, most smart MPEG-2 receivers (like the d-box) will automatically go off and search the other frequencies used by Echostar since the NIT on each transponder points to all the other transponders. The NIT can also point to transponders on other satellites, so that in the case of Echostar, the receiver would know to switch to another dish to receive programming from the Echostar 3 DBS satellite at 61.5 degrees when you tune the receiver to a channel carried on this satellite. In its own strange and totally non-standard way, Digicipher 2, uses a similar technique to allow the 4DTV receiver to know where other satellites are and turn to them when a particular channel is chosen.
Now the receiver knows all the frequencies associated with a package, there are few other PIDs that make a DVB receiver work the way it does. The optional BAT or Boquet Association Table tells the receiver about programs of the same type (such as sporting events, movies, news etc.) that are part of the package. Echostar uses this part of DVB for their "Themes" menu. The EIT or Event Information Table on PID 0012 contains a list of the programs (or events) that when interpreted by the receiver's firmware, make the program guide. The EIT allows for up to two weeks worth of programming to be sent ahead of time.
And finally, if you wondered how MPEG-2/DVB receivers know what the time is, the TDT (Time and Date Table) tells the receiver what the date and time is in Universal Time - the smartcard or non-volatile memory in the receiver contains the UTC offset, so that you see local time on the screen.
SCPC signals are transmitted pretty much the same way as MCPC, but obviously only contain one PES since they occupy less bandwidth. Because SCPC channels are normally feeds, they typically do not carry many of the DVB SI streams such as the NIT, BAT and EIT.
What about moving MPEG-2 transport streams around facilities?
At sites where MPEG-2 transport streams are processed, there are a number of different interfaces used to move transport streams between devices. It's worth mentioning some of them since they come up from time in discussions about professional-level equipment.
DVB-ASI: This is a local area (300 metres) serial interface is based on coaxial cable. The data is simplex (i.e. from one device to another - not the other way around) and runs at 270 Mbps, although due to overhead the actual data rate is around 240 Mbps.
DVB-SPI: Again, a local network typically used to interconnect professional MPEG-2 equipment. Data is sent in parallel using LVDS (Low-Voltage Differential Signaling) balanced transmission. Data rates up to 40 MBps (B = bytes b = bits) can be used with this interface over short distances (a few metres).
ATM: ATM networks are WAN, MAN or LAN (Wide, Metropolitain and Local Area) networks used by communications companies as the protocol on fibre (and other) high speed networks. ATM has 53 byte packets, 48 of which are available for payload. When using AAL1 (one of the possible standards for transmitting MPEG-2 over ATM networks), the payload for data is 47 bytes, so one 188-byte MPEG-2 packet can fit exactly in four ATM cells. This is the main reason for the 188 byte packet length used in MPEG-2. As a side-note, frequently another standard called AAL5 is used to encapsulate DVB packets over ATM networks - in this mode, two MPEG-2 packets plus 8 bytes of overhead adds up to 384 bytes which fits nicely into the payload of eight ATM cells.
Thanks to Michael Clawson and Paolo Bevilacqua for info about the AAL5 encapsulation
Time Division versus Statistical Multiplexing
When MPEG-2/DVB carriers broadcast Multiple Channels Per Carrier (MCPC), packets for each of the channels within the transponder are mixed into the higher rate stream that's carried on the transponder. This process is called multiplexing and can be done two different ways. Before we get into how the two work, it's good to have an understanding of how the uplink works.
At the uplink, signals that are to be carried on the service are first received. This can be via a number of different methods such as from another satellite, off air antenna or via a leased line circuit. As an example, many of the DISH Network local channels are sent back to Cheyene in MPEG-2/DVB format using 45Mbs T-3 leased lines.
The signals are then converted back to composite video by decoding incoming any digital inputs which may be either in MPEG-1, MPEG-2, Digicipher 1 or Digicipher 2 - obviously the analog video is already in composite format.

Next, the analog video and audio is re-encoded using very expensive encoders which generate an MPEG-2/DVB video and audio stream. This stream along with the other streams that will make the channels on the transponder and then multiplexed together and the System Information and Conditional Access streams are inserted before the resulting stream is modulated onto QPSK DVB-complaint carrier and transmitted up to the satellite.
At first, you may think that it's rather wasteful to decoded and then re-encode the signals that are already in digital format. However, this is done to allow the uplinker control of how much bandwidth his system has allocated fo the particular channel.
Also keep in mind that the DVB-SI and CA streams are transmitted in parallel across all transponders within the system (even across multiple satellites at different orbital locations). Obviously, this is wasteful from a bandwidth perspective, but it's a necessary evil to keep the EPG and authorization of the receiver working.
Graphically, the resulting stream looks something like the diagram below.
http://www.coolstf.com/mpeg/stream.gif Time Division Multiplexing
In the TDM system the bandwidth is divided up at a fixed rate for each of the streams on a particular transponder. The uplinker has to make some decisions about how much bandwidth to allocate to each channel taking into consideration the type of programming that will be carried on each channel. For example, the DISH Network system uses SR 20.000 MS/s FEC 3/4 on its transponders. This results an available bitrate on each transponder of about 28Mb/s:

20.000MS/s = 40.000Mb/s
Minus 3/4 convolutional coding = 30.000Mb/s
Minus 188:204 Reed-Solomon coding = 27.647Mb/s

gessle
05-11-08, 23:18
Given that the uplinker wants to make a certain number of channels on this transponder, he might choose a scheme like:

Movie Preview Channel 3Mb Cable Channel 3.5Mb Sport Channel 4Mb Sport Channel 4Mb Movie Channel 4.5Mb PPV Channel 4.5Mb News Channel 3Mb Audio Channel 128Kb Audio Channel 128Kb Audio Channel 128Kb Audio Channel 128Kb Audio Channel 64Kb Audio Channel 64Kb Audio Channel 64Kb Low-speed Data Service 19.2Kb High-speed Data Service 512Kb DVB-SI (EPG, authorization etc) 630Kb Firmware Update 128Kb Total 27.993Mb Unused (null PID) 7Kb
The problem with TDM is that channels that are given low bandwidth tend to contain lots of overcompression artifacts when there is too much motion, many frame changes or huge differences in luminosity. Additionally, when high bandwidth channels contain very compressable video, their bandwith is lost.
Statistical Multiplexing
Statmux in MPEG-2/DVB systems is very new - the second generation encoders only hit the streets a few months ago. The only service known in North America to be using statmux at this time is DISH Network, where it has made a huge difference to the quality of the video and at the same time allowed all the transponders FEC to be backed down to 3/4, therefore improving rain fade performance.
Statmux encoders in-effect "talk" to each other about the amount of bandwidth required for the video they are currently compressing and they share this information with a central processor that talks to the other encoders and knows some basic rules, like the amount of space to allocate for fixed rate services, like the DVB-SI etc.
The end result is that a particular channel's bandwidth utilization might be 6Mb one second and 2Mb the next, depending on how much bandwidth that particular channel needs at that time. Obviously, there is still an upper limit to the number of channels that can be transmitted on each transponder, but the number is generally increased by going to statmux encoders since the bandwidth is now shared.


When buying an MPEG-2 receiver from Europe, there are a number of terms used there that need to be understood in order to use a receiver designed for that market.
Ku-Band is King
In Europe, the most common way of transmitting feeds and video programming is via Ku-Band and not C-Band as it is here in North America. The reason for this simple - a) Ku-Band dishes are smaller and therefore easier to install b) the geographical distances there are much smaller than in North America c) if a signal is targeted towards say the Balkan states (Bulgaria, Romania, Albania etc.), a beam can be used for these services because generally no-one outside of these areas will want to receive the signals. The beam results in a stronger signal on the ground, which improves signal quality and therefore can also reduce the size of the receiving dish.
C-Band is used in Europe, however, it's typically used for Arabic feeds (again a large geographical area) and for hemispheric feeds (for example, the Deutche Welle feed on Intelsat at 1 west that covers all of Europe and Africa).
Because Ku-band is so popular in Europe, most people use an offset-style dish with a combined LNB and feedhorn (an LNBF). The LNBF uses variation of the supply voltage to switch between horizontal and vertical polarity (14v = vertical, 18v = horizontal). In the US, Echostar and DSS use the same technique to switch between left-hand and right-hand circular polarization.
Frequency Bands
In North America, Ku-Band is split into two bands. The FSS band covers 11.7-12.2 GHz and is used for some DBS services (ex-AlphaStar and Primestar), but also for video feeds between TV stations and data services. The DBS band is designed only for direct to home applications and uses 12.2-12.7GHz.
When DBS started in the Europe, the initial band was 11.2-11.7 GHz, however, this has now been expanded to cover from 10.7 to 12.7 GHz, all for direct to home services.
Intermediate Frequencies and LOs
Initial satellite receivers available in Europe received in the range 950-1450MHz. This meant that the LNB contained a local oscillator frequency of 10.25 GHz (10.25 GHz + 950 MHz = 11.2GHz). When the band was extended down to 10.7GHz, this meant that the receivers had to change in order to receive all the programming. This meant that the IF was extended with the range 950-2100MHz with an LO frequency of 9.75 GHz.
Next, along comes digital TV, which occupies the 11.7 to 12.7 GHz band. Trying build a tuner and LNB that handles the entire range of 10.7 to 12.7 is impossible, so therefore the LNB contains two LOs - one at 9.75GHz and the other at 10.6GHz. This type of LNB is called a Universal and it is switched between the two LO frequencies by the receiver modulating a 22KHz tone onto the power supply for the LNB.
Diseqc and Hot Bird
In Europe, the major orbital location for DBS services (both analog and digital) has always been the Astra slot at 19.2 east. The Astra fleet currently comprises six co-located satellites with more planned. All the satellites are owned by Astra, based in Luxembourg, who then lease the transponder time to programmers.
The other major European satellite consortium is Eutelsat (based in Paris) and they recently decided to also get into the direct to home market, much like Astra has. They currently have three high power satellites (called Hotbird) co-located at 13 east with more on the way.
Because of only a difference of 6.2 degrees between the two satellites, many Europeans that want to receive from both satellites use two LNBs pointed at the same dish with an adaptor to point each at the correct satellite. Obviously, switching between the two LNBs requires either a manual switch or something a bit more high-tech.
Diseqc (DIgital Satellite EQuipment Control) fills this gap by modulating digital commands onto the 22KHz signal that is used to switch between bands. With a Diseqc compatible receiver (like the d-box with its latest version of software), it is possible to have the receiver send a command to a switching device mounted at the dish to switch between the Astra and Hotbird LNBs without the effort of running an extra cable. With the correct external 12v switchbox and a 4 to 1 Diseqc switch box, it's possible to connect up to eight LNB inputs to many modern receivers. In the future, Diseqc will offer bi-directional communications between the receiver and equipment at the dish for features such as dish motorization and switching into circular modes.
The Astra 1D Frequency Extender (ADX)
When Astra 1D launched, it had sixteen transponders that were below the regular direct to home band, i.e. in the 10.2-10.7 GHz range. Because most receivers that were already in use couldn't tune the band, the ADX was invented. It shifts the IF frequency up or down by 500MHz.
In North America, most people use Ku-Band LNBs with a local oscillator frequency of 10.750GHz, which results in the tuning of 11.7-12.2 GHz with an IF frequency of 950-1450MHz. A few satellites (Intelsat K for example) have Ku-Band transponders below the normal North American range, so by using an ADX, the transponders below 11.2 GHz can be received by shifting the 11.2-11.7 GHz band up by 500MHz.
Typically, the maximum extra range that can be reached with a regular LNB is about 150MHz. You can tune down to about 11.55GHz, but that's enough for the transponders on Intelsat K, which include a few SCPC MPEG-2 signals.
Another North American use for ADXes is if you use a wide band LNB for Ku-Band that has a standard LO of 10.75GHz, but an output range of 950-2100 MHz. Here, you use an ADX to shift down the 11.7 to 12.2 GHz band by 500MHz to make it match the 950-1450MHz IF of most North American receivers.
The ADX does cause a couple of band edge spurious signals at the bottom of the band, but generally works very well. I've heard of people using it with a wide band Ku-band LNB on a big dish and getting a signal lock on Echostar 1/2 by shifting down the DBS band at 119 degrees. It's rather weak because of the mismatch of circular versus linear polarization though.
What can be received with Echostar and AlphaStar receivers
Not too much really. Both receivers are package receivers and therefore have fixed SR and FEC values. However, if you peruse Lyngesat, you'll most probably find something that matches.
Info about the Echostar Receiver
The Echostar receiver uses SR 20.000 with automatic FEC. Because it was designed to operate in the DBS band (12.2 to 12.7 GHz), it uses a local oscillator frequency of 11.25 GHz in the LNBF. Remember that the Echostar DBS satellites use circular polarity and uses the 14v and 18v technique to switch between right and left hand.
There are a few signals (other than Echostar's own) that will work with Echostar receivers.
One is the Microspace package on GE-1. Hook the receiver up to an LNB pointing at GE-1 Ku and tune to transponder 16. If you're using an LNB with a feedhorn, set for horizontal polarity. If you're using an LNBF, physically rotate the feedhorn by 90 degrees. You'll get a lock and the program guide will show lots of channels. Sad to say, there are all scrambled.
Echostar receivers will also lock onto ExpressVu on Nimiq Ku-Band. This isn't surprising as ExpressVu buys their receivers and LNB's from Echostar - the hardware is identical. Unfortunately, ExpressVu is scrambled, with the exception of the 30 "Galaxie" music channels. Unfortunately, you won't be able to see the music channels on the channel map unless you subscribe (or use an FTA MPEG2 receiver).
The third option is the SkyVista programming package on Telstar 5. The SkyVista programming package is a joint venture by Loral Skynet and Echostar, using EchoStar hardware. SkyVista requires the use of a KU Band LNB rather than the circular polarity FSS LNB's used by ExpressVu and EchoStar.. Like Microspace and ExpressVu, SkyVista is scrambled, with the exception of a few arabic channels.
Info about the AlphaStar Receiver
This receiver was initially made by Tee-Comm for the AlphaStar DBS service which used a symbol rate of 23.000 and FEC 2/3. Software upgrades since the demise of AlphaStar have made it work with a package uplinked by Spacecom Systems on T5 using the same SR/FEC, but this receiver is also being used for a Chineese package on T5 with SR 20.000 FEC 3/4, so obviously someone knows how to change the SR/FEC on this box by changing the firmware.
It was also used very briefly in Europe where a Dutch distributor re-wrote the firmware to use variable SR/FEC, along with making the menus in Dutch. Since the Tee-Comm 1000 uses the Nokia tuner, it can actually handle SR from 1-45MS/s. It was never sold though, since Multichoice (now Canal+) wouldn't license the CA to the distributor, which does seem rather odd, since AlphaStar used the same IRDETO CA as Multichoice.
Conditional Access - The Key to Private and Pay-TV Systems
Conditional Access (CA) is used to prevent unauthorized access to either private or pay-TV systems.
Note: As you will have read at the start of this document, this site does not provide any information relating to compromising scrambled signals. We do explain how CA systems work in general, but don't expect to find hacking information here.
How is Conditional Access on DVB-systems performed?
In the DVB specification, there are only a few of the stream types that must be transmitted without scrambling. Obviously, these only include some of the Systems Information streams such as the Program Association Tables (points to more info about each channel) and the Network Information Table (points to the other transponders used by the service). These streams must be transmitted without scrambling so that any DVB compliant receiver can at least tell "what's there". However, everything else (including the program guide streams in the EIT) can be scrambled.
Scrambling of the appropriate streams is performed at the uplink site. The MPEG-2 packets are encrypted by the usual techniques, based on a common key known to both the scrambling and decryption devices. The actual scrambling technique, i.e. how the bits are rearranged to make them nonsensical, is obviously kept a secret, as are the keys contained in both the scrambling computer and the decryption device (typically, a smartcard in DBS applications).
When a scrambled packet arrives, before it passed through to the demultiplexor, it's first sent through the CAM or Conditional Access Module. The CAM is the descrambling engine and can be either built directly into the receiver or inserted into the receiver via a PC Card (aka PCMCIA) connector. At the start of each MPEG-2 packet is a 2-bit field called the TSF or Transport Scrambling Flags - if zero or one, the packet is passed through the CAM onto the demux for display since this value indicates an un-scrambled stream. If the TSF is set to either two or three, then the packet is passed through to the CAM, which takes the key obtained from the smartcard and uses it to turn the packet packet back into an MPEG-2/DVB transport packet which can then be processed by the rest of the system.
Many people think the smartcard and CAM are the same thing. These are two different entities - the DVB transport cannot be sent through the serial card - the serial interface it way too slow! Instead, based on the card receiving authorization from the service provider (using the DVB EMM and ECM tables), the card will emit the keys required by the CAM which are in turn used to descrambled the program stream. Most smartcard's serial interfaces operate in the 9,600 to 38,400 bps range.
Obviously, the key used to scramble the channel changes over time. If you look at the serial communications between the CAM and the smart card with an oscilliscope, you will see a burst of data every few seconds. This is the CAM asking the smart card for the next set of decryption keys for the next few second's worth of video. This also explains why, on most systems, if you pull out the smart card, you'll often see a second or two worth of programming before the picture blanks out.

gessle
05-11-08, 23:21
Why is some signal scrambled and yet another isn't?
Mostly to protect distribution rights. For example, RTPi and Deutche Welle on PAS5 (58 west) are unscrambled since both of these are public information channels transmitted for worldwide distribution and for use by pretty much anyone without fee, much like in the way that NASA TV is a free channel that anyone can redistribute without cost nor license.
Dish Network transmits their "Dish Information" barker channel without scrambling - that way, even if a card swap has occurred due to a hack on their security system, when an out of the box receiver powers up for the first time, it gets the barker channel, therefore verifying the correct operation of the receiver.
However, Dish Network also transmits many other interesting streams without scrambling, such as their audio channels. Despite the lack of scrambling, they do actually charge for these channels since the Nagravision security system has the ability to "hide" channels from the user if they are not subscribed. With an "official" receiver, the channels can't be received without a subscription and it obviously saves a lot of hardware for scrambling these streams that really are "low value". In effect, this is "Poor Man's" scrambling.
How can I subscribe to a certain signal?
Generally, unless it's a signal from an established Pay-TV provider, you can't. Many signals are encrypted for private or exclusive cable distribution and as a result cannot be subscribed to even if you have the right hardware. Many feeds in Canada use the Scientific Atlanta PowerVu scrambling system and even with the right receiver and an address in Canada, you still can't subscribe to many of the channels which are available on cable.
DCII - The "other" MPEG-2 satellite standard
Well, OK, there's DSS in North and South America along with ISDB in Japan, but one of the major video distribution methods in North America is Motorola's Digicipher II standard.
What's Digicipher?
Digicipher is Motorola's proprietary video distribution system. The first version was a totally non-standard system called Digicipher I and was one of the first digital video compression systems available in the market. The largest Digicipher I user was the Primestar direct-to-home system which closed down in 2000 after the company was purchased by DirecTV. There are two or three other DCI multiplexes available in North America - mostly feeds for South America that are receivable on global beams.
Digicipher II is Morotola's MPEG-2 based distribution system. It's used by about 70% of "cable" channels in North America to distribute their video to cable headends, other satellite companies like DirecTV and Dish Network and also available to backyard dish owners via Motorola's 4DTV satellite receiver products. DCII is also used by Canada's StarChoice direct-to-home service.
DCII includes an uncompromized encryption and authorization system.
What's different between DVB and DCII?
Both systems are based on the MPEG-2 standard. Both use the MPEG-2 transport stream format, so both have a Program Association Table (PAT) and Program Map Tables (PMTs) along with elementary data multiplexed onto various PIDs.
The video format used by DCII can be just as varied as DVB - 720x480, 480x480, 576x480 and so on. DCII also supports 4:2:2 video and HDTV like DVB.
Moving onto audio, DVB's primary audio format is MPEG-1 Level 2, also called Musicam. DVB optionally supports the AC3 standard from Dolby (otherwise known as Dolby Digital) but DCII requires AC3 for all audio streams. This doesn't mean that all DCII channels are using 5.1 surround - most channels transmit "2.0" format encoded with analog Dolby Suround.
Where DVB and DCII are really different is in how the channel definitions get into the receiver. DCII was designed before the DVB standard was ratified and General Instrument (now a part of Motorola) designed their own scheme without any interfacing to the work being done by the DVB team was doing and as a result, we have two totally different standards to deal with. This part of a digital TV system is called the SI or System Information.
What SI differences are there between DCII and DVB?
During this discussion, keep in mind the two major places where DCII and DVB are used: DCII in the consumer realm where a receiver is hooked up to a motorized dish and DVB in the Dish Network model where an electronic switch is used to switch between stationary dishes pointed at differental orbital locations.
So, the fresh receiver gets turned on and due to factory programming and correct installation, is able to receive a "homing" channel. The homing channel contains all the info needed by the receiver to tune any one of the channels that the receiver can receive.
In the case of DVB, this means a Network Information Table (NIT - used to tell the receiver about other transport streams - contains frequency, symbol rate, orbital location etc), the Service Description Table (SDT - used to tell the receiver the names and types of programming available on each channel) along with the Time and Date Table (TDT - tells the receiver about programming events on the channels - used to build the electronic program guide).
DCII is a little more complicated. First, there's a thing called the Satellite Definition Table (SDT - tells the receiver about the name and orbital locations of satellites), next there's the Modulation Mode Table (MMT - lists the different symbol rate, FEC and modulation types used by DCII) and then the Carrier Definition Table (CDT - this just lists the L-Band frequencies required to tune a channel) followed by the Transponder Defintion Table (TDT - lists the polarity and CDT table index). Finally, the Virtual Channel Table (VCT) is the thing that ties everything together. This lists each channel on the entire DCII system across multiple satellites and contains an index the appropriate TDT, SDT, MMT and CDT tables so that the receiver can select the correct channel analog or digital. If the channel is digital, the VCT also contains the MPEG-2 Program Number (although it's called Service Number in Motorola parlence) so the receiver can process the MPEG-2 PAT and PMT to acutally display the program. Whew!
A simpler way to look at the differences is that DVB uses a flat list (much like a comma seperated file with lots of repetition) whereas DCII uses a very relational table structure (much like a SQL database over satellite).
What are these DCII modulation types?
DVB is pretty straight forward - it's pretty much always QPSK modulation at variable symbol rates and FEC coding. There have been enhancements to the DVB standard to support BPSK, 8PSK and 16QAM modulation modes. BPSK is like QPSK but only transports one bit per time period and is therefore more robust on degraded links. 8PSK and 16QAM increase the data rate by using more than two bits per time period but require a much more robust link than QPSK.
DCII has a number of different modes. First, there's regular-QPSK and Offset-QPSK. Offset-QPSK is quite similar to normal QPSK but one of the bits in the symbol is delayed by one bit period and phase changes are limited to 90 degrees, making performance in a non-linear environment much better than QPSK. All DCII signals with a symbol rate less than 19.51 MSps use OQPSK - anything above uses QPSK. In the DCII system, the encoders and receivers have pre-programmed symbol rates. For example, the 3.25 MSps rate supports one 576x480 video stream and one audio stream; the 4.88 MSps rate supports two 352x480 video streams and associated audio streams and so on.
There's one other variable for a DCII signal - it's mux mode. As you probably know from reading this document, the higher the symbol rate, the higher the actual bitrate that can be carried across the channel but this also depends on the FEC coding rate - the less fraction of the datarate used for error correction, the higher still the bitrate, so a 19.51 MSps 3/4 stream carries less data per second than a 19.51 MSps 7/8 stream.
In regular QPSK, the data is recovered from both the I and Q phases that make up a QPSK symbol, i.e. each bit on the I and Q phases ends up as a serial bit in the transport stream. Because of limitations in either the QPSK demodulator or the transport stream demultiplexor, most DCII streams with a rate above 19.51 MSps 4/5 FEC coding operate in "split" mode. In split mode, the I phase contains a transport stream and the Q phase contains another different transport stream. It's not really true QPSK but more like dual-BPSK. The Mode Modulation Table sent as part of the DCII SI contains an indication as to whether the transmission is using QPSK or OQPSK and whether it's the regular "combo" mode or split mode.
Thanks to Mark Hemstad for info about OQPSK.
There is one exception to this rule. Some DCII signals use a symbol rate of 29.27 MSps and yet operate in combo mode. One assumes that Motorola was able to correct the limitations that caused them to invent split mode, however, the bad news is that the only receiver that can do this high-speed mode (Motorola calls it MegaStream) is the DSR-4800 which is an expensive commercial receiver. It's worth pointing out that CBS uses MegaStream for it's HDTV network feeds which explains why they can't be received by consumer DCII receivers and the 4DTV HD decoder.
What's the "channel map" issue with DCII?
Remember when you first turn on a new DCII consumer receiver, it needs to be tuned to a homing transponder so it can download all the appropriate tables that allow it to tune channels. Because 4DTV receivers don't allow manual control of the tuning and video parameters, the only way for a receiver to get told what channels are out there is for Motorola to include them in the channel maps they send on the homing transponders.
Motorola used to be quite friendly towards the 4DTV receiver owner and included many channels such as PBS's PBS You and PBS Kids as part of the 4DTV channel maps. However, despite their unscrambled transmission on satellite, PBS insisted that Motorola remove them from the maps being sent to 4DTV receivers and as a result, 4DTV receivers can only tune one of the many PBS feeds that are available and free-to-air.


Why is some signal scrambled and yet another isn't?
Mostly to protect distribution rights. For example, RTPi and Deutche Welle on PAS5 (58 west) are unscrambled since both of these are public information channels transmitted for worldwide distribution and for use by pretty much anyone without fee, much like in the way that NASA TV is a free channel that anyone can redistribute without cost nor license.
Dish Network transmits their "Dish Information" barker channel without scrambling - that way, even if a card swap has occurred due to a hack on their security system, when an out of the box receiver powers up for the first time, it gets the barker channel, therefore verifying the correct operation of the receiver.
However, Dish Network also transmits many other interesting streams without scrambling, such as their audio channels. Despite the lack of scrambling, they do actually charge for these channels since the Nagravision security system has the ability to "hide" channels from the user if they are not subscribed. With an "official" receiver, the channels can't be received without a subscription and it obviously saves a lot of hardware for scrambling these streams that really are "low value". In effect, this is "Poor Man's" scrambling.
How can I subscribe to a certain signal?
Generally, unless it's a signal from an established Pay-TV provider, you can't. Many signals are encrypted for private or exclusive cable distribution and as a result cannot be subscribed to even if you have the right hardware. Many feeds in Canada use the Scientific Atlanta PowerVu scrambling system and even with the right receiver and an address in Canada, you still can't subscribe to many of the channels which are available on cable.
DCII - The "other" MPEG-2 satellite standard
Well, OK, there's DSS in North and South America along with ISDB in Japan, but one of the major video distribution methods in North America is Motorola's Digicipher II standard.
What's Digicipher?
Digicipher is Motorola's proprietary video distribution system. The first version was a totally non-standard system called Digicipher I and was one of the first digital video compression systems available in the market. The largest Digicipher I user was the Primestar direct-to-home system which closed down in 2000 after the company was purchased by DirecTV. There are two or three other DCI multiplexes available in North America - mostly feeds for South America that are receivable on global beams.
Digicipher II is Morotola's MPEG-2 based distribution system. It's used by about 70% of "cable" channels in North America to distribute their video to cable headends, other satellite companies like DirecTV and Dish Network and also available to backyard dish owners via Motorola's 4DTV satellite receiver products. DCII is also used by Canada's StarChoice direct-to-home service.
DCII includes an uncompromized encryption and authorization system.
What's different between DVB and DCII?
Both systems are based on the MPEG-2 standard. Both use the MPEG-2 transport stream format, so both have a Program Association Table (PAT) and Program Map Tables (PMTs) along with elementary data multiplexed onto various PIDs.
The video format used by DCII can be just as varied as DVB - 720x480, 480x480, 576x480 and so on. DCII also supports 4:2:2 video and HDTV like DVB.
Moving onto audio, DVB's primary audio format is MPEG-1 Level 2, also called Musicam. DVB optionally supports the AC3 standard from Dolby (otherwise known as Dolby Digital) but DCII requires AC3 for all audio streams. This doesn't mean that all DCII channels are using 5.1 surround - most channels transmit "2.0" format encoded with analog Dolby Suround.
Where DVB and DCII are really different is in how the channel definitions get into the receiver. DCII was designed before the DVB standard was ratified and General Instrument (now a part of Motorola) designed their own scheme without any interfacing to the work being done by the DVB team was doing and as a result, we have two totally different standards to deal with. This part of a digital TV system is called the SI or System Information.
What SI differences are there between DCII and DVB?
During this discussion, keep in mind the two major places where DCII and DVB are used: DCII in the consumer realm where a receiver is hooked up to a motorized dish and DVB in the Dish Network model where an electronic switch is used to switch between stationary dishes pointed at differental orbital locations.
So, the fresh receiver gets turned on and due to factory programming and correct installation, is able to receive a "homing" channel. The homing channel contains all the info needed by the receiver to tune any one of the channels that the receiver can receive.
In the case of DVB, this means a Network Information Table (NIT - used to tell the receiver about other transport streams - contains frequency, symbol rate, orbital location etc), the Service Description Table (SDT - used to tell the receiver the names and types of programming available on each channel) along with the Time and Date Table (TDT - tells the receiver about programming events on the channels - used to build the electronic program guide).
DCII is a little more complicated. First, there's a thing called the Satellite Definition Table (SDT - tells the receiver about the name and orbital locations of satellites), next there's the Modulation Mode Table (MMT - lists the different symbol rate, FEC and modulation types used by DCII) and then the Carrier Definition Table (CDT - this just lists the L-Band frequencies required to tune a channel) followed by the Transponder Defintion Table (TDT - lists the polarity and CDT table index). Finally, the Virtual Channel Table (VCT) is the thing that ties everything together. This lists each channel on the entire DCII system across multiple satellites and contains an index the appropriate TDT, SDT, MMT and CDT tables so that the receiver can select the correct channel analog or digital. If the channel is digital, the VCT also contains the MPEG-2 Program Number (although it's called Service Number in Motorola parlence) so the receiver can process the MPEG-2 PAT and PMT to acutally display the program. Whew!
A simpler way to look at the differences is that DVB uses a flat list (much like a comma seperated file with lots of repetition) whereas DCII uses a very relational table structure (much like a SQL database over satellite).
What are these DCII modulation types?
DVB is pretty straight forward - it's pretty much always QPSK modulation at variable symbol rates and FEC coding. There have been enhancements to the DVB standard to support BPSK, 8PSK and 16QAM modulation modes. BPSK is like QPSK but only transports one bit per time period and is therefore more robust on degraded links. 8PSK and 16QAM increase the data rate by using more than two bits per time period but require a much more robust link than QPSK.
DCII has a number of different modes. First, there's regular-QPSK and Offset-QPSK. Offset-QPSK is quite similar to normal QPSK but one of the bits in the symbol is delayed by one bit period and phase changes are limited to 90 degrees, making performance in a non-linear environment much better than QPSK. All DCII signals with a symbol rate less than 19.51 MSps use OQPSK - anything above uses QPSK. In the DCII system, the encoders and receivers have pre-programmed symbol rates. For example, the 3.25 MSps rate supports one 576x480 video stream and one audio stream; the 4.88 MSps rate supports two 352x480 video streams and associated audio streams and so on.
There's one other variable for a DCII signal - it's mux mode. As you probably know from reading this document, the higher the symbol rate, the higher the actual bitrate that can be carried across the channel but this also depends on the FEC coding rate - the less fraction of the datarate used for error correction, the higher still the bitrate, so a 19.51 MSps 3/4 stream carries less data per second than a 19.51 MSps 7/8 stream.
In regular QPSK, the data is recovered from both the I and Q phases that make up a QPSK symbol, i.e. each bit on the I and Q phases ends up as a serial bit in the transport stream. Because of limitations in either the QPSK demodulator or the transport stream demultiplexor, most DCII streams with a rate above 19.51 MSps 4/5 FEC coding operate in "split" mode. In split mode, the I phase contains a transport stream and the Q phase contains another different transport stream. It's not really true QPSK but more like dual-BPSK. The Mode Modulation Table sent as part of the DCII SI contains an indication as to whether the transmission is using QPSK or OQPSK and whether it's the regular "combo" mode or split mode.
Thanks to Mark Hemstad for info about OQPSK.
There is one exception to this rule. Some DCII signals use a symbol rate of 29.27 MSps and yet operate in combo mode. One assumes that Motorola was able to correct the limitations that caused them to invent split mode, however, the bad news is that the only receiver that can do this high-speed mode (Motorola calls it MegaStream) is the DSR-4800 which is an expensive commercial receiver. It's worth pointing out that CBS uses MegaStream for it's HDTV network feeds which explains why they can't be received by consumer DCII receivers and the 4DTV HD decoder.
What's the "channel map" issue with DCII?
Remember when you first turn on a new DCII consumer receiver, it needs to be tuned to a homing transponder so it can download all the appropriate tables that allow it to tune channels. Because 4DTV receivers don't allow manual control of the tuning and video parameters, the only way for a receiver to get told what channels are out there is for Motorola to include them in the channel maps they send on the homing transponders.
Motorola used to be quite friendly towards the 4DTV receiver owner and included many channels such as PBS's PBS You and PBS Kids as part of the 4DTV channel maps. However, despite their unscrambled transmission on satellite, PBS insisted that Motorola remove them from the maps being sent to 4DTV receivers and as a result, 4DTV receivers can only tune one of the many PBS feeds that are available and free-to-air.

gessle
05-11-08, 23:22
How does DSS handle things like the program guide, network tables etc?
The DSS system uses a thing called the Master Program Guide (MPG). This stream has all of the information found in the EIT, SDT, NIT, etc. in the DVB system. It tells the IRD what satellite transponders are available and what code rates they have (like DVB's NIT), it tells the IRD which virtual channels exist ("viewer channels") and which PIDs and PID types comprise each channel (rather like the DVB SDT).
The single MPG gives all of the PID and transponder assignments for all channels and all transponders, and so the MPG is a "one-stop shop" vs. the DVB approach of many smaller tables. The MPG also has two hours worth of programs and program titles, and then "points" to extension guides for data out to two days in the future. New US DIRECTV IRDs also have a feature called "Advanced Program Guide", which works a little more like the DVB EIT and allows program info out to weeks in the future.
What are the technical differences between DSS and DVB transmissions?
DSS DVB Transmission Format QPSK QPSK Viterbi Code Rates 1/2, 2/3, 6/7 1/2, 2/3, 3/4, 5/6, 7/8 Multiplex Proprietary - MPEG-2 like (has PIDs) MPEG-2 standard System Information Proprietary DVB Audio Musicam (MPEG-1 layer 2) or AC3 Musicam (MPEG-1 layer 2) or AC3 Video MPEG-2 standard 4:2:0 (MP@ML) MPEG-2 standard 4:2:0 (MP@ML)

gessle
05-11-08, 23:26
1) MPEG

stands for Moving (or Motion) Picture Experts Group & is an organisation of interested parties.It's run in similar manner to JPEG (Joint Picture Expert Group) -JPEG being for still images;there is also a standard known as M-JPEG (Moving JPEG) but this is intended more for the needs of the security industry. How about the fact that discservers, and corresponding high-end editing suites are only just moving from Motion Jpeg to MPEG (4:2:2). As an intra coding only system it has been and still is widely used in these types of video applications.
MPEG describes a form of compression for digital data where the data represents moving images of a TV like-nature.The standard also allows for audio datastreams sync'd with the video.MPEG1 is common on IBM PC's (& other platforms) using *.mpg files. Xing, Mediamatics & others supply software players for these and all but the cheapest PC VGA cards seem to have some hardware support for MPEG1 files; normally you need a Pentium PC to have much chance of playing mpeg1 files at reasonable speeds (25 frames per second or more). Anyway,mpeg1 isn't used for satellite TV;the industry needed a faster,more flexible & efficient method. For broadcast use, less tendency to pixellation or "blockiness" was desired with fewer "artefacts" -technical/marketing term for unwanted material on the screen (that's a bit like calling a software bug an "anomaly" !!). Now what the satellite industry wanted was to squeeze more channels into the bandwidth taken up by a satellite transponder. Analogue satellite TV uses around 27-36 MHz of bandwidth for its FM video + audio FM subcarriers; this is for each channel. So the operators want to put 5,10 or more separate channels, via a digital datastream into a similar bandwidth. This allows many more channels or needs fewer transponders to transmit a given number of channels. To give flexibility,the actual compression ratio can be varied between "Studio Quality" and "Video recorder quality". Studio needs 12 MBits/second data,broadcast needs 8 MBits, VHS needs 2 MBits/s.I don't want this faq to become too technical but read DVBFAQ.TXT from Markus Kuhn for technical info. Just remember that the compression ratio can be varied to cope with the needs of the supplier of the video information. The digital data from several channels can be multiplexed into an MPEG Transport Stream,along with various (compressed) audio channels (which can include digital surround sound & multiple languages). A package with several channels modulated onto 1 carrier is often referred to as a "bouquet" which is, I guess, a marketing term for such a multiplex or package. The compression ratio used can be different for different channels within the multiplex and can vary over time. The lowest compression ratio will occupy the highest bandwidth but will be needed when the video content requires much detail with lots of changes & movement occurring. A good example might be an athletics event ; you would have people running around the track , people throwing javelins and others just walking slowly or sitting down. Consequently there will not be a lot of redundancy in the whole scene so if too much compression is applied, some movement could easily be displayed jerkily. A studio news broadcast, when they are simply showing the newsreader and his desk, will compress very efficiently & occupy a relatively small bandwidth. So the newscast will be allocated a lower bit-rate & compressed more. The whole process of dynamically allocating bandwidth per channel in a multiplex is known as Statistical Multiplexing & uses quite advanced mathematical algorithms to do its allocation processing.
Incidentally MPEG1.5 is a hybrid (falling between 1 & 2) but any mpeg2 receiver SHOULD be backwards compatible with older MPEG revisions. Actually MPEG1.5 isn't a real (ratified) standard but covers several proprietary systems which tend to improve on MPEG1 . Examples include "System2000" from NTL & "Orbit" from Scientific Atlanta.
1 interesting point is that you can't compress "noise" with current MPEG schemes.Imagine watching a film via digital (mpeg) satellite TV where,within the plot of the film,the camera zooms in to show a TV screen switched on but with no antenna connected. You expect to see nothing but noise ("snow)" on the picture.This signal is entirely random & so can't be compressed -there's no repeating pattern/redundancy in the signal. Apparently a future MPEG version (mpeg4 ?) will have some kind of algorithm built in to get around this problem. It looks like the next "official" (ratified) standard will be MPEG4.
Mpeg2 is also used for DVD (digital video disk or digital versatile disc) & other digital video delivery systems including cable,fibre & digital terrestrial TV.
MPEG4, when it comes, is a scalable encoding/compression system so that mini versions could output to the tiny screens on mobile phones and PDAs whilst higher resolutions & larger object sizes would output to televisions. It's more flexible than MPEG2 to allow this object-based scalability as well as interactivity where desired. It introduces its own new acronyms like AVO (Audio/Visual Object) & DMIF (Delivery Multimedia Integration Framework). MPEG4 allows for 2D & 3D images & user selection (or ungrouping) of parts of the whole scene - where the originator has permitted this. It will require more powerful processors to implement these procedures in satellite receivers. The picture below attempts to illustrate the concept.
http://uib.no/hff/smi/ksv/mpeg4obj.jpg


Modulation & Error Coding

The real world is analogue so we have to find a way of transmitting our mpeg2 transport stream as information on a (non-digital) carrier wave.A traditional analogue satellite transmission varies a carrier FREQUENCY in sympathy with the video signal -"frequency modulation" or FM. Similar to this ,one can vary the PHASE instead of the frequency - "Phase Modulation".Now we could code our digital signal (which consists of simply binary or "ones & zeros") directly as phase modulation in which case 0 degrees (our carrier reference frequency) could represent a binary 0,whilst 180 degrees phase shift=binary 1; there would be a practical difficulty in keeping track & always changing 180 degrees as there will be natural phase variations over the transmission path. To solve this we can instead make the phase changes cumulative -i.e. make the phase changes refer to the previously signalled state rather than 0/180 degrees absolute. This is known as DPSK -differential phase shift keying. Now those of you who know of schemes used for digital transmission, in modems for example,will know that DPSK is somewhat inefficient. There are various schemes that allow the data rate to be doubled,quadrupled (or more) whilst maintaining the original signalling rate.Thus Quaternary (or Quadrature) phase shift keying uses a 2-bit symbol (instead of previously described 1-bit) based on 4 possible phases. At the same time,0 degrees is avoided to prevent long periods of unmodulated carrier which could cause problems in part of the circuitry -too complex to discuss here. So we use typically 45,135,225 & 315 degrees.We now have a greater data rate in bits per second than our actual baud (signalling) rate. This can ,in fact be further extended by using a constellation of 8 or 16 phases & beyond -although tolerance to noise (required signal to noise ratio for a suitably low bit error rate) & to interference increases as the data rate rises. [This is one reason why computer modems have trouble at high data rates on a poor line & your 56K modem ends up communicating at 31,200 or less]. Other digital transmission media can still use mpeg2 but change from qpsk to a different modulation scheme.Normally this is QAM for cable (although qpsk can be used for the return path back to the operator) whilst digital terrestrial TV uses CODFM -coded orthoganal digital frequency modulation -with either 2048 or 8192 individual carriers each seperately modulated.The choice of type of modulation is made based on the sort of problems most prevalent for the medium,e.g.terrestrial is more subject to multipath interference ("ghosting" in analogue TV) & CODFM is fairly resistant to this.The same type of modulation is also used for DAB (Digital Audio Broadcast). The main figure of merit for a qpsk demodulator is the minimum Eb/No that the receiver can tolerate to deliver a specified BER (Bit Error Rate) to the MPEG section. Eb/No is the ratio of energy per bit to the noise available at the demodulator. So since satellite signals are inherently noisy,a low order modulation scheme is used with lots of error correction. In fact the DVB adopted what is known as a "concatenated FEC" scheme which means that multiple error correction types are used together - in this case "convolutional" & "block" coding are both used. Viterbi coding is a form of convolutional coding (also used in modems) & the "code rate" refers to the ratio of the number of bits coming out for a given number of bits going in. So 3/4 means 3 bits come out for every 4 bits going into the decoder. The DVB uses 1/2 code rate for channels with lots of noise (low Eb/No). The error correction comes from the redundant coding data that is transmitted. The constraint length (k) is the number of bits over which the code is computed ; for DVB K=7. The operator decides which code rate to use & the receiver must either scan for the right rate or be told by the user (manual entry). The block coding scheme used is called Reed-Solomon usually abbreviated to R/S, with additional coding by interleaving blocks of bytes. 204/188 code is used which means 188 bytes come out for 204 in - the remainder being parity bytes.

gessle
05-11-08, 23:28
D2MAC :- I am sometimes asked whether D2MAC transmissions (still used by Nordic countries)are digital. In fact the "A" in MAC stands for "Analogue" - MAC = Multiplexed Analogue Component. The (analogue) video luminance & chrominance signals are sent at separate times which avoids the interference seen when they are all sent at the same time in PAL. So the 2 signals cannot mix with each other to generate false alias signals which can show up in PAL - like flashes or stripes of colour often seen on a newsreader's (plain grey or checked) suit or tie. Howebver, there IS a digital part to D2MAC - the sound & the video synchronisation information is sent in digital packets. For the audiophiles, note that the audio is only 14 bit so it isn't quite CD-quality. So a D2MAC signal is a "hybrid", part digital & partly analogue.


3) DVB

Like MPEG groups there is a DVB group -Digital Video Broadcast, made up of interested parties, sharing information & setting the standards.It's somewhat like the VESA group for PC graphics. DVB was set up by the EBU (European Broadcast Union) to set the standards for digital video transmission.They have published these via ETSI (European Telecommunications Standards Institute) who also set standards for devices such as GSM telephones. In fact there are several DVB standards for different transmission media.Some of these are : DVB-S Satellite DVB-C Cable DVB-T Terrestrial DVB-SI Specification for Service Information DVB-CI Common Interface for conditional access They've settled on using a subset of MPEG2 for their compression of the video & audio. I've pasted in below a definition of the requirements to be met to claim that your IRD (Integrated Receiver Decoder i.e. satellite box) is DVB compatible :-

To be DVB compliant a Satellite or a Cable receiver must, according to DVB Document A001-revision 1, at least fulfill the following key features:


Systems

MPEG-2 Transport Stream is used
Service information is based on MPEG-2 Program Specific Information
Scrambling is as defined by CA Technical Group
Conditional Access uses the MPEG-2 CA_descriptor


Video

MPEG-2 Main Profile at Main Level is used (1.5-15 Mbits/s)
The frame rate is 25 Hz
Encoded pictures may have either 4:3, 16:9 or 2.21:1 aspect ratio (4:3 is the normal TV format, 16:9 is the widescreen format and 2.21:1 is the cinemascope format that is use in the movie theaters)
IRDs will support 4:3 and 16:9 and optionally 2.21:1 aspect ratios
IRDs must support the use of pan and scan vectors to allow a 4:3 monitor to give a full-screen display of a 16:9 coded picture
IRDs must support a full screen display of 720 x 576 pixels (and a nominal full-screen display of 704 x 576)
IRDs must provide appropriate up conversion to produce a full-screen display of 544 x 576 and 480 x 576 and a nominal full-screen display of 352 x 576 and 352 x 288 pixels.


Audio

MPEG-2 Layer I and Layer II must be supported by the IRD
The use of Layer II is recommended for the encoded bitstream
IRDs must support single channel, dual channel, joint stereo and the extraction of at least a stereo pair from MPEG-2 compatible multichannel audio
IRDs must support sampling rates of 32 kHz, 44.1 kHz and 48 kHz
The encoded bitstream will not use emphasis


Note that American DSS,DirecTV etc. systems are NOT DVB-compliant & won't work in Europe.I do know of an attempt by someone in the USA to modify a European Nokia digital receiver to decode DigiiCypher2 transmissions -but,at the time of writing,this has not been successful. EPG (Electronic Programme Guide) :- A feature of most digital satellite receivers. Essentially a programmable guide to what's on each channel with further program information when supplied by the channel. Many include some quite useful information like what's on next in addition to what's on now and a brief description of the programme content. There's specific provision within the Mpeg2 structure for this information to be transmitted ; however not all operators make use of it.


4) Block Diagram (simplified)

http://uib.no/hff/smi/ksv/skiz2.gif Some observations on the afforementioned diagram : DAC = Digital-to-analogue converter ADC = Analogue-to-digital converter The video encoder typically contains 4 or more DACs which run at video rates & quality. This infers 8 bit video DAC's (not cheap). 3 are needed for RGB; another is needed for composite video out (PAL or SECAM). Some use 10 bit DAC's & the difference *may* be visible by viewing sharp transitions like black to white -hint : have a look at the On Screen Display if you want to try to spot this effect. I have not included the conditional access module for simplicity. See section 5 for more detail on this (including a digram). Complex IC's are needed for many of these blocks. A qpsk demodulator/ADC/Viterbi decoder can easily cost around $7-$13 in manufacturers volumes! The mpeg transport demultiplexer & decoders cost even more!I haven't included the CPU & memory (usually around 1-3Meg. is needed & some of this may be fast,expensive SRAM). Perhaps you can now see why the digital receivers cost a lot more than the analogue ones!!! It's worth noting that on Astra,a Network Information Table (NIT) is transmitted every 10 seconds on every DVB/mpeg transponder. The information sent includes the FEC,S/R,frequencies etc.


5) Equipment Needed

First of all, a universal lnb is recommended as digital receivers for Europe are optimised for these. A universal lnb will have low phase noise (required so as not to confuse the qpsk modulator) & 2 local oscillators,1 at 9.75 GHz & 1 at 10.6 GHz. The default is to enable the 9.75 GHz oscillator whilst a 22KHz tone generated by the receiver enables 10.6 GHz. Any receiver made for European digital reception may work up to a point **BUT** (big problem) many receivers are sold for use on a particular operator's "bouquet" (multiplex) of channels & often have internal software that prevents you receiving anything else!!!! All Pace receivers up to late 1997 seem to suffer from this and,according to a recent French magazine report,so do Sagem boxes sold for the French TPS (Television Par Satellit) bouquet. You also need the relevant Conditional Access Module (CAM) for any subscription channel along with appropriate smartcard (which could include a pirate card -these started to appear in summer 1997 although many were knocked out via ECM's from the operators). There are several different conditional access schemes in use by the different operators & each system needs the relevant CAM (as well as the smartcard for subscription channels). IRDETO was the commonest in Europe but now SECA seems to have overtaken it. (See diagram of a generic CA (conditional access) system as used in an IRD)



See diagram of a generic CA (conditional access) system as used in an IRD) http://uib.no/hff/smi/ksv/CARCVR2.jpg
Just to explain a some of the acronyms used in the diagram :-
ACS = Access Control System
ECM = Entitlement Control Message
EMM = Entitlement Management Message
CW = Control Word
MMI = Man-Machine Interface (smartcard reader in this case).
DVB-CI was a "cop-out" in that they could have specified that all DVB receivers used the same form of conditional access.

gessle
05-11-08, 23:31
Normal CAM modules use PCMCIA connections -a technology borrowed from laptop computers. This should allow you to unplug one module & insert another to switch from, for example, IRDETO to Viaccess.However this isn't a simple 5-minute task & the internal software isn't guaranteed to support the change!!Add to that the difficulty in easily obtaining CAM modules other than the one supplied within the IRD ,so this isn't going to be an easy option for many people. Of course,it gets easier with receivers that have 2 CI CAM slots. There is a group known as OKAPI trying to cut through the Conditional Access mess! OKAPI = Open Kernel for Access to Protected Interoperable interactive services). They consider : a)Simulcrypt -proprietary systems & common scrambling algorithm = interoperabilty.
b) Multicrypt -proprietary systems,common scrambling algorithm & DVB CI = openness & equitabilty
c) Equicrypt (from OKAPI) -TTP (Trusted Third Party),Public Key Cryptography,common smartcard DVB CI = openness, equitability AND interoperabilty.
Multicrypt and Simulcrypt
Multicrypt transmissions allow two different encryption systems to co-exist in the same receiver.The MPEG transport stream is sent sequentially through different modules that are inserted into the CI & each CAM will receive its entitlement messages.
Simulcrypt,on the other hand,allows different decoders with different CAMs (not necessarily CI compatible) to decrypt valid entitled channels (i.e. channels for which a valid smartcard is present in the CAM). This adds complexity to the service provider's equipment but allows the use of existing receivers without modification.Any individual IRD selects the entitlement information it requires whilst ignoring entitlement messages that are destined for other IRDs that use a different decryption system.Therefore,a CI compatible IRD is NOT required.However,global security is necessarily weaker.


6) DiSEqC

This stands for DIgital Satellite EQuipment Controller. DiSEqC was an idea dreamed up by the Eutelsat satellite team in order to make it easier for users in Europe to watch multiple satellites. In particular, they wanted more people to watch the Hotbird group of satellites at 13 degrees east whilst, at that time, Astra at 19.2 degrees east had the largest audience. Many people used 1 dish & 2 LNBs carefully positioned at the dish feed ; this worked and required some form of LNB switch at the dish end but controlled from indoors. Otherwise 2 separate lengths of IF cable were needed to run from the dish to the indoor receiver. So Eutelsat, based in Paris, successfully demonstrated the DiSEqc system. Normally the receiver acts as the DiSEqC controller sending commands to switches or even intelligent LNBs via the same coaxial cable that provides LNB power and acts as the IF downlead. It was a logical progression as Universal LNBs using the absence or presence of a 22KHz tone to select between 2 local oscillators were already proving a great commercial success. DiSeqC modulates the (existing) 22KHz tone with digital code words ; in other words, the coaxial feed is used as a digital communication bus with DiSEqC commands superimposed on the tone. The idea was presented by Eutelsat in 1996 & test units shipped to interested manufacturers that year. A low cost microcontroller was used and a UK firm built the original evaluation boards as well as providing technical support to triallists. The hardware cost of designing DiSEqC into a receiver turned out to be minimal - about 1 US dollar. Simple Master/Slave implementation does not even need full 2-way communication as this uses a simple "receive only" LNB or switcher at the dish end. Some flavours of DiSEqC are:

Mini DiSEqC - for simple control of block LNBs.
DiSEqC V1.0 - One-way signal only, no detection/acknowledgment normally used.
DiSEqC V1.2 - Allows positioner/motor control of a DiSEqC-compatible motor. For these rotators, the settings are stored within the motor unit - the receiver is just the means for setting up the motor in the first place by sending the right DiSEqc information to the (outdoor) motor unit.
DiSEqC V2.0 - This will allow full 2-way communication which will allow auto setup, diagnostics & other switching functions to be implemented.

Eventually, the self-installing intelligent LNB could become a reality using DiSEqC 2.x. There's more information and specifications on Eutelsat's website (www.eutelsat.org) as well as some on Philips Semiconductor website concerning compatible ICs.

7) Some receiver information

Some of this info I have collected from others , info on the Pace DVR500 ,D-Box, Nokia 9200, Seleco & RSD is from my own personal experience. The latest receivers do now have teletext -although not all operators or channels send teletext data. Also look out for receivers with a "MacroVision" video encoder fitted. MacroVision is a form of copy protection which prevents recording on a VCR.Macrovison Corporation have managed to get this implemented for DVD (Digital Video Disc) & it's also likely to appear on Digital IRD's. It will help protect PPV screenings of films possibly screened before they appear in the video hire shops!! As usual ,Hollywood is again exerting its influence. Most manufacturers now have Macrovision licences. Successive versions of Macrovision have each added more complexity (and so are harder to hack). Macrovision V 7 uses 3 processes that exploit differences between TV and VCR architectures ... AGC (Automatic Gain Control) pulses in the VBI (Vertical Blanking Interval)
Back Porch pulses (to upset synchronisation) -i.e. pseudo-sync and agc pulses are inserted onto the video output.
Colour Striping using phase reversal of the colourburst signal.

gessle
28-11-08, 11:28
According to the half trasmissivo (cable, satellite, spar), the available bandwidth for the transmission by satellite depends on technical considerations. In fact the relationship signal-noise and the echo, considerably vary among signals coming from satellite, cable or spar. In the receipt by satellite, the relationship signal-noise can be very small (10 dbs or less), in how much the signal originates from a transmitter positioned to more than 36000 Kms from the receiver, but it is not corrupt from echo.
In the receipt by cable, the relationship signal-noise is taller (30 dbs) but the signal is affection from due echi to multiple walks, interference and meaningful variations of the ampleness of the signal. These are the motives that you/they force to different choices for different means transmitted you. It needs besides to consider the compatibility with the existing analogical transmissions.
The gang of frequencies devoted to the analog transmission by satellite is generally between 27 and 36 Mhzes in Europe. The digital transmission inherits this situation and generally has to use the same gang of frequencies of channel of its analogical counterpart. The standardization to which he comes is that published by ETSI (European Telecommunication Standards Institute).
PANNING ON THE SYSTEM OF TRANSMISSION DVB

The signals audio video that is wanted to spread you/they are codified in formed MPEG-2 and subsequently multiplexatis come way to get packets of 188 byteses. The packets are made to pass through a randomizzatore that tries to make the most possible equiprobabili the symbols inside the flow of data. Such trial, known as randomization, has the purpose to eliminate the woodpeckers from the ghost of the I signal. You/he/she is done, if necessary, the scrambling to condition the access to the information. The length of the packet is increased then of 2 byteses, from an error-correcting code type Reed Solomon (RS 201,188), that allows the correction in receipt of a maximum of 8 wrong bit in presence of casual errors. Subsequently to the coding Reed Solomon, the data are again mixed (interleave) to outdistance them among them, so that to avoid that a sequence of errors brought closer affairs too much belonging bit to the same symbol. In the case of the satellite, a further error-correcting code is applied, the code convoluzionale of Viterbi, that multiplies the bit.rate of an inclusive factor among 1.14(Rc = 7/8) and 2(Rc =1/2). The data are subsequently formatted (symbol mapping), filtrates and converts in the analogical signals The and Q. Questi last is modulated.


(QPSK for satellite and QAM for the cable) to the if (intermediary frequency, that we point out with fi). You intermediary frequency is converted (up conversion) in the frequencies of channel RF (radio-frequency) related to the half trasmissivo. In the satellite before the diffusion of the signal to the consumers, a further conversion of the frequency happens in the gang KU (from 10.7 to 12.75 GHzes). You direct diffusion away cable is relatively rare in Europe. You/they are usually used systems that understand a satellite and a device that develops the functions of demodulations QPSK and rimodulazione QAM, necessary to suit the signal for the cable. The other receiver is not but the implementation of the operations of the complementary operations to that just described. In the case of the satellite an initial conversion of the frequency is served as the LOW NOISE CONVERTER (LNC), where the signal is brought in the field of frequencies understood between 950 and 2150 MHzes for then to pass to the intermediary frequency of around 480 MHzes. The coherent demodulation to the frequency IF returns analogical signals The and Q. Dopo a conversion A/D the signals you/they are filtered and reformatted (symbol demapping), remixed (de-interleaving) and the error correction recovers the packet di188 bytes. After the randomizzazzione the demultiplexter selects the channel chosen by the consumer, if necessary you/he/she is made the descrambling and the coding MPEG-2 it reconstructs the signals video and audio. In the case of terrestrial transmission the DVB is based on the modulation 2k/8k OFDMs (ETSI/EBU 300 744). This type of modulation performs the distribution of a tall bit-installments in an elevated number of carrying ortogonali (number that varies among the few hundred to few thousand), every of which he/she works to a low bit-installments - The same principle is used for the DAB (DIGITAL Audio BROADCASTING) that it uses 2K OFDMs. His/her principal advantage is the good behavior in the case of mobile receipt.


CHARACTERISTICS OF THE SYSTEM

For the transmissions satellitari the only possible coding is the QPSK (4-QAM). This depends on the elevated distorsion introduced by the half trasmissivo,che in the case of the DVB-S it is the ether. Infatti,valori typical of SNR (signal to noise Ratio) they are of the 10db order for satellitare drawn her/it. Under these conditions, a transmission to 64-WAM door a BER (Bit Error Rate) theoretical from the order of 10(elevato to the -1), while with coding Qpsk the ber is around of 5 orders of best greatness.

Forced therefore it is the choice of the coding.

The lnb is connected to the decoder through a coaxial cable with transmission asymmetrical fullduplex.

The decoder asks for a datum channel sending some signals to the LNB.

The presence or less than a tone to 22Khz selects the tall or low gang, while the polarization (orizz. or vert. ) of the channel to be visualized you/he/she is planned through a tension of 13 or 18 V. L' lnb it sends on the coaxial cable one of the 4 selected streams

Tall gang (With Tone to 22 KHz)Bandas Bassa (Without tone to 22 Khz)Polarizzaziones Orizz. (13 V)Streams 1Stream 2Polarizzazione Verts (18 V)Streams 3Stream 4

Sintonizzatore: it has the assignment to tune in the frequency of the channel that is wanted to visualize.


Demodulatore QPSK: retransform the harmonic signal modulated in phase, in the corresponding binary sequence.


FEC: law and it uses the bit of redundance to correct the possible errors in the packets DVB/MPEG2.


Demultiplexer: In base to the value of the header of every packet it decides whether to ignore the packet, to send him/it to the CAM, to directly send him/it to the decoder MPEG or, if it deals with packets of data to let them manage to the CPU.


CAM: Form of conditioned access. It is the part of the decoder able to perform the descrambling, that is it is able to put again in clear the signal criptato. It deals with a card PCMCIA (aka PC-CARD), that can be replaced with a CAM with another algorithm of descrambling (SECA, NDS, etc). to perform the descrambling uses some use of a smart card.

N.B.: In the case of events pay for view the cam warns through modem the manager that the consumer is looking at the program. The provider will deal him with to debit the cost of the event.


DECODER MPEG2: it brings in form not compressed the flow audio/video.


DAC: It converts the flow audio/video in analogical, ready to be connected to a TV, by scart for instance.


EEPROM: Memory rom riprogrammabile electronically, in which the necessary values are contained for the tuning of every channel.

gessle
28-11-08, 11:30
FORWARD ERROR CORRECTION

http://library.thinkquest.org/C0122280/english/objects/Image6.gif (http://library.thinkquest.org/C0122280/english/graphic.php?object=id1)
The transmission satellitare is subject to a high-level of trouble, therefore it is necessary to implement a form of correction of the errors. Being an unidirectional channel and gives the nature realtime and broadcast of the message it is not possible to ask the delay of the wrong packets to the satellite, therefore together with the byte of information you/they are sent some byte redundant for the correction. This technique has called FEC (Forward Error Correction)Nel our case you/they are used 2 layers of FEC.
The first called Viterbi is express in fractional number. The fraction expresses anymore the relationship among the number n of bit of data in entrance and the number n the bit used for the correction (es 2/3: every 3 received bit 2 are of data and 1 you/he/she is used for the correction).
The second layer of FEC has called Reed-Solomon tails (R/S). The r/s is usually 188/204, that is every 204 bytes, 188 are of data and the remainders they are used for the error correction. This same chain of FEC in the correction of errors is used for the CD-audios and the DVDs.
Every drawer decides the different values of FEC that will engrave on the gang (therefore possiblità to send more or less channels for every transponder) and on the quality.
In more to effectively correct burst of errors (that is more wrong bit cosecutivi) the flow is sent in a different order as shown in figure and riodinato after the transmission, from the IRD (interleaving/deinterleaving).

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ADC: They convert the analogical signals in binary sequences.
ENCODER MPEG2: it compresses the digital flow in packets MPEG2.
CRIPTATORE: It codifies the packets so that to be able to be interpreted only from who has paid the service.
CROSS-INTERLEAVER: It adds bit of redundance for the correction of the errors. Being a transmission broadcast, it is not possible after the comparison of an error from the decoder to ask for the corrupt packet. Therefore redundant information are added for assuring the expeditious data integrity.
MULTIPLEXER: It sends to the only exit, to turn, each of the present flows in the a lot of entrances (TDM: multiplazione in division of time). You multiplazione can be type deterministic (a gang pre-arranged for every channel) or type statistic (gang allocata dynamically for every in operation channel of the complexity of the images).
MODULATOR QPSK: Modulator of phase (and ampleness) to 4 stadiums of a carrying harmonica and a digital modulante.

gessle
28-11-08, 11:31
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The analogical flows (video and audio) digitalizzatis come, that is you/they are converted in a sequence of 0 and 1 through a converter A/D and compressed with the algorithm MPEG2. If necessary criptatis come. These flows (anymore some information for the decoding and for the synchronism) multiplexatis together come in a stream DVB/MPEG2 to form the PES (Packetized elementary stream). Every PES is distributed in packets of transport TPS long 188 byte, containers a byte of synchronism, 3 byte of prefix (PID), of which currently only the 2 byte less meaningful you/they are used, and 184 byte said profits payload.
1 SYNC3 PID184 PAYLOAD
Every packet therefore, according to the standard DVB/MPEG-2 it is identifiable through his/her PID. The value of the PID of every packet of the PES is decided by the drawer. Nevertheless there are some packets with PID consosciuto that has some particular meanings. Everything this is established in the document DVB-him (Service information).
XXH00 00HPAT (Program Association Table)
It furnishes Us the relationship among the CHs ID (Identificativo of the channel) of all the channels and the Pid PMT associated to every channel.
XXHYY YYHPMT (Program Association Table)
Containing Chart all the values of the PIDs (audio, video, dates, clock, ecm/emm, etc) necessary for the vision of a channel.

http://library.thinkquest.org/C0122280/english/objects/Image12.gif (http://library.thinkquest.org/C0122280/english/graphic.php?object=id6)
The providers send in uplink to the satellite these packets modulated in QPSK. Multiplexatis together often come more channels to the purpose of utlizzare the whole gang.
The satellite trasla toward the frequencies of downlink the harmonic signal modulated in phase, changes, if necessary, its polarization and it rebroadcasts him/it I pour earth.
To earth a receiving system satellitare will deal him with to make to visualize on a common television a datum channel.

OPERATIONS OF AN IRD

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LNB: Low Noise Block. You/he/she is placed on the fire of the parabolic spar. Its assignment is that to amplify the signal and traslare the gang of the frequencies received by the satellite, currently inclusive among 10.700 and 12.750Mhz, in a gang of lower frequencies denominated 1° IF (first intermediary frequency) inclusive among 950 and 2150



=va urma=

gessle
02-02-09, 20:44
Sateliti

I Introducere
Satelitii sunt corpuri ceresti care se rotesc in jurul altui corp ceresc, insotindu-l in cursul miscarii sale de revolutie. Dupa originea lor, ei se impart in doua mari categorii: naturali si artificiali.
In astronomie, satelitii naturali se definesc ca fiind corpuri ceresti secundare care executa o miscare de rotatie in jurul unei planete sau stele. Cel mai cunoscut satelit este cel al Terrei, Luna – desi cele doua sunt destul de apropiate ca marime pentru a fi considerate un sistem. Miscarea majoritatii satelitilor este directa, de la vest la est si pe aceeasi directie ca planete in jurul carora orbiteaza. Doar cativa sateliti ai marilor planete se rotesc in sens invers; probabil ca acestia au fost captati in campul lor gravitational dupa o anumita perioada de la formarea sistemului solar. De exemplu, Pluto, care se roteste in jurul Soarelui pe o orbita independenta se crede a fi un satelit deviat a lui Neptun. Recent s-a descoperit ca, la randul lui, si Pluto are un satelit. (fig 1)
Satelitii artificiali sunt obiecte plasate cu un scop bine definit pe o orbita in jurul unei planete. De la lansarea primului satelit arificial in 1957, mii de astfel de “luni create de om” au fost trimise pe orbita Pamantului. In zilele noastre, ei joaca un rol important in industria comunicatiilor , in strategia militara si in studiile stiintifice ale Terrei si Universului. 58754ool51edu1g
II Scurta istorie
Cativa dintre primii sateliti au fost proiectati pentru a opera in mod pasiv. In loc sa transmita activ semnale radio, ei serveau doar la a reflecta semnale care erau directionate spre ei de catre statiile de pe sol. Semnalele erau reflectate in toate directiile ,astfel incat sa poata fi receptionate de catre statiile din toata lumea.
In zilele noastre, satelitii folosesc in mod exclusiv sisteme de operare active, in care fiecare din ei poarta propriul echipament transmisie-receptie. Sute de sateliti de comunicatii sunt in prezent pe orbita. Ei primesc semnale de pe o statie de pe sol, le amplifica, apoi le retransmit pe o frecventa diferita la alte statii. Satelitii folosesc o gama de frecvente masurate in hertzi, mai precis benzi de frecventa de aproximativ 6 GHz.
Primul satelit activ, Score, lansat in 1958 de catre Statle Unite, era echipat cu un aparat de inregistrare a mesajelor primite in timpul trecerii pe deasupra unei statii de transmisie. Acestea erau retransmise cand satelitul se afla deasupra statiei de receptie. Telstar1, lansat de Compania Americana de Telefon si Telegraf in 1962, oferea transmisie tv directa intre SUA, Europa si Japonia, si putea de asemenea asigura redarea catorva sute de statii radio. od754o8551eddu
Alt satelit, Echo 1, lansat de catre SUA in 1960, era construit dintr-un balon de plastic aluminizat cu diametrul de 30m. In 1964 a fost lansat Echo 2, care avea un diametru de 41m. Capacitatea acestor sisteme era limitata de necesitatea transmitatorilor puternici si antenelor mari de pe sol.
III Tipuri si componente ale satelitilor artificiali
Inginerii au proiectat multe tipuri de sateliti, fiecare realizat pentru a servi unui anumit scop sau misiune.
De exemplu, telecomunicatiile si industria teleradiodifuziunii folosesc satelitii de comunicatii pentru a transporta undele radio, tv si semnalele telefonice pe distante mari fara a fi necesare cabluri sau relee de microunde. Satelitii pentru navigatii arata locatia obiectelor de pe Terra, in timp ce satelitii meteorologici ajuta la realizarea buletinelor meteo. Guvernul SUA foloseste sateliti de supraveghere pentru a monitoriza activitatile militare. Satelitii stiintifici servesc ca platforme cu baza in spatiu pentru observarea Pamantului, Lunii si altor planete, comete, galaxii, oferind o gama variata de aplicatii.

Sateliti de comunicatii
Majoritatea primilor sateliti includeau un oarecare echipament de comunicatie. NASA a lansat primii sateliti de telefonie si televiziune, AT&T’s Telastar 1, in 1962.Departamentul de Aparare al SUA a lansat Syncom 3 in 1964. Acesta a fost primul satelit care a avut o orbita geostationara. Din 1957 au fost lansati peste 300 sateliti de comunicatii.Cei din prezent ofera servicii de comunicare audio-video si de transmitere a datelor.
Satelitii de navigare
Satelitii de navigare ajuta la pozitionarea navelor si chiar a automobilelor echipate cu receptori radio speciali. Un asemenea satelit emite continuu semnale radio catre Pamant, care contin informatii pe care un receptor radio de la sol le converteste in informatii despre pozitia satelitului. Receptorul analizeaza mai departe semnalul pentru a afla directia si viteza satelitului.
Marina SUA a lansat primul satelit de navigare, Transit 1 B, in 1960. Air Force-ul american opereaza cu un sistem numit NAVSTAR GPS (Global Positioning System) care consta intr-un ansamblu de 24 de sateliti. In functie de receptor si metoda folosita GPS poate furniza informatii despre pozitionare cu o acuratete de la 100 m la mai putin de 1 cm.
Sateliti meteorologici
Satelitii meteorologici poarta camere video si alte instrumente indreptate catre atmosfera terestra. Acestia pot furniza avertismente in legatura cu instabilitatea vremii si contribuie foarte mult la prognoza meteorologica. NASA a lansat primul satelit TIROS 1, in 1960, care transmitea aproximativ 23000 de fotografii ale Terrei si ale atmosferei. Administrartia Nationala a Oceanelor si Atmosferei (NOAA) opereaza cu trei sateliti care colecteaza date pentru prognoza vremii pe termen lung. Acesti trei sateliti nu au o orbita geostationara; mai degraba, orbitele ii duc pe deasupra polilor la o altitudine relativ redusa.
Sateliti militari
Multi dintre satelitii militari sunt similari celor comerciali, dar ei transmit date codificate pe care numai un receptor special le poate descifra. Satelitii de urmarire fotografiaza la fel ca si ceilalti sateliti dar camerele acestora au o rezolutie mai mare.
Armata SUA opereaza cu o varietate de sisteme de sateliti. Sistemul de Aparare prin Sateliti de Comunicatie este alcatuit din cinci aeronave in orbita geostationara care transmit date audio si video intre locatiile militare.
Satelitii stiintifici
Satelitii care orbiteaza in jurul Pamantului pot furniza date privind harta Terrei, marimea si forma sa si pot studia dinamica oceanelor si a atmosferei. Savantii utilizeaza de asemenea satelitii pentru a cerceta Soarele, Luna, alte planete, comete, stele si galaxii. Telescopul spatial Hubble este un observator general lansat in 1990. Unii sateliti stiintifici orbiteaza in jurul altor corpuri ceresti decat Pamantul.
Celulele de energie solara montate pe panouri mari, atasate satelitului furnizeaza energie pentru receptie si transmitere.
SERVICII
Satelitii comerciali furnizeaza o gama larga de servicii.Programele de televiziune sunt transmise international, oferind astfel sanse fenomenului "globalzarea satelor"("global village." ).Acestia transmit de asemeni semnale catre sistemele de televiziune prin cablu sau catre antenele "farfurie".Satelitii Intelsat poarta acum peste 100 000 de circuite telefonice,un numar din ce in ce mai mare fiind transmisii digitale.
Organizatia International a Satelitilor Mobili(INMARSAT), fondata in 1979 este o retea mobila de telecomunicatii , ce ofera transmisii digitale ale datelor, telefonie si fax, diferite servicii intre nave maritime, facilitati ~offshore~ in toata lumea.In prezent isi extinde proprietatile pentru a oferi transmisii fax sau voice avioanelor pe rute internationale.

gessle
02-02-09, 20:45
PROGRESE TEHNICE RECENTE
Sistemele satelitilor de comunicatii au intrat intr-o perioada de tranzitie ~from point-to-point high-capacity trunk communications between large, costly ground terminals to multipoint-to-multipoint communications between small, low-cost stations.~.Dezvoltarea metodelor de acces multiplu a facilitat aceatat tranzitie.Cu TDMA, fiecarei statii de pe sol ii este transmis un timp slot pe acelasi canal utilizat in comunicatii; toate celelalte statii monitorizeaza aceste slot-uri si selecteaza directia de comunicatii spre acestia.Amplificand o singura cale de frecventa in ~repeater~ fiecarui satelit, TDMA asigura cea mai eficienta cale de utilizare a surplusului de enrgie al unui satelit.
O tehnica numita refolosirea ~(reuse)~ frecventei permite satelitului sa comunice cu un numar mare de statii de pe sol folosind aceeasi frecventa, transmitand unde inguste catre fiecare dintre acestea.Latimea undelor poate fi modificata pentru a acoperi arii mari ca SUA sau mici ca aprox. 6 judete ale Romaniei.Doua statii amplasate destul de departe una de alta pot primi diferite mesaje pe aceeasi frecventa.Antenele satelitilor au fost create special pentru a transmite mai multe unde in directii diferite, folosind acelasi emitator.
O metoda pentru interconectarea mai multor statii de pe sol, situate la distante foarte mari una fata de alta, a fost demonstrata in 1993, odata cu lansarea de catre NASA a satelitului ACTS (~Advanced Communications Technology Satellite~).Acesta foloseste tehnologia ~hopping spot beam~ pentru a combina avantajele reutilizarii frecventei, ale ~spot beams~ si ale TDMA. Concentrand energia transmiterii semnalui satelitului, ACTS poate folosi statiile de pe sol care au antene mai mici, reducand astfel cerintele de energie.
Conceptul de comunicatii ~multiple spot beam~ a fost demonstrat cu succes in 1991 odata cu lansarea Italsat, construit de catre Consiliul de Cercetari Italian.
De asemeni se pot folosi si raze laser, dar acestea au o rata de transmisie limitata deoarece ele sunt aborbite si imprastiate in atmosfera. Aparetele laser ce opereaza cu lungimile de unda albastre-verzi, ce pot patrunde in apa, sunt folosite pentru comunicatiile intre sateliti si submarine.
Ultima descoperire in domeniu este folosirea retelelor de sateliti mici ce urmeaza o orbita joasa (2000km sau chiar mai putin) pentru a oferi comunicatie telefonica la nivel global.Telefoanele speciale care comunica prin acesti sateliti permit utilizatorilor sa acceseze in mod regulat reteaua si sa efectueze convorbiri din orice loc de pe glob.
LANSAREA SATELITILOR
Plasarea satelitilor pe orbita necesita o cantitate colosala de energie, ce trebuie sa vina de la un vehicul sau dispozitiv de lansare.Satelitul trebuie sa ajunga la altitudinea de cel putin 200 km si la o viteza de peste 29 000 km/h (8km/s) pentru a putea fi pozitonat cu succes pe orbita.Acesta primeste aceasta combinatie de energie potentiala(in functie de altitudine) si de energie cinetica (in functie de viteza) de la arderea unor combustibili chimici.
Primul nivel al rachetei consta in motor, care furnizeaza o cantitate uriasa de energie. Acest nivel ridica de pe locul de lansare in prima parte a zborului intregul vehicul de lansare, toata cantitatea de combustibil, corpul rachetei si satelitul.Dupa ce motoarele folosesc tot combustibilul, primul nivel se separa de restul vehicului si cade pe Pamant.In acest moment intra in functiune al doilea nivel, care furnizeaza energia necesara pentru a ridica satelitul pe orbita.In cele din urma si acest nivel se desprinde de restul vehiculului.
Continuarea procesului de lansare este diferita in functie de misiunea satelitului.De exemplu daca acesta trebuie sa urmeze o orbita geostationara, care poate fi atinsa numai la o distanata de aproximativ 35 000 km de Pamant, un al treilea nivel al rachetei furnizeaza energia necesara pentru a pozitiona satelitul pe orbita sa finala.Dupa aceasta, un alt un alt motor cu reactie intra in functiune si ofera satelitului o orbita circulara.Fiecare ardere la motorul cu reactie are loc la un moment precis si dureaza atat timp cat satelitul ocupa pozitia potrivita in spatiu.
In 1990 Statele Unite au inceput sa lanseze cativa sateliti de pe aeronave ce zburau la altitudini mari.Aceata metoda inca necesita puterea motorului cu reactie al vehiculului de lansare, deoarece acesta nu poate depasi modulul fortei de frecare cu partea densa a atmosferei de la altitudini joase, folosind mai putin combustibil.Oricum, marimea rachetei este limitata de marimea si forta aeronavei, astfel putand fi lansati doar satelitii mici.
O alta metoda folosita este aceea de a lansa satelitii de pe nava-mama(space-shuttle).Acesta poate transporta satelitii mari, si deoarece este deja pe orbita la lansarea acestuia, astronautii pot verifica daca satelitul a "supravetuit" rigorilor lansarii.Ea poate aduce de asemeni satelitii pe Pamant pentru a-i repara.




OPERATIUNI IN SPATIU
Deoarece satelitii trebuie sa reziste lansarii si trebuie sa opereze in mediul aspru al spatiului, ei necesiata o tehnologie unica si durabila.Ei trebuie sa-si transporte sursa de putere deoarece nu o pot primi de pe Pamant.Satelitii trebuie sa ramana pozitionati pe aceeasi directie sau orientare pentru a-si indeplini misinea.Temperatura lor trebuie sa fie constanta intre patrea in care bate Soarele si cea in care este frig.Ei trebuie sa reziste la radiatii sau coliziunii cu micrometeorii.Majoritatea satelitilor au montate computere care ajuta la efectuarea operatiilor si la indeplinirea misiunii.

gessle
02-02-09, 20:47
1)PUTEREA
Un satelit isi produce puterea necesara pe toata durata misiunii, care poate fi extinsa la 10 ani sau chiar mai mult.Cea mai folosita sursa este o combinatie a fotocelulelor cu reincarcarea bateriilor. Panourile cu fotocelulele trebuie sa fie foarte mari pentru a produce puterea de care are nevoie satelitul.De exemplu, panourile telescopului spatial Hubble se de aproximativ 290 m2 si furnizeaza cam 5 500 watt, in timp ce un alt satelit, Global Positioning System(GPS) cu o suprafata de 4,6m2 furnizeaza 700 watt.Panourile arata ca niste aripi care se deprind de pe satelit in momentul in care ajunge pe orbita finala.Bateriile ofera putere inainte de a se descide panourile sau atunci cand razele solare nu ajung la ele.
2)ORIENTAREA
Orientarea unui satelit este directia pe care o are fiecare componenta. Acesta isi mentine panourile solare tot timpul spre Soare.In plus, antenele satelitului si senzorii sunt mereu orientati spre Pamant sau spre alte obiecte.De exemplu, satelitii meteorologici sau de comunicatii au antenele si camerele orientate spre Pamant, in timp ce telescoapele spatiale sunt directionate spre obiectele astronomice pe care oamenii de stiinta vor sa le studieze.Una din metodele folosite pentru orientare este folosirea unor mici motoare cu reactie, a unor roti care rotesc satrelitul si a unor magneti ce interactioneaza campul magnetic al Pamantului ce ajuta la orientarea corecta a satelitului. Motoarele cu reactie pot face modificari mari intr-un timp scurt, dar nu sunt cea mai buna solutie cand stabilitatea intoarcerii este critica. De asemeni acestea necesiata combustibil, si astfel durata de viata a unui satelit depinde de limita de combustibil a motoarelor.
Roata satelitului joaca rolul unui giroscop.Miscarea de rotatie a acesteia face satelitul sa stea pe o singura directie, iar miscarea rotii il va face sa se intoarca.Roata precum si magnetii sunt mai inceti, dar sunt excelenti pentru stabilitatea pe care o confera, precum si pentru ca necesita doar o sursa electrica de energie.
3)DIFUZAREA CALDURII
De vreme ce orbiteaza in jurul Pamantului, satelitul intalneste zone cu caldura intensa si zone cu o temperatura scazuta, deoarece alterneaza momentele in care este cu fata spre Soare si cele in care se ascunde de acesta. Echipamentul electronic de pe satelit creaza de asemeni caldura care poate cauza o avarie. Pe Pamant radiatiile de caldura pot fi transportate. In schimb, in spatiu unde nu exista aer care sa treaca pe deasupra satelitului si sa transfere caldura prin convectie si cum nu exista un alt corp caruia acesta sa-i poata ceda caldura, el trebuie sa-si controleze caldura.
Deseori satelitii folosesc radiatoare in forma de panouri ~louvered~, incluzand si panourile care se inchid si se deschid pentru a controla cantitatea de caldura.Pentru a preveni incalzirea pronuntata de Soare a unor puncte, satelitul se poate roti astfel incat caldura sa se imprastie pe toata suprafata.
4)RADIATIILE COSMICE SI PROTECTIA DE MICROMETEORITI
Satelitii trebuie sa suporte efectele radiatiilor si , mereu, loviturile micrometroritilor, in special in timpul misiunilor de durata.Atmosfera Pamantului blocheaza majoritatea radiatiilor cosmice care afecteaza microprocesoarelor computerelor de pe sol.Orice satelit, de asemeni, trebuie sa-si protejeze computerele.Radiatiile din spatiu fac unele materiale se devina fragile, si astfel unele portiuni ale satelitului se pot strica mai usor dupa o expunere indelungata. Panourile solare produc din ce in ce mai putine energie din cauza efectelor radiatiilor si a impactului cu micrometeoritii.


ORBITELE SATELITILOR
Trasaturile definitorii ale orbitei sunt forma, altitudinea si unghiul care il face cu Ecuatorul Pamantului. Acestea sunt alese pentru a servi cat mai bine misiunii satelitului. Majoritatea sunt circulare dar sunt unii sateliti care au orbite eliptice. Altitudinea unei orbitei determina timpul necesar satelitului sa executa o miscare de revolutie in jurul Terrei si proportia in care planeta este vizibila satelitului in acel moment. Satelitii trec peste diferite nivele ale latitudinii Pamantului in functie de unghiul orbitei lor luand ca sistem de referinta Ecuatorul. In plus, majoritatea se misca in sens invers aclor de ceasornic , privind de pe Polul Nord.
A. Orbita geostationara ecuatoriala(GEO)
Satelitii care au o orbita geostationara ecuatoriala, orbiteaza in jurul Pamantului de-a lungul Ecuatorului, la o altitudine specifica, in acelasi timp in care Terra efectueaza o rotatie completa. Ca rezultat, acestia stau deasupra unei regiuni mereu. Altitudinea orbitei de 5,6 ori mai mare decat circumferinta Ecuatorului, adica de aproximativ 35 800km. Satelitii care transmit emisiuni televizate, in direct, au o astfel de orbita.Cu toate acestea doar cativa sateliti pot furniza semnal pe toata suprafata Terrei. De asemenea, in supravegherea militara sau meteorologica se folosesc sateliti cu o orbita geostationara ecuatoriala.
B. Orbita joasa a Terrei(LEO)
Un satelit cu o orbita joasa se poate intalni la o altitudine de 2 000km sau mai putin. Aproape orice satelit intra pe acesta orbita dupa ce este lansat. In cazul in care misiunea necesita o alta orbita, acesta se deplaseaza cu ajutorul rachetelor. Orbita de altitudine mica minimizeaza cantitatea de combustibil necesara. De asemenea el poate furniza imagini de supraveghere mai clare, evitandu-se centurile de radiatii Van Allen. Are nevoie de semnale mai salbe pentru a putea comunica cu Pamantul, care ajung mai repede la destinatie, oferindu-le o proprietate destul de importanta in transmiterea datelor.
C. Orbita medie a Terrei(MEO)
Satelitii ce utilizeaza acesata orbita se intalnesc la altitudinea de aproximativ 10 000km si combina avantajele orbitelor LEO si GEO. Orbita medie este folosita in general pentru satelitii de navigatie si comunicatii.
D.Orbita polara
Satelitii cu orbite polare orbiteaza Pamantul la unghiuri de 90* fata de Ecuator si fata de poli. Acestea se pot intalni la orice altitudine, dar cei mai multi sateliti folosesc si orbita LEO. Doi sateliti apartinand Administreatiei Nationala a Oceanelor si a Atmosferei furnizeaza informatii despre vreme pentru toate zonele Globului la fiecare 6 ore. De asemenea, acestia realizeaza harti ale nivelului de ozon ale atmosferei, incluzand si zonele de deasupra polilor. LANDSAT este un satelit apartinand Guvernului SUA care opereaza pe o orbita polara. Oamenii de stiintra il utilizeaza pentru a studia diferite fenomene ale agiculturii, cum ar fi defrisarile forestiere.

gessle
02-02-09, 20:48
E. Orbita ~de sincron solara~~Sun-Synchronous~
Un satelit cu o astfel de orbita trece pe deasupra unui punct al Pamantului in acelasi moment in care Soarele este in aceasi pozitie pe cer. Acesta are o orbita retrogarda (in sensul acelor de ceasornic in jurul Terrei), la un unghi de aproximativ 98* fata de Ecuator. Aceasta orbita este utila pentru satelitii care fotografiaza Pamantul, deoarece Soarele va fi mereu la acelasi unghi fata de locul fixat pe sol.
Cateva dintre cele mai mari luni ale sistemului solar sunt la fel de mari cat cele mai mici planete:
Pe primul rand : Terra, Marte, Mercur si Luna Pamantului.
Al 2-lea rand: Io si Europa: satelitii lui Jupiter
Al 3-lea rand: Ganymede si Callisto: satelitii lui Jupiter
Al 4-lea rand: Venus si satelitul
lui Saturn, Titan.
Satelitii au revolutionat comunicatiile, facand legaturile telefonice si transmisiunile „in direct” ceva obisnuit.Un satelit primeste un semnalul(scria: microwave signal) de la o statie emitatoare de pe Pamant (uplink) care amplifica si retransmite semnalul spre o statie de receptie, la o frecventa diferita (downlink). Un satelit de comunicatii este pe orbita ...(geosynchronous ), adica el se roteste cu aceesi viteza cu care se roteste Pamantul in jurul axei sale. Astfel satelitul ramane relativ in aceeasi pozitie si nu va pierde legatura cu statia de receptie.Echo si Echo II au fost primii sateliti de comunicatii lansati de SUA in anii 1960. Acestia au pregatit drumul pentru construirea altor sateliti de comunicatii, mult mai sofisticati.
Telstar a fost unul dintre primii sateliti activi pentru comunicatii, lansat pe orbita de catre SUA in 1962. A transmis primele emisuni de televiziune „in direct” intre SUA si Europa. De asemeni a transmis si convorbiri telefonice.Satelitul de comunicatii Syncom 4 a fost lansat de pe Discovery. Satelitii moderni primesc, amplifica si retransmit informatiile inapoi spre Pamant, spre statiile de televiziune, telefax, telefon sau radio.Syncom 4 urmeaza o orbita (geosynchronous), ceea ce inseamna ca are aceeasi viteza cu Pamantul, ramanand in pozitie fixa deasupra Pamantului. Acest tip de orbita ofera posibilitatea de a mentine legaturile neintrerupte intre statiile de pe Pamant.Operatiile facute de satelitii de comunicatii sunt monitorizate din camere de control, cum este cea din imaginea alaturata, de unde pot fi facute mici modificari pentru a se putea pozitiona pe orbita, si astfel comunicatiile pot fi intotdeauna verificate. Daca apar probleme tehnicienii le pot rezelva sau pot transfera comunicatiile la un alt satelit.Cele mai multe statii de meteorologie folosesc informatiile furnizate de sateliti.Imagini cum este aceasta arata modul in care evolueaza vremea.Aceata este in permananta monitorizata si fotografiata de catre satelitii din spatiu.Dupa prelucrarea imaginilor, meteorologii pot determina temperatura, presiunea sau viteza vantului.Meteorologii utilizeaza informatii primite de la sateliti, cum ar fi GOES.Acesta ia date despre atmosfera si oceane. O camera din dotarea sa supravegheaza mereu Pamantul. In imagine, satelitul GOES-C este pus intr-o capsula pentru a putea fi transportat in spatiu.Un numar de 24 de sateliti GPS orbiteaza Pamantul oferind date utile atat armatei, cat si unor orase importante. Fotocelulele ofera energia necesara satelitului.Lansarea pe orbita a unui satelit GPS cu ajutorul rachetei Delta.



===va urma===:)

gessle
02-02-09, 20:53
Sistemul solar, Planetele si satelitii lor, Planete telurice, Planetele indepartate


Universul
Universul reprezinta totalitatea de energie si materie, inclusiv Pamantul, galaxiile si continutul intergalactic.
Soarele 28834vgf78gni6y
Datorita faptului ca se afla atat de aproape, Soarele 28834vgf78gni6y este steaua cea mai bine cunoscuta.

Astronomii disting chiar detaliile de la suprafata sa ( cele mai mici au o intindere de 150 km ). In comparatie cu Pamantul Soarele 28834vgf78gni6y este gigantic, volumul sau ar putea cuprinde 1 300 000 de planete ca a noastra, iar de alungul diametrului sau s-ar putea alinia la 109. Soarele 28834vgf78gni6y
este o imensa sfera de gaz foarte cald a carui masa o depaseste de gn834v8278gnni
300 000 de ori pe cea a Pamantului. La suprafata forta gravitationala
este de aproximativ 28 de ori mai puternica de cat cea de pe Pamant,
totusi Soarele 28834vgf78gni6y nu e decat o stea foarte obisnuita. Pentru astronomi, este
o adevarata sansa sa poata studia o stea atat de banala, tot ceea ce afla prin studierea Soarelui ii ajuta sa inteleaga mai bine si celelalte stele.

Fotosfera

Lumina orbitoare a Soarelui provine de la un invelis de grosime mai mica de 300 km, fotosfera. Aceasta este cea care da impresia ca Soarele 28834vgf78gni6y are o margine bine delimitata, temperatura sa este de aproximativ 6000 grade C. Vazuta prin telescop ea se prezinta ca o retea de celule mici stralucitoare, sau granule, eflate intr-o permanenta miscare. Ficare granula este o bula de gaz de marimea unei tari ca Franta, ea apare se transforma si dispare in aproximativ 10 minute.


Planetele si satelitii lor
Cele 9 planete principale ale sistemului solar se invartesc in jurul Soarelui in sensul acelor de ceasornic , la distante cuprinse intre minimum 45.9 milioane de km in cazul planetei Mercur si maximum 7.4 miliarde de km in cazul planetei Pluto . Planetele telurice sunt cele mai apropiate de Soare . Planetele gigant se afla mai departe iar si mai departe , planetele indepartate .
Mercur , Venus , Pamant si Marte , cele patru planete situate cel mai aproape de Soare sunt planetele telurice : ele sunt alcatuite din roci destul de dense . Suprafata lor - numita crusta sau scoarta – este solida . Ele sunt de talie mijlocie : diametrul lor este pana la 5000 km in cazul celei mai mici (Mercur) si sub 13000 in cazul celei mai mari (Pamantul) . Aceste planete au evoluat mult de cand s-au format . Ele au pierdut invelisul initial de gaz usor , iar atmosfera lor actuala provine de la gazul din interiorul acestor planete . Relieful lor s-a modificat pe parcursul timpului .
Planetele gigant , situate dincolo de Marte , Jupiter si Saturn sunt mai voluminoase decat planetele telurice . Ele reprezinta adevarte planete gigant . Diametrul lui Jupiter este de aproape 11 ori mai mare decat cel al Pamantului ; cel al lui Saturn de 9 ori mai mare . Dar densitatea lor este mult mai mica : aceste planete sunt in esenta sfere de gaz . Aceste planete nu au o suprafata solida ci doar un nucleu de roci si gheata . Ele au evoluat putin de cand s-au format si si-au pastrat invelisul initial : o atmosfera densa pe baza de hidrogen si heliu ( doua gaze usoare ) . Au o miscare rapida de rotatie ( in 10 pana la 16 h ) si sunt inconjurate de inele de materie .
Dupa Jupiter si Saturn urmeaza cele trei planete care sunt cel mai departe de Soare : Uranus , Neptun , si Pluto . Uranus si Neptun nu sunt atat de mari ca Jupiter . Ele sunt formate in principal din gaze usoare si sunt inconjurate de inele . Se crede ca interiroul lor contine o cantitate insemnata de gheata . Pluto , cea mai indepartata este un caz aparte : ea se aseamana planetelor telurice prin dimensiunea ei mica (un diametru de 2300 km ) si planetelor mari prin densitate scazuta . Cu exceptia lui Mercur si a lui Venus , principalele planete ale sistemului solar au unul sau mai multi sateliti . Astazi se cunosc in total 61 . Dintre acestia 27 au fost descoperiti datorita fotografiilor realizate de sondele spatiale . In functie de dimensiune satelitii pot fi clasificati in trei categorii . Cei mai mari sunt Luna , cei patru sateliti ai lui Jupiter ( Io , Europa , Ganimede si Calisto ) , satelitul cel mai mare al lui Saturn (Titan) si principalul satelit al lui Neptun (Triton) . Ei au un diametru de peste 3000 de km . Unii ca Luna si Calisto sunt formati din roci ; altii dintr-un amestec de gheata si roci . Satelitii de dimensiuni mijlocii au un diametru intre 200 si 1600 km . Ei se afla in jurul planetelor Saturn , Uranus , Neptun si Pluto . Majoritatea sunt formati dintr-un amestec de gheata si roci . In sfarsit minisatelitii , cu forma neregulata si o marime mai mica de 200 km ( cei mai mici chiar de cativa km ) , constituie a treia categorie . Cei mai cunoscuti sunt cei doi sateliti ai planetei Marte : Phobos si Deimos

gessle
02-02-09, 20:54
Planete telurice
Chiar daca la prima vedere cele 4 planete telurice ( Mercur , Venus , Pamantul si Marte ) sunt diferite , ele se aseamana prin dimensiuni si structura . Inca de la inceputul anilor ’60 , sondele spatiale au fost trimise spre Venus si Marte pentru a le studia .
Terra
Pamantul se afla la aproximativ 150 de milioane de km de Soare . El efectueaza miscarea de revolutie in aproape 365.25 zile , iar cea de rotatie in jurul propriei sale axe in 23h 56min 4sec . Aceasta este cea mai voluminoasa dintre cele patru planete telurice : ea are un diametru putin mai mare de 12700 km . In jurul Pamantului se afla aer , un amestec de gaz continand 78% azot si 21% oxigen . Specificul Pamantului consta in faptul ca este singura planeta pe care apa poate ramane lichida , favorizand astfel aparitia si dezvoltarea vietii . Aceasta apa , care erodeaza treptat rocile contribuie si la modificarea reliefului pe suprafata terestra. Temperatura cea mai ridicata pe Pamant este de +58 grade in Libia , iar cea mai scazuta de –89.9 grade in Antarctica . Pamantul are un singur satelit: Luna.
Luna


Diametrul este de 3476 km
Masa de 81,3 ori mai mica decat a Pamantului
Volumul de 50 ori mai mic decat al Pamantului
Departarea fata de Pamant este de 356400 km la pigeu si 406700 km la apogeu
Densitatea: 3,34 g/cm3
Atmosfera este practic absenta
Temperatura circa 150 grade C pe partea insorita si 1380 pe partea umbrita
Perioada de revolutie (in jurul Pamantului) este egala cu perioa-da de rotatie (in jurul axei sale) ca urmare are indepartata me-reu aceasi emisfera catre Pamant.
Aselenizarea primilor pamanteni a avut loc la 21 iulie 1969
Regiunile plate mai intense poarta numele de “mari” si “oceane” (Marea Linistei, Oceanul Furtunilor) si sunt delimitate de lanturi muntoase cu denumiri similare celor de pe Pamant (Alpi, Caucaz, Carpati).


Mercur
Mercur se afla la 58 milioane de km de Soare si face inconjurul acestuia in 88 de zile . Cum aceasta planeta este situata aproape de Soare si se invarteste lent in jurul propriei sale axe ziua este foarte cald (pana la 400 de grade) , iar noaptea foarte frig . Aceasta este cea mai mica dintre planetele telurice ( 4880 km in diametru ) . Mercur este practic lipsit de atmosfera pentru ca la fel ca Luna nu este suficient de greu pentru a retine un invelis de gaz . Absenta atmosferei a facut ca , pe parcursul a miliarde de ani , sa fie lovit de mici corpuri care circulau in spatiu . Mercur nu are nici un satelit cunoscut .
Venus

Situata la 108 milioane km de Soare , Venus isi parcurge orbita in 225 de zile . Rotatia in jurul propriei sale axe este foarte lenta , dureaza 243 de zile si are loc de la est la vest , in sens invers fata de rotatia celorlalte planete . Cu un diametru de 12100 km Venus este cu foarte putin mai mica decat Pamantul , dar atmosfera sa este foarte diferita : in principal aceasta este compusa din 96% gaz carbonic si 3.5% azot . Este inconjurata de un val gros de nori repartizati in 3 straturi situate la o altitudine intre 50 si 70 km . Unii dintre acestia provoaca ploi de acid sulfuric , o substanta chimica foarte periculoasa . Pe Venus temperatura este foarte ridicata . De fapt , gazul carbonic acumulat in atmosfera actioneaza sub efectul razelor Soarelui ca geamurile unei sere : temperatura la sol ajunge pana la 460 grade . Suprafata lui Venus este plina de platouri vulcanice . Se pare ca multi vulcani sunt inca activi . La fel ca Mercur , Venus nu are sateliti .

Marte
Planeta Marte este situata la aproximativ 228 milioane km de Soare . Ea inconjoara Soarele 28834vgf78gni6y in 687 de zile si se invarteste in jurul propriei sale axe in 24 h 37 min . Diametrul sau (6800 km) reprezinta putin mai mult decat jumatate din diametrul Pamantului . Din cauza slabei ponderabilitati ( o treime din cea a Pamantului ) ea nu a mai putut retine decat un invelis atmosferic neinsemnat . Acesta contine 95.6% gaz carbonic , 2.7% azot , 1.6% argon si urme de oxigen . Fiind mai departe de Soare decat Pamantul , Marte este o planeta mai rece : temperatura la sol scade in mod curent la –50 grade si nu depaseste niciodata 20 de grade . La fel ca Venus Marte pastreaza urmele unei intense activitati vulcanice : aici pot fi observati cei mai mari vulcani ai sistemului solar , cu o inaltime de peste 20 km . Suprafata desertica si stancoasa prezinta o frumoasa culoare rosiatica . De fapt rocile contin un oxid de fier care le da o culoare oarecum asemanatoare cu cea a ruginei . Uneori au loc furtuni violente care ridica nori de praf . In jurul lui Marte se invartesc doi sateliti de dimensiuni mici : Phobos si Deimos .

gessle
02-02-09, 20:56
Planetele gigant
Dincolo de Marte se afla doua planete gigant : Jupiter si Saturn . Usor vizibile si cu ochiul liber , ele au fost urmarite inca din antichitate . Cele mai concrete informatii in privinta lor au fost furnizate de sondele americane Voyager care le-au survolat intre 1979-1981 . Spre deosebire de Pamant , Jupiter si Saturn nu au o suprafata solida : aceste doua planete sunt doua imense sfere de gaz .
Jupiter
Jupiter este cea mai mare dinte toate planetele sistemului solar : are un diametru de 11 ori mai mare decat cel al Pamantului , o masa de 318 ori mai mare si un volum de 1300 de ori mai mare . Jupiter se afla la 778 milioane km de Soare . Acest gigant este inconjurat de o atmosfera densa pe baza de hidrogen si heliu , in care circula nori formati tot din gaze solidificate sau lichefiate : in special metan si amoniac . Cum el se invarteste foarte repede in jurul propriei sale axe ( mai putin de 10 h ) acesti nori se intind la ecuator si il acopera ca niste brauri . Norii aflati la exteior au aspect stralucitor , ceilalti , in schimb , sunt intunecati . Aceste formatiuni noroase sunt foarte turbulente : s-au observat turbioane enorme , care se modifica mai mult sau mai putin rapid . Unele dintre ele formeaza o imensa pata rosie , care i-a intrigat mult timp pe astronomi : este un uragan permanent , de patru ori mai mare decat Pamantul . Nivelul superior al norilor este foarte rece ( -148 grade ) , dar cu cat se coboara spre interiorul planetei , temperatura si presiunea cresc . In centrul lui Jupiter , temperatura atinge 30000 grade iar presiunea de 100 de milioane de ori mai mare decat la suprafata Pamantului . Jupiter are 16 sateliti cunoscuti . Patru dintre acestia sunt sateliti mari , cu o talie comparabila cu cea a lunii : Io , Europa , Ganimede si Callisto . Ceilalti sunt sateliti , cu un diametru de cateva zeci de kilometri . Sondele americane Voyager au produs o adevarata surpriza dezvaluind faptul ca pe Io , unul din cei patru sateliti principali ai lui Jupiter , exista numerosi vulcani activi , chiar daca suprafata sa este inghetata . Atrasa , pe de-o parte de planeta gigant Jupiter si , pe de alta de trei sateliti mari ai acestei planete materia situata in interiorul satelitului Io este in permanenta deformata si incalzita . Ea tasneste periodic la suprafata prin niste vulcani mari , cum este vulcanul Pele . Uneori lava de sulf este aruncata cu peste 3000 km/h la o inaltime mai mare de 200 km.

Inelele lui Jupiter
Sunt mai putin spectaculoase decat cele ale lui Saturn . Inelul principal are marginea exterioara la aproximativ 57000 kmde cei mai inalti nori ai atmosferei . Cu o inaltime de aproximativ 6000 km , el se prelungeste spre planeta intr-un halo difuz si , in partea opusa printr-un inel exterior mare
Saturn
Alt gigant , Saturn , are un diametru de 9,5 ori mai mare decat cel al Pamantului , de 95 de ori masa acestuia si de 750 de ori volumul lui . Saturn este situat la 1,4 miliarde de kilometri de Soare . La fel ca Jupiter , acesta este o sfera gazoasa care se invirteste foarte repede in jurul propriei sale axe ( in putin mai mult de 10 ore ) . Dar Saturn este mai putin des deoarece contine mai mult hidrogen : Saturn ar putea sa pluteasca pe apa ! Norii care il inconjoara sunt animati de miscari foarte violente : adevarate cicloane . La fel ca Jupiter , Saturn are o sursa de caldura interna : el emite aproape de trei ori mai multa enrgie decat cea primita de la Soare . In jurul lui Saturn s-au descoperit 18 sateliti , printre care unul gigantic numit Titan , mai mare decat planeta Mercur .
Inelele lui Saturn
Marea particularitate a lui Saturn consta in sistemul de inele care il inconjoara ; acesta este atat de amplu incat poate fi perceput chiar si cu o luneta de amatori . Galileo Galilei il intrezareste inca din 1612 , dar abia olandezul Huygens va fi cel care va intelege pentru prima oara fenomenul , in 1659 . De pe Pamant nu s-u descoperit decat sase inele , dar fotografiile realizate de sonda Voyajer au demonstrat ca ele sunt de ordinul miilor . Ele formeaza in jurul lui Saturn , in planul ecuatorului sau , un fel de disc imens , cu diametrul de 300000 km , dar cu o grosime de numai un kilometru . Dupa pozitia lui Saturn in functie de pamint si de soare , noi vedem aceste inele mai mult sau mai putin inclinate . Atunci cand ele apar pe muchie sunt atat de subtiri incit nu le mai vedem . Aceste inele sunt alcatuite din blocuri de gheata si pulberi care seinvirt in jurul planetei ca niste sateliti mici .

gessle
02-02-09, 21:00
Planetele indepartate
Dincolo de planetele gigant au fost descoperite alte planete de mari dimensiuni : Uranus si Neptun . Foarte indepartate , aceste planete sunt greu de studiat de pe Pamant . Ele sunt cunoscute mai bine de cand au fost survolate de sonda americana Voyajer 2 : Uranus in 1986 , Neptun in 1989 . In privinta lui Pluto , de acesta nu s-a apropiat nici o sonda spatiala , raminind astfel destul de misterioasa.
Uranus

In 1781 , Uranus a fost observat prin telescop din intamplare de catre astronomul englez William Herschel , care a crezot la inceput ca este o cometa . El are de 4 ori masa pamintului si de 15 ori masa acestuia . Se afla la 2,8 miliarde de km de soare . Mai mic si mai dens decat Jupiter si Saturn , Uranus este inconjurat la fel ca acestia de o atmosfera densa , pe baza de hidrogen si heliu . Insa atmosfera lui contine si un gaz care ii da o frumoasa culoare albastra : metan . Uranus este un adevarat ghetar : temperatura lui coboara sub –200 grade . Se crede ca nu contine hidrogen lichid metalic ci un nucleu de roci acoperit de un invelis dens de gheata . El este inconjurat de 10 inele de pulberi intunecate , care se desfasoara la o distanta intre 42000 si 51000 km de centrul planetei . In jurul lui Uranus au fost reperati 15 sateiti : cei mai mari , in numar de 5 au fost observati de pe Pamant , ceilalti au fost descoperiti de catre sonda Voyajer 2 .
Neptun
Neptun a fost descoperit in anul 1846 , chiar in locul in care astronomul francez Urbain Le Verrier a calculat ca ar trebui sa se afle , fiindca numai prezenta sa putea explica anumite anomalii ale miscarilorlui Uranus . Neptun se afla la o distanta medie de 4,5 miliarde de km de Soare . Prin aspectul talia si masa sa , Neptun este o adevarata sosie a lui Uranus , dar atmosfera lui estemai agitata . La diferite altitudini s-au observat nori deplasati de vanturi de peste 1000 km/h . Formatiunea cea mai spectaculoasa este o pata mare , intunecata , de marimea Pamantului . Ea aminteste de marea pata rosie a lui Jupiter . Aceasta este un uragan enorm , al carui turbion are peste 600 km/h . La altitudine mai mare circula nori luminosi , foarte rapizi , formati fara indoiala din cristale de gheata di metan . Din cauza indepartarii mari fata de Soare , Neptun primeste de 900 de ori mai putina enrgie solara decat Pamantul . In acelasi timp , s-a constatat ca el emite de 2,7 ori mai multa energie decat primeste . Nu se cunoaste sursa acestei calduri interne , dar ea explica vilentele miscari ale atmosferei . Datorita lui Voyajer 2 , au fost identificate in jurul lui Neptun 3 inele cufundate intr-un disc de pulberi ; particularitatea celui din exterior este aceea ca reprezinta 3 arcuri mai conturate , de-a lungul carora exista mai multa materie . Neptun are 8 sateliti cunoscuti . Cel mai mare , Triton , este corpul cel mai rece observat vreodata in sistemul solar . Temperatura la sol este de –228 grade .
Pluto
Cand a fost descoperi , in 1930 , Pluto era cea mai indepartata planeta din sistemul solar . Dar , cum orbita sa are forma unei elipse foarte alungite , distanta de soare variaza intre 4,4 si 7,4 miliarde de km . Astfel , din 1979 , Pluto se afla mai aproape de Soare decat Neptun iar acest lucru a durat pana in martie 1999 . Cu un diametru mai mic de 2500 km , el este de proportii mai reduse decat Luna . Vazut de pe Pamant , dimensiunile sale sunt echivalente cu cele ale unei monede vazute de la o distanta de zeci de km ! Nu a fost survolat de nici o sonda si ramane prea putin cunoscut . Se crede ca este format dintr-un nucleu de roci , inconjurat de un invelis de gheata . Suprafata sa ar putea fi acoperite cu azot si metan inghetate . Planeta ar avea o atmosfera rarefiata care contine metan . Unii cred ca aceasta planeta este un fost satelit al lui Neptun . Ea ar fi devenit libera I urma coliziunii cu un alt corp . In 1978 i s-a descoperit un satelit : Charon . Diametrul sau , de ordinul a 1200 km , reprezinta aproape jumatate din cel al lui Pluto . In sistemul solar , nu exista alte exemple de satelit proportional atat de mare in raport cu planeta sa .
Viitorul sistemului solar
Sistemul solar este menit sa dispara . De fapt , de cand Soarele a inceput sa straluceasca energia sa (lumina si caldura) rezulta din reactiile nucleare care transforma hidrogenul intr-un gaz ceva mai greu , heliul . Dar in mai putin de 5 miliarde de ani tot hidrogenul aflat in centrul sau va disparea . Noi fenomene se vor declansa si Soarele va creste in dimensiuni : se va transforma intr-o stea gigantica rosie . Pamantul va deveni atunci un adevarat cuptor : temperatura de la suprafata va atinge in jur de 2000 grade Celsius si din aceasta cauza rocile se vor transforma in roca fierbinte ! Cu mult inainte de acestea oceanele vor fi secat si intreaga viata va fi disparut . Dupa ultimele tresariri , Soarele 28834vgf78gni6y va inceta sa mai creasca . Materia se va contracta pentru a da nastere unei stele mici , de dimensiune Pamantului , dar cu o densitate deosebita ; o pitica alba care se va stinge treptat lasand sistemul solar in frig si intuneric .
Spectroscopia a dat informatii despre compozitia chimica si miscarea obiectelor astronomice. De-a lungul lungul secolului 20 constrirea unor telescoape din ce in ce mai mari a permis cunoasterea structurii galaxiilor si a unor parti din galaxii. Au fost construite clase noi de echipament astronomic sensibil la variatia radiatiilor electromagnetice.

gessle
02-02-09, 22:43
TELEVIZIUNEA PRIN SATELIT

Televiziunea prin satelit se foloseste de sateliti situati pe orbite eliptice (cu un tur complet de circa 12 ore) sau geostationare, dotati cu antene parabolice mari (diametrul de 9-12 m), pentru un semnal mai curat. Programele TV transmise prin satelit sunt receptionate de o serie de statii de sol si distribuite prin emitatoare si translatoare pentru acoperirea unui anumit teritoriu sau sunt receptionate direct de catre telespectatori folosind antene individuale. Statiile de sol folosesc emitatoare cu putere de cca 5-10 kW, antene parabolice de cca 20-25 m si sunt dotate cu aparatura necesara de urmarire a evolutiei satelitilor.
De obicei uplink-ul si downlink-ul sunt facute in benzi de frecventa diferite (C sau Ku), pentru evitarea interferentelor.
Satelitii care au banda C folosesc au in jur de 24 de canale de receptare-emitere cu o latime de banda de 36-50 Mbit/s, air cei care au banda KU au pana la 32 de canale receptie-emitere. Pentru evitarea interferentei, satelitii geostationari trebuie sa aiba o distanta intre ei de 2 grade sau de 1 grad, pentru cei cu banda C, respectiv banda Ku. Asta inseamna ca exista un numar limita de sateliti geostationari pentru fiecare banda, obtinut prin impartirea celor 360° ale Pamantului la 2, respectiv la 1.
Satelitii destinati in special televiziunii sunt impartiti in doua categorii:


sateliti DBS (Direct Broadcast Satellite). Acestia sunt utilizati in servicii de tip DTH (Direct To Home), adica transmisiunea se face direct catre echipamentul folosit de utilizator. Receptia canalelor TV prin satelit in locuinte folosind antena parabolica se face prin acest serviciu. Satelitii folosesc antene parabolice mici, cu diametre de 40-60 cm si polarizare circulara, iar frecventele sunt cele din partea superioara a benzii Ku. Cateva companii furnizoare de servicii DTH sunt: Sky Digital (Marea Britanie), Premiere (Germania), DirecTV (SUA).
sateliti FSS (Fixed Service Satellite). Acestia opereaza in banda C si portiunile joase ale benzii Ku si folosesc antene parabolice mai mari, cu polarizare lineara, dar cu putere mai mica. Satelitii FSS acopera aproape toate tipurile de servicii de telecomunicatii: transmisiuni catre statii de radio sau televiziune, transmisiuni telefonice sau de date, transmisiuni video in direct, gazduiri de videoconferinte sau invatamant la distanta, transmisiuni catre furnizorii de televiziune prin cablu. Prin serviciul TVRO (Television Receive Only), satelitii FSS reusesc sa furnizeze servicii DTH mai ieftine decat cele ale satelitilor DBS.

Inca un concept des folosit este cel de canal free-to-air, adica acel canal de televiziune care nu are nici un cost de receptare. In Romania de exemplu, astfel de canale free-to-air sunt: Antena 3, Pro TV, Prima TV, TVR international, Realitatea TV, B1TV(fta/conax) etc.
Constelatii de sateliti de televiziune mai importante poarta urmatoarele nume: Intelsat, Americom, Sirius, HotBird, Astra, Arabsat, Spaceway, PanAmSat, Amos.



http://sateliti.pluto.ro/TVradio/undestaantena.gif

gessle
02-02-09, 22:48
CE SUNT SATELITII ?

Satelit este orice obiect care parcurge o traiectorie circulara (care poarta numele de orbita) in jurul altui obiect.
Satelitii Pamantului pot fi naturali (Luna) sau artificiali (construiti de om, apoi lansati pe orbita).
Exista mai multe tipuri de sateliti artificiali, in functie de obiectivele pe care le au de atins:


sateliti astronomici - folositi pentru cercetare astronomica;
sateliti de telecomunicatii - folositi pentru a facilita comunicatiile la distanta;
sateliti de recunoastere - folositi mai mult in scopuri militare si de spionaj;
sateliti de observare a Pamantului - folositi pentru studii geografice;
sateliti meteorologici - folositi pentru masurari si prognoza meteorologica;
statii spatiale - compuse din mai multe module; transportul materialelor si echipajelor catre si de la statia spatiala este efectuat de alte nave spatiale.

De asemenea, exista mai multe tipuri de orbite, dupa care se clasifica si satelitii artificiali:


Satelitii subsincroni (aflati pe o orbita joasa, la o distanta mai mica de 35786 km de la nivelul marii). Perioada de rotatie a a acestora in jurul Pamantului este mai mica decat perioada de rotatie a Pamantului.
Satelitii sincroni (aflati pe orbita GEO, la distanta de 35786 km de la nivelul marii).
Satelitii geostationari (aflati pe orbita GSO) sunt sateliti sincroni a caror orbita se afla in planul ecuatorial al Pamantului si a caror miscare are acelasi sens cu sensul de rotatie a Pamantului. Satelitii geostationari au, practic, o pozitie fixa in raport cu un punct de pe suprafata pamantului (de ex. in raport cu statia de sol).
Satelitii aflati pe orbite eliptice. Aceste orbite au aparut din cauza dezavantajelor pe care le aveau zonele Pamantului aflate la latitudini mai mari de 60° in cazul in care comunicau cu un satelit geostationar. Cea mai folosita orbita eliptica este Molniya, aflata deasupra Rusiei.


http://sateliti.pluto.ro/orbitfig.gif

gessle
02-02-09, 22:55
Symbol Rate

This page refers to the outlink multi-Mbit/s carrier from the hub which is shared amongst all vsat users to download internet web pages etc. The carrier is similar to a DVB-S carrier which carries several MPEG TV programmes.
The carrier on the satellite is made up of a sequence of joined together pulses to make a continuous signal. Each pulse is a symbol. According to the modulation method each symbol represents 1, 2 or 3 etc bits of transmission rate data.
In phase shift keying (PSK) modulation each pulse is a burst of carrier signal with its sinewave zero crossing point timing adjusted forwards or backwards in time to constitute a phase shift. Phase shifts of 180 deg apply in BPSK, 90 deg in QPSK etc. A phase shift of 90 deg represents a time shift of 1/4 of a full cycle of the sinewave. The closer the spacing phase shifts, the more difficult it is to distinguish between them at the receive end, so for for each higher order PSK schemes more carrier to noise ratio is required.
As a general rule if you have bandwidth to spare, then use a lower order modulation or a higher rate FEC (like 1/2 or 2/3) to spread the signal out. If you have power to spare then use a higher order modulation and/or lower rate FEC (like 3/4 or 7/8). Ideally you want to use all of both the available bandwidth and power simultaneously to obtain the highest user information rate.
If you use larger receive dishes you will always be able to increase the system capacity. If you are doing a point to point link it is worth using larger dishes - spend more on the antennas and used advanced modulation technique modems, like Vipersat CDM-570L, to save on the space segment costs. If you have thousands of receive dishes then the aggregate cost of these is significant and you will want to allow smaller receive dish sizes even though this reduces system capacity and increases space segment costs.
Forward error correction

Forward error correction is applied to the customer's information data at the transmit end.
so transmission data rate = customer information rate x 1/ (FEC rate).
FEC rate is typically in the range 1/2 to 7/8 so the transmission data rate is always significantly more than the customer information rate.
This page provides a key formula:
SR = Symbol Rate
DR = Data Rate = the information rate. This is the same as the customer information rate if there is no framing, supervisory, conditional access or encryption overhead added to the data stream in the modem. DVB modems add significant overheads.
CRv = Viterbi forward error correction (FEC) Code Rate. Eg. 1/2, 2/3, 3/4, 5/6, 7/8
CRrs = Reed Solomon forward error correction (FEC) Code Rate. Eg. 188/204
If some other type of FEC coding method is chosen, such as Turbo coding, just use whatever FEC rate is selected (e.g. 5/16, 21/44, 3/4, 7/8, 0.95 )
m = modulation factor (transmission rate bits per symbol). BPSK=1, QPSK=2, 8PSK=3, 8QAM =3, 16QAM=4 etc
Formula: SR = DR / (m x CRv x CRrs)

DVB-S carrier bandwith


The bandwidth of the carrier at the -3.8 dB points is approx the same as the symbol rate.
The bandwidth of the carrier at the -12 dB points is approx 1.28 times the symbol rate.
The expression "occupied bandwidth" is used to refer to a bandwidth 1.19 times the symbol rate, approx -10 dB points..

The allocated bandwidth, i.e. spacing between carriers needs to be approx 1.35 to 1.4 times the symbol rate. If you put the carriers too close together you will start to see more adjacent carrier interference. If you put them too far apart you will waste expensive bandwidth, so choose some compromise that includes some, but not too much interference. A suggested adjacent carrier interference allowance in link budgets is 28 dB on each side. You choose. If you can avoid high spectral density carriers adjacent to low spectral density carriers it will help.
For example: Symbol rate=27.5 Msym/s. Bandwidth = -1 dB 20.9 MHz, -2 dB 24.2 MHz, -3 dB 26.25 MHz, -3.8 dB 27.5 MHz, -4 dB 27.7 MHz, -6 dB 30.3 MHz, -12 dB 35 MHz.

gessle
02-02-09, 22:58
Some examples:
Modulation and FEC rate and FEC coding method Minimum threshold Eb/No
(BER=10E-8)
Add an operating margin to this for clear sky set up, depending on C or Ku band and rain area. Information rate
bit/s Symbol rate.
per information bit rate
(e.g. 1 Mbit/s info x 0.667 = 667 ksps) Occupied bandwidth Hz at -10 dB points.
1.19 times the symbol rate Allocated bandwidth Hz (suggested carrier to carrier spacing)
1.35 times the symbol rate QPSK 1/2 rate FEC Viterbi 7.2 dB 1 1 1.19 1.35 QPSK 21/44 FEC Turbo 3.1 dB 1 1.048 1.246 1.414 QPSK 3/4 rate FEC Turbo 4.3 dB 1 0.667 0.793 0.9 QPSK 7/8 FEC Turbo 4.4 dB 1 0.571 0.68 0.77 8-PSK 3/4 rate FEC Turbo 6.7 dB 1 0.444 0.53 0.6 16-QAM 3/4 rate FEC Turbo 8.1 dB 1 0.333 0.397 0.536 16-QAM 7/8 rate FEC Turbo 8.2 dB 1 0.286 0.340 0.386

gessle
12-02-09, 14:20
Broadcast Engineering Basics


This is mainly about broadcast engineering, and the elements of the system that software developers don't usually see. This is by no means complete, and is nothing more than a sketch to show you some of what goes on in the rest of the system. A typical digital TV transmission setup looks something like this:
The components that make up a typical broadcast system.
http://www.interactivetvweb.org/images/tutorials/dtv_intro/head_end.gif This equipment is normally all connected together using high-speed connections like SDI (Serial Digital Interface) or ASI (Asynchronous Serial Interface) which are standard in the TV field. In addition to this, all of the equipment will me connected via ethernet to a control system and monitoring equipment to make sure that nothing goes wrong (or that if something does go wrong, the viewer doesn't see it). There will normally be a large number of some of these components, including some redundant spares in the event of problems. A typical head-end will contain many MPEG encoders and multiplexers, for instance. Now that we've seen how it's put together, let's examine each of these components in more detail.
The encoder

The encoder is used to take an analog signal and convert it to MPEG-2. This is more commonly used in live shows - for other shows, we may have a selection of pre-encoded MPEG streams that we can play out from a dedicated playout system. This playout system is usually a highly customized PC or workstation with a large high-speed disk array and a number of digital interfaces for transmitting the data to the rest of the transmission system.
An encoder can generate two types of MPEG stream. Constant bit-rate streams always have the same bit-rate, no matter what the complexity of the scene they contain. If the signal is too complex to be coded at the specified bit-rate, the quality of the encoding will be reduced. If the scene takes less data to code than the specified bit-rate, it will be stuffed with null packets until the correct bit-rate is reached. This makes later parts of the processing easier, because the fact the bit-rate does not change makes things easier to predict later, but it does waste bandwidth.
Most encoders can now produce variable bit-rate MPEG streams as well. In this case, the bit-rate of the stream can be adjusted dynamically, as more or less bandwidth is needed to encode the images with a given picture quality. Since some scenes take significantly more bandwidth to encode than others, this lets the picture quality be maintained throughout a show while the bandwidth changes. The fact that the bit-rate of the stream can change doesn't mean that it will reach higher levels than a constant bit-rate encoding of the same stream of course: the operator can usually set the maximum bit-rate that the encoder can use, and the encoder will reduce the quality of the encoded output, if necessary to meet this.
Most broadcasters today use variable bit-rate encoding because it offers better quality while using lower bandwidth. In particular, variable bit-rate encoding lets us make maximum use of the available bandwidth at the multiplexing stage.




The multiplexer

One MPEG stream on its own isn't much use to us as a TV broadcast. Even several MPEG streams aren't terribly useful, because we have no way of associating them with each other. What we really need is a single stream containing all the MPEG streams needed for a single service, or ideally multiple services. A transport stream, in other words.
The multiplexer takes one or more MPEG streams and converts them into a single transport stream. The input streams may be individual elementary streams, transport streams or even raw MPEG data - most multiplexers can handle a range of input types.
The multiplexer actually does a number of jobs - multiplexing the data is one of the more complex of these, for a variety of reasons. Each transport stream typically has a fixed bandwidth available to it, which depends on the transmission medium and the way the transmission network is set up. One of the jobs of the multiplexer is to fit a set of services in to this bandwidth. The easy way of doing this is to use constant bit-rate MPEG streams, because then the operator knows exactly how much bandwidth each stream will take, and setting up the multiplexer is easy. This gets pretty inefficient, though, since some streams may be using less than their share of the bandwidth, while others may need to reduce the picture quality in order to fit in their allocated share. This wasted space is a real problem, since the transmission costs are high enough (especially in a satellite environment) that you want to make maximum use of your bandwidth.
The way round this is to use variable bit-rate MPEG streams and a technique known as statistical multiplexing. This system takes advantage of the statistical properties of the multiplexed stream when compared to the properties of the several independent streams. While the bit-rate of each individual stream can vary considerably, these variations are smoothed out when we consider ten or fifteen streams (video plus audio for five to seven services) multiplexed together. Each stream will have different bit-rate needs at each point in time, and these differences will partially cancel one another out at any given time. Some streams will need a higher bit-rate than average at that time, but others will probably need less than average. This makes the bit-rate problems easier to handle, since they are now less severe. By maintaining a separate buffer model for each stream, the multiplexer can decide how to order packets in the most efficient way, while making sure that there are no glitches in any of the services.
At some points, the streams being multiplexed may have a bit-rate that is higher than the available bandwidth. A statistical multiplexer will use another one of the statistical features on MPEG streams to handle this situation. Since most MPEG streams only reach their peak bandwidths at fairly wide intervals for fairly short periods, delaying one or more of the streams will move the peak to a point where the bandwidth is available to accommodate it. This is another reason to maintain a buffer model for each stream - to ensure that these peaks are not moved to a point where they would cause a glitch in the service.
In some older statistical multiplexing systems, the multiplexer and encoders are connected and can communicate with one another. In particular, the multiplexer can provide feedback to the encoders and set the bit-rate that they encode their streams at. The feedback from the multiplexer means that if one stream needs more bandwidth than it's currently getting, the bandwidth for that stream can be increased temporarily at the expense of the others. This doesn't use true variable bit-rate encoding, since in many cases the streams are actually constant bit-rate streams, where the bit-rate used to encode them changes from time to time.
Despite appearances, this system is less flexible than true statistical multiplexing, because if the total bit-rate of the streams is higher than the available bandwidth, then the quality of one of the streams must be reduced. This isn't necessary in the case of the latest generation of statistical multiplexers, where these peaks can often be moved slightly to accommodate them. The other place where flexibility is lost is in the need for a connection between the encoder and the multiplexer. In practical terms, this means that the multiplexer and encoder have to be on the same site, or at least that the encoder feeds only one multiplexer at a time. In these days of remote processing, that can cause problems. Without this need, a network can handle streams where they have no control over the encoder, such as streams from remote sites, from other networks or from a playout system. This offers some big advantages in terms of bandwidth saving.

gessle
12-02-09, 14:25
Conditional access (CA)

Since we may not want to give our content away for free, we need some way of encrypting our services. This is handled by the conditional access (or CA) system. The algorithm that's used for this is proprietary to each CA vendor, although there are some open (but not publicly-known) algorithms such as the DVB Common Scrambling Algorithm. Manufacturers are understandably nervous about disclosing the algorithms they use, because the costs of having the algorithm cracked are huge - in some European markets, as much as 30% of subscribers were believed to be using hacked smart cards at one point. Even the DVB Common Scrambling Algorithm requires STB manufacturers to sign a non-disclosure agreement before they can use it.
In a DVB system, scrambling can work at either the level of the entire transport stream, or on the level of individual elementary streams. There's no provision for scrambling a service in its own right, but the same affect is achieved by scrambling all of the elementary streams in a service. In the case of scrambled elementary streams, not all of the data is actually scrambled - the packet headers are left unscrambled so that the decoder can work out their contents and handle them correctly. In the case of transport stream scrambling, only the headers of the transport packets are left unencrypted - everything else is scrambled.
As well as encrypting the data that's supposed to be encrypted, the CA system adds two types of data to the stream. These are known as CA messages, and consist of Entitlement Control Messages (ECM) and Entitlement management Messages (EMM). Together, these control the ability of individual users (or groups of users) to watch scrambled content. The scrambling (and descrambling) process relies on three pieces of information:


The control word
The service key
The user key

The control word is encrypted using the service key, providing the first level of scrambling. This service key may be common to a group of users, and typically each encrypted service will have one service key. This encrypted control word is broadcast in an ECM approximately once every two seconds, and is what the decoder actually needs to descramble a service.
Next, we have to make sure that authorized users (i.e. those who have paid) can decrypt the control word, but that only authorized users can decrypt it. To do this, the service key is itself encrypted using the user key. Each user key is unique to a single user, and so the service key must be encrypted with the user key for each user that is authorized to view the content. Once we've encrypted the service key, it is broadcast as part of an EMM. Since there is a lot more information to be broadcast (the encrypted service key must be broadcast for each user), these are broadcast less frequently - each EMM is broadcast approximately every ten seconds.
Encapsulating code words and service keys in ECMs and EMMs.
http://www.interactivetvweb.org/images/tutorials/dtv_intro/ecm_emm.gif One thing to note is that the encryption algorithms used may not be symmetrical. To make things easier to understand we're assuming that the same key is used for encryption and decryption in the case of the service and user keys, but this may not be the case.
When the receiver gets a CA message, it's passed to the CA system. In the case of an EMM, the receiver will check whether the EMM it intended for that receiver (usually by checking the CA serial number or smart card number), and if it is, it will use its copy of the user key to decrypt the service key.
The service key is then used to decrypt any ECMs that are received for that service and recover the control word. Once the receiver has the correct control word, it can use this to initialize the descrambling hardware and actually descramble the content.
While not all CA systems use the same algorithms (and it's impossible to know, because technical details of the CA algorithms aren't made public), they all work in basically the same way. There may be some differences, and the EMMs may or instance be used for other CA-related tasks besides decrypting service keys, such as controlling the pairing of a smart card and an STB so that the smart card will work correctly in that receiver.
In order to generate the EMMs correctly, the CA system needs to know some information about which subscribers are entitled to watch which shows. The Subscriber Management System, or SMS, is used to set which channels (or shows) an individual subscriber can watch. This is typically a large database of all the subscribers that is connected to the billing system and to the CA system, and is used to control the CA system and decide which entitlements should be generated for which users. The SMS and CA system are usually part of the same package from the CA vendor, and are tied together pretty closely.
The ECMs and EMMs are broadcast as part of the service (see the introduction to MPEG if you're unclear on the concept of a service). The PIDs for the CA data are listed in the Conditional Access Table (CAT), and different PIDs can be used for ECMs and EMMs. This makes it easier for remultiplexing, where some of the CA data (the ECMs) may be kept, while other data (the EMMs) may be replaced.

Error correction and error prevention

Before we can transmit our signal we need to make sure that it will be received correctly. This means some way of identifying and correcting errors in the stream. To do this we add some extra error correction data to the MPEG packets, in order to allow us to correct data. The most common requirement in DTV systems is for an MPEG stream to be quasi-error free (QEF), which means a bit error rate of approximately 1x10-10, or one erroneous bit every 1 hour of video for a 30 Mbits/sec stream. Since we have to be able to correct the errors in real-time, the process is called Forward Error Correction (FEC)
Different transmission mechanisms (cable, satellite or terrestrial) all have different characteristics including different noise levels. A satellite signal for instance can have a lot of errors introduced by conditions in the atmosphere. A terrestrial signal may have errors introduced by reflections from buildings, or by the receiving aerial not being aligned correctly. These different conditions mean that very efficient error correction mechanisms are needed. DVB and ATSC systems all use Reed-Solomon encoding to add a first layer of protection. This adds a number of parity bytes to each packet. Typically, this 16 parity bytes are added to a 188-byte packet, which means that an 8-byte error can be corrected. Larger errors can be detected but not corrected.
Once this is done, a further layer or error correction coding is added to improve things still further. Common coding mechanisms at this stage are trellis coding and viterbi coding. These exploit the fact that data is not sent one bit at a time, but is instead sent as 'symbols' that can carry several bits of data. In trellis coding, symbols are grouped together to form 'trellises.' For a group of three symbols, a modulation scheme that stores eight bits per symbol can store 512 separate values. By using a subset of these as 'valid' values, the network operator can introduce some extra redundancy into the signal. The effect of this is that each symbol may carry fewer bits of data, but for every group of three symbols, it's possible to correct one erroneous symbol by choosing the value for that symbol that gives a valid trellis. This is the approach used by US digital terrestrial systems. DVB systems use Viterbi coding instead, which is a modification of trellis coding that uses a slightly different algorithm to find the best matching trellis.
To strengthen the error correction, another technique called interleaving may be added. This helps avoid situations where a burst of noise (for example, a lightning strike causing electrical interference) can corrupt data past the point where FEC can fix it. After the data has FEC added, but before it is transmitted, the data is written to a RAM buffer and then read out in a different order. For instance, if we assume that our RAM buffer is a two-dimensional array with ten rows and ten columns, the data may be written to the buffer starting at row 1 and working down to row 10, then read from starting at the top of column 10 and working back to column 1. This means that bytes from the same packet (which will share error correction) are spread over a longer transmission period and are less vulnerable to burst noise.
At the receiver, the process is reversed, and the original order of the bytes can be restored. The interleaving scheme described here isn't the only possible one, and other (more memory-efficient) techniques will often be used instead.
Once we've added error correction, we need to do one more thing before it can be prepared for transmission. If the digital bitstream contains a large run of 1's, then there will be a (small) current flowing in the transmission and reception equipment. This is a Bad Thing, and so some randomization is needed to make sure that there is never a long run of 1's or 0's in the bitstream and to disperse the energy in the signal across all of its bandwidth. To do this, a simple randomizer is used, as shown in the diagram below. The process is symmetrical, so the same hardware is used to de-randomize the signal in the receiver.
A logical diagram of the DVB randomizer.
http://www.interactivetvweb.org/images/tutorials/dtv_intro/dvb_randomizer.gif Every eight transport packets, the randomizer is reset and its register is loaded with the bit sequence 100101010000000. Of course, the randomizer and the de-randomizer must both reset themselves at the same point in the stream, or the input can't be recreated. This is done using the sync bytes from the transport packets. These are not scrambled, so the start of a packet can always be identified, and at every eighth packet, the value of the sync byte is inverted (from 0x47 to 0xB8). This is the signal for the de-randomizer to reset itself, making sure that both the randomizer and the de-randomizer are synchronized correctly.

gessle
12-02-09, 14:30
Modulating the signal

Now we have a digital stream that is almost ready for broadcast. However, we can't directly broadcast digital data - first we have to modulate it - convert it to an analog signal so that we can broadcast it using radio signals or electrical voltages in a cable.
As we've already seen, each of the different transmission mechanisms has different characteristics, and different strengths and limitations. So, each type of signal uses a different modulation scheme. The modulation scheme is just the way of converting digital information into an analog signal so that it can be transmitted. I'm not going to examine these in too much detail, because it's really not interesting to us as MHP developers. The table below describes which modulation scheme is used by each of the transmission mechanisms in a DVB environment.


Cable and satellite use a similar modulation scheme (it's actually the same scheme, with different parameters). The main difference is that satellite signals are more prone to errors and so use a less efficient way of sending the data that provides a bigger difference between symbols, making correct demodulation easier. Terrestrial broadcasts use a different scheme in order to provide a much stronger resistance to errors caused by reflected signals.

The modulation is carried out by a device called, surprise, surprise, a modulator. This takes the digital transport stream as an input, and produces an analog output that can be passed onto the transmission equipment. The modulator is the last stage in the process that takes a digital input - after this, everything is analog and we're into the world of radio engineering.
Typically, signals are modulated to a lower frequency than they are broadcast at. Since the broadcast frequencies can be very high (up to 30GHz in the case of satellite transmissions, and up to 950MHz for cable signals), modulating the signals at these frequencies can be hard. So, what happens instead is that the frequencies are modulated at a lower frequency, which is then converted to a higher frequency before transmission. This is done using an upconverter. Basically, this does nothing else except convert the signal from one frequency to another, much higher, frequency. In this case, that other frequency is the one used by the network that you're broadcasting on. Each transport stream will be broadcast on a different frequency, and so the upconverter will have different settings for each transport stream that it handles.

Once you have a modulated signal, the signal is ready for transmission. All you need then is a transmitter, and antenna (in the case of terrestrial or satellite) or a cable network, and an audience...:)

gessle
11-12-09, 21:25
LNB Teorie- Constructie-Performante-Exemple

LNB sau LNC este precurtarea de la Low Noise Block sau Low Noise Convertor si reprezinta unul din componentele importante ale
instalatiei de satelit de care depinde si performanta acesteia .
LNB-ul este un amplificator si convertor de zgomot foarte mic pentru semnalul captat de antena si primit de la feedhorn
si sondele pozitionate in ghidul de unda.Teoretic daca am avea un LNB mai bun ar trebui sa putem folosi o antena mai mica ca
diametru pentru a receptiona anumite posturi Tv dar diferenta intre un LNB de 0,2 dB si unul de 0,5 dB nu este prea vizibila practic
mai ales ca in majoritatea zonelor tarii ,semnalul mai ales pe DTH-uri este destul de puternic.
In momentul cand zgomotul LNB-ului este mai mare semnalul util este ,,inecat’’ si nu mai poate fi extras la un nivel convenabil
pentru a fi procesat de celelalte blocuri ale LNB-ului si mai departe in receptor.
este trimis spre doua preamplificatoare de zgomot foarte redus comutabile de semnalul de 22 khz al receptorului ce comanda o
anumita polaritate in functie de cum este on anumit post dorit.
Un al doilea bloc de amplificare ridica nivelul acesteia in asa fel sa compenseze pierderile pe etajul filtrului ,acesta avand rolul de
a lasa sa treaca numai spectrul necesar extragerii semnalului util pentru al trimite spre mixer.
Blocul Mixer executa o coborare a frecventei semnalului de la 10,7 Mhz-12,7 Mhz pana la intervalul 950-2150 Mhz ce poate fi transmis
pe un cablu coaxial spre receptorul de satelit .Utilzand aceste frecvente mai joase de coborare pe cablu pierderea de nivel pe cablu
este inca la o valoare relativ mica pentru a putea fi procesat de receptorul digital.Dupa cum se observa frecventa de coborare pe cablu
este totusi pana la de doua ori mai mare decat cea din zona CATV ( 50 Mhz-900 Mhz) si deci ne impune folosire unor cabluri coaxiale
cu pierderi mai mici in frecventa si daca se poate de lungimi mai mici.
Urmeaza un ultim bloc de amplificare mare a semnalului converit necesar a compensa eventualele pierdeii pe cablu si aducerii la un
nivel convenabil receptorului. Alimentarea LNB-ului si transmiterea semnalului de 22 Khz se face pe acelasi cablu coaxial de coborare
spre receptor .Exista in LNB un bloc simplu de alimentare stabilizata ce aduce tensiunea la valorile impuse montajului .
Mai avem cele doua oscilatoare comutabile de semnalul de 22 Khz necesare mixerului ,aceste sunt facute cu cristale speciale ce au
frecventa de rezonanta si stabilitate adaptate la montajul LNB-ului.Aceste oscilatoare trebuie sa fie extrem de stabile in functie de
temperatura ambianta ,posibile vibratii,tensiunea de alimentare ,de aceasta depinzand stabilitate semnalului de iesire.
Exista si posibilitate comandarii Lnb-ului de oscilatoare tip PLL mai stabile sau externe ,in cazul unor necesitati profesionale.
Din punct de vedere al performantelor LNB –urilor putem aminti cei mai importanti parametrii de care poate depinde calitatea acestora :
1. Zgomotul LNB-ului care se poate situa intre 1 dB si 0,1 dB in zilele noastre marja de fabricatie este intre 0,6 si 0,1 Db
cu cat avem zgomotul mai mic cu atat LNB-ul e mai bun,ca totul ce este scris sa fie si real(sau cat mai real).
2. Amplificarea LNB –ului care se situeaza in marja 50-60 Db trebuie sa fie mai mare la unul mai bun.
3. Stabilitatea in banda a amplificarii .Deoarece banda este cuprinsa intre 10,7 Mhz-12,7 Mhz ar putea aparea o variatie in
Banda a amplificarii mai ales la marginile spectrului ceea ce ar face sa nu putem capta anumite frecvente si implicit canale
Tv din aceasta cauza.
4. Efectul de microfonie ,acesta depinzand de cat de stabil este oscilatorul local al LNB-ului la vibratiile antenei determinate
de instabilitatile atmosferice ( vant ,ploaie etc)
5. Intermodulatia este un efect ce poate aparea in cazul utilizarii pe un satelit mai puternic si semnalul de intrare in preamplificatorul
de intrare il ineaca aparind intermodulatii .Se poate face o atenuare a semnalului dar nu e prea convenabila daca folosim o antena
mobila cu semnale diferite.
Din punct de vedere constructiv putem avea mai multe posibilitati :
1. Carcasa din metal impotriva coroziunii
2. Carcasa din plastic
3. Cu mufa F sau N de iesire a semnalului ,in ultimul timp mai mult F.
4. Cu mufa de iesire coliniara cu axa LNB –ului si intr-o parte ( mai agreata pentru a nu patrunde apa ).


Exista si LNB in banda C ce este cuprinsa in intervalul :3,4 -4,2 Ghz ,acestea se folosesc mai rar Romania deoarece tarile
europene nu prea folosesc acest tip de transmisie .Exista si avantaje ale acestei benzi cum ar fi influenta mult mai mica a
conditiilor atmosferice asupra stabilitatii semnalului si aria de raspandire a semnalului pe o suprafata mai mare si deci putere
de emisie ceva mai mica a satelitului.Dintre dezavantaje cel mai important ar fi necesitatea unei antene mai mari la receptia
a semnalului deci si costuri mai mari .Alt dezavantaj major ar fi si interferenta in anumite zone cu frecvente transmise terestru
intre anumite statii ale unor televiziuni locale si chiar ale radarelor.
Se folosesc in general de tarile asiatice si americane ce au de acoperit o suprafata mare de teren si unde ploile sunt mai dese,
prin zona noastra ajung si semnale din Rusia ,China ,Pachistan ,India ,tari arabe si chiar spre west din tari sudamericane.


Din punct de vedere constructiv LNB –urile le putem imparti in patru categorii mai importante si raspandite :
1.LNB-uri Universale cu flansa
2.LNB-uri Universale offset
3.LNB-uri de banda C
5. LNB-uri circulare
Acest tip de LNB poate fi de banda C sau Ku cu flansa sau offset si are o imagine asemanatoare cu cele de mai sus
singura diferenta fiind in dispunerea sondelor , intercalarii sau scoaterii unor placi depolarizatoare in ghidul de unda
Polaritatea si benzile LNB-urilor
Schimbarea polaritatilor LNB-urilor universale se face prin comanda din receptorul de satelit cu o tensiune variabila astfel :
A . Poarizarea verticala V se obtine cu o tensiune de comanda in intervalul 12,5 V-14,5 V
B . Polarizarea orizontala H se obtine cu o tensiune de comanda in intervalul 15,5 V-18 V
Pentru accesarea benzilor superioare semnalul de 22 Khz dat de receptor joaca un rol esential in cazul LNB-urilor universale astfel :
C. Banda superioara de la 11,6-12,75 Mhz cu polaritate V se obtine prin adaugarea unui semnal suplimentar de 22 Khz la varianta A
D. Banda superioara de la 11,6-12,75 Mhz cu polaritate H se obtine prin adaugarea unui semnal suplimentar de 22 Khz la varianta B

gessle
11-12-09, 22:15
Antene si Feedhornuri


Caracteristicile antenelor

Antenele sunt printre cele mai importante componente ale sistemelor de comunicatii.Prin
definitie antena este dispozitivul ce transforma smnalul ce vine printr-un conductor in unde
electromagnetice in spatiul liber si invers din spatiul liber sunt preluate de antene si
transmise de cabluri spre dispozitivele ce transforma semnalul captat intr-un semal util(
audio ,video,date ,etc).Majoritatea antenelor sunt dispozitive rezonante ce lucreaza eficient
pe o banda relativ ingusta .Cand semnalul este primit de o antena ,aceasta il va emite pe o
anumita directie in spatiu.Graficul reprezentarii distributiei in spatiu a radiatiei se
denumeste caracteristica radiatiei.
Impedanta de intrare
Pentru un transfer optim de energie ,impedanta antenei si cablului de conectare trebuie sa
fie aceeasi.In general impedanta liniilor de transmisie este de 50 ohm ,iar cand exista o
diferenta trebuie sa intercalam un circuit adaptor.
Atenuarea de adaptare
Atenuarea de adaptare este o alta modalitate de cuantifica pierderea .Este fractie
logaritmica masurata in dB care compara puterea puterea reflectata de antena cu puterea
primita de feed de la linia de transmisie.
Return Loss (in dB) =20log10 SWR/ SWR-1
Largimea de Banda
Largimea de banda a unei antene se refera la domeniul de frecventa in care antena
functioneaza corect. Se poate transcrie matematic in procente ale frecventei centrale a
benzii astfel :
BW= 100 x Fh-Fl /Fc
Unde Fh este frecventa cea mai inalta a benzii , Fl este frecventa cea mai de jos a benzii iar
Fc Este frecventa centrala a benzii.Diferite tipuri de antene pot avea largimi de banda
diferita.
Directivitatea si castigul
Directivitatea este posibilitatea antenei de a focaliza energia intr-o anumita directie,sau de a
receptiona energie din directia cea mai buna.Intr-o situatie statica este posibil folosind
directivitatea antenei pentru concentrarea spotului intr-o directie dorita.Insa intr-un sistem
dinamic unde transferul nu e fix ,antena ar putea radia egal in toate directiile,aceasta fiind
denumita antena omnidirectionala .
Castigul nu este definit printr-o cantitate fizica cu ar fi V ,volti ,W,wati ci o marime
raportata.Castigul este exprimat fata de o antena standard numita antena izotropica ce
radiaza egal in toate directiile.Aceasta antena nu exista in realitate este numai un concept
teoretic pentru comparatie.Orice antena reala va radia energie mai multa intr-o directie dar
si ceva pe alta directie.Uzual suntem interesati de castigul maxim pe care antena radiaza
sau receptioneaza energie.
Modelul radiatiei pentru antena terestra
Radiatia sau modelul radiatiei descrie intensitatea relativa a campului in diferite directiei
ale antenei la o distanta constanta.Modelul radiatiei este tridimensional ,dar este uzual
reprezentata bidimensional.

Modelul radiatiei pentru o antena satelit
Dupa cum se vede in imaginea de mai sus o antena parabolica are un unghi de deschidere
foarte ingust ceea ce face ca daca antena se misca 2-3 grade stanga- dreapta semnalul sa
scada vizibil si chiar sa dispara de tot,depinzand de satelitul pe care suntem pozitionati ,
daca acesta este puternic ,gen Hotbird o scadere cu 20-30 % a semnalului nu afectea prea
mult majoritatea posturilor .In cazul unui satelit mai slab gen Astra 1 sau Nilesat chiar o
scadere de 10 % poate face sa dispara semnalul.Aceste scaderi pot aparea usor la o antena
mobila daca acesta nu se pozitioneaza exact pe satelit.
Daca avem o antena mai mare nivelul relativ creste si unghiul se mai ingusteaza ,si invers.
Deasemenea o antena este cu atat mai buna cu cat castigul relativ este mai mare .
La o antena bine facuta lobii secundari sunt de nivel mai mic.
Largimea fascicolului

Prin largimea fascicolului unei antene se intelege in mod curent injumatatirea puterii
acestuia.Jumatate din putere exprimata in dB este -3 d B/
Lobii secundari
Cunoscand faptul ca cea mai mare a radiatiei se divide la lobii secundari , directivitatea
castigului este invers proportionala cu banda ,atunci cand scade banda creste castigul ,si
invers.
Nulul
In modelul radiatie ,nulul reprezinta zona unde efectiv puterea radiatiei e minima.
Polarizarea
Polarizarea este definita ca fiind orientarea campului electric a undelor electromagnetice.
Polarizarea este descrisa in general ca o elipsa .Doua cazuri speciale ale acestuia ar fi
polarizarea liniara si cea circulara.In polarizarea liniara vectorul campului electric ramane
in acelasi plan tot timpul. La polarizarea circulara vectorul campului electric se roteste spre
directia de propagare facand o rotatie completa la fiecare ciclu RF.Rotatia poate fi spre
stanga sau spre dreapta.
Raportul fata -spate
Este util sa cunoastem ca raportul fata-spate este raportul dintre maximul de directivitate al
antenei pe directivitatea proprie fata de directia inversa la 180 grade.
Antena parabolica este o forma clasica de antena reflector folosita de receptoarele digitale
de satelit pentru receptionarea semnalului.Acest tip de antena are proprietatea ca razele
sosite in paralel cu axa de simetrie sunt reflectate intr-un punct comun numit focar .

O antena satelit este formata dintr-un reflector parabolic cu un feed mic aflat in focar unde
sunt reflectate undele incidente.Reflectorul este o suprafata metalica totala sau o insertie
pe un reflector parabolic.
Antenele tip prime focus au proprietatea de a avea un punct focar distinct .Undele venite
paralel cu axa de revolutie sunt reflectate intr-o zona numita focar, unde se afla feed-ul.
Antenele tip offset sunt antene de regula cu castig mai mic si au o forma mult mai
complexa .Forma reprezinta o decupare dintr-o suprafata a unui paraboloid mult mai mare.
La antenele tip Cassegrain undele reflectate de suprafata reflectorului principal in
reflectorul secundar sunt la randul lor reflectate in focar aflat in zona centrala .
Am putea zice teoretic ca fiind perfect ,dar in realitate antena genereaza o zona numita
focar care poate fi mai mica sau mai mare ,in functie de cat de precisa este realizata
parabola din punct de vedere a curburii,si de cat de corect a fost asmblata aceasta.
Este sarcina feedhornului de a reuni semnalul sosit in vecinatatea punctului focal si al
dirija spre primul etaj de de amplificare al convertorului de zgomot redus numit LNB
Miscarea moleculara genereaza un zgomot de fond care se intinde pe tot spectrul
electromagnetic ,inclusiv pe cel al frecventelor de satelit.Temperatura de zero absolut
sau 0 K ,este temperatura de zgomot unde miscarea moleculara se opreste.Chiar spatiul
cosmic temperatura este mai mare de zero absolut cu cateva grade in jur de 30 K.In
atmosfera temperatura de zgomot este in jur de 290 K.
Cu toate ca antena parabolica este inclinate spre spatiul ,,rece’’ pentru a receptiona
semnalul de la satelit,feedhornul este intors inapoi spre antena si spre spatiul din afara
acesteia .Prin urmare este extrem de important cum feedhornul ,,ilumineaza ‘’ reflectorul
Daca feedhornul ,,supralumineaza “ parabola ,feedhornul va vedea deasemenea si zgomotul
tare atmosferic din jurul acesteia .In acest caz temperatura de zgomot atmosferic se va
combina cu temperatura sistemului de receptie satelit astfel ca va scadea intensitatea
semnalului dorit.
Daca feedhornul va ,,subilumina” parabola ,acesta nu poate vedea o portiune exterioara
de antena si se va reduce astfel castigul acesteia .

Fedhornul se construieste de regula din duraluminiu ,dar poate fi facut si din alte metale
cum ar fi cupru,alama ,bronz ,dar aceste pot fi mai scumpe .Cercurile scalare conduc
semnalul venit din marginea exterioara a feedu-lui spre centrul acestuia unde se situiaza
ghidul de unda spre LNB.Dimensiunea si toleranta ghidului de unda sunt in legatura directa
cu lungimea de unda a semnalului de microunde si este conceput pentru fiecare dispozitiv
separat.Raportul intre distanta focala si diametrul antenei ,F/D , este o alta
caracteristica a antenei care direct impact asupra performantelor feedhornului.
Distanta intre inelele scalare si ghidul de unda adeschis este de multe ori ajustabila de catre
instalator pentru a putea potrivi cu F/D –ul antenei .Antenele prime-focus au F/D cuprins
intre :0,25-0,45.

Calcularea focarului unei antene se face in urmatorul fel ,unde D este diametrul
antenei parabolice ,d este adancimea acesteia masurate cum se indica acolo( F=D*D/16*d) .
O alta caracteristica a antenelor de satelit reprezinta apertura .Aceasta reprezinta proiectia
suprafetei frontale a unei antene ,in cazul unei antene prime focus este suprafata interioara
marginita de cercul cu diametrul D.
Castigul antenelor de satelit
Folosind formula pentru aria cercului,aria aperturii reflectorului parabolic este :
A= Π x D ² / 4
Iar castigul unei antene parabolice este dat de formula :
G =este in Db si λ lungimea de unda a undelor incidente , Π este 3,14
Dupa cum se observa castigul antenei este proportional cu apertura sau cu patratul
diametrului acesteia .
Lungimea de unda λ = c / f unde c reprezinta viteza de propagare a undei in mediul
respectiv ,se poate lua in vid ca 300 000 km /s ,iar f este frecventa acesteia.
Dupa inlocuirile respective se poate trage concluzia finala :
Castigul antenei parabolice este proportional cu
- eficienta acesteia
- cu patratul diametrului
- cu patratul frecventei
De aici ne dam seama de ce o antena in banda C unde frecventa este mai mica are castigul
mai mic si trebuie sa marim diametrul pentru compensare.
Antenele prime-focus sunt mai usor de construit si de reglat pe satelitul dorit dar au doua
mari dezavantaje :
-Feedhornul si suportul acestuia blocheaza o parte din undele ce vin spre antena
-Feedhornul priveste spre antena sub un unghi in care poate intercepta zgomotul di spatele
reflectorului., pentru a compensa acest lucru trebuie scazuta iluminarea antenei si deci
poate scadea si eficienta acesteia.Suportul feedhornului se poate face cu 3 sau patru tije
legate intre ele cu un suport specific ,sau un suport singular central rigid ce tine LNB –ul
in focarul predeterminat.In acest caz este foarte dificil de a regla LNB-ul in focar si poate
aparea o departare a acestuia din focar datorita greutatii suportate de o singura prindere.
La antenele cu motor feedhornul se poate misca la o deplasare de pe un satelit pe altul,sau
un vant puternic.
Pozitia inelelor scalare poate fi reglabila fata de tubul central pentru reglarea iluminarii si
implicit a zgomotului captat din exteriorul antenei...

Exista un tabel care ne arata cat de departe de gatul feedului trebuie sa fie inelele scalare
functie de raportul F/D
F/D --------Distanta in cm
0,42 ---------0.3 cm
0,40 ---------0,81 cm
0,38 ---------1,32 cm
0,36 ---------1,82 cm
0,34 ---------2,33 cm
0,32 ---------2,84 cm


Pentru determinarea practica a focarului unei antene in cazul in care nu mai avem suportul
original al LNB-ului sau vrem sa verificam daca este corespunzator ,lipim pe fiecare sfert
de antena niste bucati de oglinda sau alt material reflectorizant la marginea acesteia
pozitionam antena spre soare sau spre o alta sursa de lumina intensa coliniara cu axa
centrala a acesteia .In zona unde ar trebui sa fie focarul va aparea o pata reflectata care
trebuie sa fie cat mai concentrata .
Pierderi ale castigului antenelor parabolice
Tolerantele de executie a reflectorului parabolic se poate aprecia in figura mai de jos
tolerantele de executie ale reflectorului se poate aprecia in figura ,unde raportul rugozitate
supra lungimea de unda si fata de diferite rapoarte F/d adica fata de tipuri de antene mai
deschise sau mai inchise. Putem deduce ca :
-la o antena mai adanca sau focarul mai apropiat pierderea este mai accentuata la acceiasi
marime a rugozitatii
-cu cat scade frecventa cu atat efectul rugozitatii e mai mic,adica la o antena de banda C
efectul e mai redus decat la una de KU.
Pierderi mici ale castigului de ordinul 0,1-0,3 d B pot aparea si din cauza pierderii ohmice
In materialulu reflectorului,mai ales la cele din fibra sau plasic acoperite cu metalizare.Din
punct de vedere al executiei mecanice antenele parabolice pun probleme deosebite care
cresc exponential cu diametrul antenei.Problema principala este toleranta profilului
parabolic fata de cel teoretic,cat si plasarea feedhornului in focar,tolerante ce se micsoreaza
cu crestera frecventei,exemplu la 10 Ghz este necesara o toleranta de +- 1mm pentru o
antena cu un diametru de 1,2 m si F/d =0,3.

Legaturi pe semnal in domeniul microundelor
Comunicatiile in microunde prezinta diferente majore fata de cele in unde scurte sau
ultrascurte ,in primul rand datorita atenuarii mult mai mare de programare si datorita
nereflectarii microundelor de catre straturile superioare ale atmosferei.
Precipitatiile atmosferice introduc o atenuare suplimentara ,care depinde de intensitatea
fenomenului(ploaie,nori,ceata,ninsoare),atenuare ce devine importanta la frecvente de peste
10 Ghz. Datorita propagarii fara reflexii ,in principiul emitatorul si receptorul trebuie sa fie
plasate la limita de vizibilitate directa .Pierderile mari in atmosfera se compenseaza cu
antene directive cu castig mare,posibil de realizat datorita lungimii de unda mici.
Ca urmare a folosirii antenelor directive alinierea antenelor se face mai dificil ,deoarece
deschiderea unui fascicol de 10 Mhz pentru o antena cu diametrul de 1,2 m este de 1,6
grade la 3 dB.
Atenuarea de propagare in spatiul liber se poate calcula cu relatia experimentala :
-unde d este distanta intre antene in metri
- λ este lungimea de unda in metri
A{d B}= 96,6 +20 f / d
Conform acestei relatii rezulta ca pentru fiecare dublare de distanta implica adaugare a
6 d B la atenuarea totala .
Atenuare introdusa de precipitatii poate fi estimata cu relatia :
Ap{d B/Km }= γ R n
Unde: R–este intensitatea precipitatiilor in {mm/min}
γ si n sunt coeficienti depinzind de frecventa astfel :
γ= 2,3 si n =1,189 la f= 11,7 Ghz
γ=3,8 si n=1,116 la f = 15,25 Ghz
Practic o ploaie moderata poate determina o atenuare de 0,2-0,5 d B/Km ,iar o ploaie
torentiala 1-2 d B /Km la frecventa de 11 Ghz.Pentru frecvente sub 10 Ghz atenuarea
datorata precipitatiilor scade foarte mult

Reflectorul parabolic se executa din tabla de duraluminiu cu nervuri de rezistenta pe spate ,
din fibra de sticla metalizata sau prin inglobarea in stratul de fibra a unei folii sau plasa de
cupru.Stratul de metalizare trebuie sa aiba o grosime de minimum 0,2 mm continua pe
toata suprafata.Daca se foloseste plasa este necesar ca dimensiunile ochiurile sa respecte
relatia :
P < λ /20
Unde λ este lungimea de unda in mm .
Din aceasta relatie se deduce ca la frecvente mai joase se poate ca dimensiunile ochiurilor
sa fie mai mare si invers.
Se stie din practica ca la antenele de banda C unde frecventa de lucru e mai mica se
folosesc antene cu ,,gaurele ‘’,tip Mash.

Parabole Offset
Antena offset este un tip de antena parabolica ce se foloseste foarte mult in receptia
programelor de satelit de majoritatea populatiei.Este asa zisa receptie de satelit tip DTH.
aceste instalatii satelit larg raspandite folosec acest tip de antena denumita offset deoarece
este de o constructie relativ simpla ,sunt in general de un diametru sub 1 m,au o eficienta
foarte buna ,peste medie ,forma face ca zapada sa nu se aseze pe suprafata sa si deci
semnalul sa nu fie mult afectat .In general sunt construite din tabla de fier pentru ca pretul
sa fie cat mai mic.
Forma sa speciala este de fapt data din principiul de la care sa pornit pentru a putea
functiona .

Antenele offset sunt foarte folosite la DTH-urile actuale deoarece sunt in general mai
mici ,de regula sub 1m ,eficienta lor este mai mare comparativ cu antenele rotunde la
acelasi diametru.Sunt mai simple din punct de vedere al constructiei mecanice ,ceea ce
permite obtinerea unui pret mai mic de vanzare.Se asambleaza mai rapid deci o eficienta
a muncii de instalare mai mare.
La diametre peste 1,5 m ar fi de preferat cele rotunde datorita faptului ca la aceste marimi
antenele offset sunt destul de greu de executat exact pentru un castig maxim ,si pot fi
deformate usor la transport sau asamblare mai ales ca din economie tabla din care este
executata este destul de subtire,sub 2 mm.
Antenele offset se preteaza mult mai bine la receptionarea mai multor sateliti simultan
prin atasarea de LNB-uri pe un dispozitiv specific ,usor de adaptat si reglat.
Exista si antene offset semiprofesionale si profesionale din aluminiu executate de firme
mai de renume si cu un castig garantat ,putand depasi pretul receptorului digital .


http://www.qtl.co.il/img/copy.png

gessle
17-02-10, 13:27
MODULATOARE



ŞI



DEMODULATOARE


In scopul transmiterii la distanţă a semnalelor se utilizează semnale variabile in timp cu modulaţie in amplitudine (MA) sau cu modulaţie in frecvenţă (MF).
Un semnal periodic sinusoidal numit semnal purtător (sau purtătoare)
vp (t) = Vp cos w0t ,
este caracterizat prin amplitudinea V p şi frecvenţa f 0 (sau pulsaţia
w0=2 л f0 ).
Prin intermediul semnalului purtător se pot transmite diferite informaţii, exprimate prin valoarea amplitudinii purtătoarei sau valoarea frecvenţei.
Informaţia poate fi binară ,alocand o valoare pentru zero logic şi o valoare pentru unu logic. Spre exemplu dacă se recepţionează o putătoare cu amplitudinea de 10 V spunem că s-a recepţionat cifra 1, iar dacă amplitudinea este de 5 V spunem că s-a recepţionat 0 – logic.
Un alt exemplu se referă la transmiterea numerică, prin linia telefonică, unde un semnal cu frecvenţa de 2025 kHz este interpretat drept 0 - logic , iar un semnal cu frecvenţa de 2225 kHz este interpretat drept 1 – logic.
In multe din aplicaţii informaţia este reprezentată de un semnal variabil in timp , care are mai mult de două valori distincte (cum este in cazul semnalului binar) numit semnal modulator.
Dacă semnalul modulator acţionează asupra amplitudinii purtătoarei, frecvenţa purtătoarei fiind constantă, se obţine un semnal modulat in amplitudine (MA).
Dacă semnalul modulator acţionează asupra frecvenţei purtătoarei, amplitudinea purtătoarei fiind constantă, se obţine un semnal modulat in frecvenţă (MF).


Dacă semnalul modulator acţionează asupra fazei purtătoarei, amplitudinea purtătoarei fiind constantă, se obţine un semnal modulat in fază (MF). Dacă semnalul modulator acţionează asupra fazei purtătoarei şi asupra amplitudinii purtătoarei, se obţine un semnal modulat in cuadratură.
Transmiterea informaţiei prin atmosferă, in prezenţa unor semnale puternic pert 21421y245v urbatoare, se face cu ajutorul semnalelor modulate in frecvenţă, pentru că frecvenţa este mai puţin afectată decat amplitudinea semnalului la recepţie semnalul informaţional va fi decodat cu mai puţine erori .




Semnale modulate in amplitudine



Spunem că un semnal este modulat in amplitudine dacă semnalul util – numit semnal modulator, notat m - modifică amplitudinea Vp a purtătoarei.
Purtătoarea vp este un semnal sinusoidal
vp (t) = Vp cos w0t ,

a cărei frecvenţă (şi pulsaţie ω0) este constantă.
Presupunand că semnalul modulator este cosinusoidal
v m(t) = Vm cos (ωmt ) ,

atunci semnalul vp devine semnal purtător al informaţiei pe care o conţine semnalul modulator vm şi semnalul vp modulat de vm poate fi scris sub forma

v MA (t) = (V p + k a v m) cos (ω0t )
numit semnal modulat in amplitudine cu modulaţie MA normală.
Forma de undă a unui semnal modulat in amplitudine cu modulaţie MA normală este prezentată figura de jos:


http://www.scritube.com/files/fizica/22_poze/image002.jpg






Semnalul modulat in amplitudine poate fi descompus matematic
vMA = Vp cos (ω0t) + ka vm cos (ω0t)

in doi termeni din care primul „Vpcosω0t” reprezintă purtătoarea iar al doilea „Kavmcosω0t” reprezintă tot un semnal modulat in amplitudine.
S-a notat cu k a - factorul de comprimare.


Uneori nu se transmite purtătoarea vp şi spunem că avem un semnal modulat in amplitudine cu purtătoare suprimată

vMA.PS (t)=kavmcosω0t .
In condiţiile unui semnalul modulator armonic (vm= Vm cosωmt ) se poate scrie

vMA (t) = (Vp+kaVmcosωmt)cosω0t =Vp(1+mcosωmt)cosω0t ,

putand defini gradul de modulaţie
mhttp://www.scritube.com/files/fizica/22_poze/image004.gif=http://www.scritube.com/files/fizica/22_poze/image006.gif .
Matematic semnalul modulat in amplitudine poate fi exprimat astfel

vMA(t) =Vpcosω0t+mVpcosωmtcosω0t
=Vpcosω0t +http://www.scritube.com/files/fizica/22_poze/image008.gifcos(ω0+ωm)t +http://www.scritube.com/files/fizica/22_poze/image008.gif cos(ω0-ωm)t ,
incat să fie evidenţiate
Vpcosω0t - purtătoarea,
http://www.scritube.com/files/fizica/22_poze/image008.gifcos(ω0+ωm)t - componenta laterală stangă
http://www.scritube.com/files/fizica/22_poze/image008.gif cos(ω0-ωm)t - componenta laterală dreaptă.
Se constată că semnalul purtător al informaţiei Vm (definit prin
Vm = http://www.scritube.com/files/fizica/22_poze/image010.gif) este prezent in componentele laterale ale semnalului modulat.


In figura de mai jos sunt reprezentate semnalele spectrale asociate unui semnal modulat in amplitudine (complet sau „normal” ).


http://www.scritube.com/files/fizica/22_poze/image012.jpg


Transmiterea semnalului modulat in amplitudine se poate face astfel :
- Cu modulaţie normală =>
vMA(t) = (Vp+kavm)cosω0t ;

-Fără purtătoare (MA- cu purtătoare suprimată) =>

vMAPS(t) = kavmcosω0t ;
-Fără purtătoare şi fără una din componentele laterale
(MA-PS-BLU), numită cu purtătoare suprimată şi bandă laterală unică =>
vMABLU(t) = http://www.scritube.com/files/fizica/22_poze/image008.gifcos(ω0+ωm)t
vMABLU(t) = http://www.scritube.com/files/fizica/22_poze/image014.gif cos(ω0+ωm)t ;

- Fără purtătoare cu o banda laterală şi un rest al celeilalte benzi laterale.

Observaţia 1 Puterea de emisie se repartizează

P= http://www.scritube.com/files/fizica/22_poze/image016.gif+2http://www.scritube.com/files/fizica/22_poze/image018.gif=http://www.scritube.com/files/fizica/22_poze/image020.gif+2http://www.scritube.com/files/fizica/22_poze/image022.gif,

o parte pe purtătoare (primul termen din relaţie) şi egal pe cele două componente laterale.
Expresia repartizării puterii a condus la introducerea sistemelor de transmisie a semnalelor modulate fără purtătoare şi numai cu una din benzile laterale (scade puterea necesară şi emiţătorul va avea un preţ de cost mai mic).
Numai că orice caştig (la emiţător) se soldează cu pierderi in altă parte (la receptor), in sensul că receptoarele pentru semnale MA-PS-BLU vor avea o schemă mai complicată decat cele pentru recepţia semnalelor MA normală – datorită faptului că trebuie să refacă purtătoarea – ceea ce conduce şi la un preţ de cost mai mare.

Observaţia 2 Concluziile desprinse considerand un semnal modulator armonic (ca mai sus) se extind şi in cazul semnalelor modulatoare de alta formă, pentru că orice semnal modulator v m se descompune intr-o sumă de semnale armonice
v m(t) = http://www.scritube.com/files/fizica/22_poze/image024.gif
a căror frecvenţă este multiplul frecvenţei semnalului nesinusoidal.


http://www.qtl.co.il/img/copy.png

gessle
17-02-10, 19:12
Demodularea (detecţia) - este procedeul de extragere a informaţiei utile , adică a semnalului modulator , din semnal recepţionat.
Avand in vedere că sunt mai multe procedee de transmitere a semnalului modulator , schemele electronice de detecţie sunt specifice tipului de modulare.
In cazul transmisiei “normale” (atat a purtătoarei cat şi a celor două benzi laterale) se utilizează doua tipuri de detectoare
-detectorul de valoare medie;
-detectorul de valoare de varf.
Dacă se utilizează un sistem care nu transmite şi purtătoarea (cu purtătoare suprimată) la recepţie aceasta trebuie să fie refăcută şi apoi, in etapa următoare, să se facă extragerea semnalului util.
Spre exemplu dacă semnalul recepţionat este MA-PS-BLU (cu modulaţie a amplitudinii, cu purtătoare suprimată şi bandă laterală unică) demodularea se realizează cu un aparat realizat conform schemei din figura de mai jos:

http://www.scritube.com/files/fizica/22_poze/image026.jpg


Semnalul modulat
vMA-PS = VP m f(t) cos ω0t,

care in cazul modulaţiei cu semnal sinusoidal ( f(t) = cosωmt ) se poate rescrie sub forma
vMA-PS =VP m cos ωmt cos ω0t

este inmulţit, in blocul “X”, cu purtătoarea

v0= V0cosω0t.
La ieşirea blocului de multiplicare se obţine semnalul produs notat “ v “

v= vMA-PS VpV0 m cos ωmt cosω0t =
v= http://www.scritube.com/files/fizica/22_poze/image028.gifcos ωmt [1+cos(2ω0t)] ,
v = http://www.scritube.com/files/fizica/22_poze/image028.gifcos ωmt + http://www.scritube.com/files/fizica/22_poze/image028.gifcos ωmt cos (2ω0t)

care se compune dintr-un semnal de joasă frecvenţă

http://www.scritube.com/files/fizica/22_poze/image030.gif http://www.scritube.com/files/fizica/22_poze/image032.gifcosωmt ,
şi un semnal de frecvenţă mare

http://www.scritube.com/files/fizica/22_poze/image030.gif http://www.scritube.com/files/fizica/22_poze/image028.gifcos ωmt cos (2ω0t) .
Filtrul trece-jos FTJ de la ieşirea circuitului de multiplicare are rolul de a permite trecerea semnalului de joasă frecvenţă, blocand componentele cu pulsaţia 2ω0 .


Circuite multiplicatoare analogice
Circuitele de multiplicare au rolul de a realizarea produsului a doua semnale vx(t) şi vy(t) pentru a obţine

v0(t)=k vx(t) vy(t).
Principiul de funcţionare al unui circuit de inmulţire se bazează pe caracteristica de intrare exponenţială a tranzistorului
IE=I0http://www.scritube.com/files/fizica/22_poze/image063.gif.,

unde IE este curentul de emitor al tranzistorului, VBE este tensiunea
bază –emitor, iar celelalte mărimi sunt constante pentru un tranzistor şi o temperatură dată.


Daca VBE se modifica la VBE+∆VBE atunci IE se modifica de la IE la
IE + ∆IE aşa incat, prin diferenţiere, avem


∆IE = http://www.scritube.com/files/fizica/22_poze/image067.gif∆VBE = I0 http://www.scritube.com/files/fizica/22_poze/image069.gif http://www.scritube.com/files/fizica/22_poze/image063.gif ∆VBE =http://www.scritube.com/files/fizica/22_poze/image069.gif IE ∆VBE http://www.scritube.com/files/fizica/22_poze/image071.gif

∆IE= http://www.scritube.com/files/fizica/22_poze/image073.gif∆VBE ,
unde VT=http://www.scritube.com/files/fizica/22_poze/image075.gif este tensiunea termică a tranzistorului
In schemele de multiplicare nu se foloseşte un singur tranzistor pentru modelarea ecuaţiei de mai sus ci se preferă montajele diferenţiale,
pentru a realiza o compensare a variaţiei parametrilor tranzistorului cu temperatura.



...mai multe la coltul electronistului(in curand).