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.