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: