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UNDERSTANDING MPEG-2 DIGITAL VIDEO COMPRESSION By Mark Long

Published at the SHOWTIME/Gulf DTH Web Site  © copyright 1996 Mark Long.

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THE DIGITAL SATELLITE TV HANDBOOK is an entire course in digital satellite TV technology, complete with self-grading exams and supporting IBM-PC compatible software on CD-ROM.

Dateline Asia

Compressed Digital Video (CDV) is the driving force behind the world-wide revolution in satellite delivered Direct to Home (DTH) TV program distribution. CDV television signals are transmitted in an abbreviated format that dramatically reduces the amount of frequency bandwidth required without substantially degrading the quality of the received pictures and sound. The introduction of CDV technology is causing a dramatic decline in the operational costs for TV service providers. The result has been a global explosion in the number of new satellite delivered DTH TV services, including, news, sports, movies, Pay Per View (PPV) special events, educational programming, and narrowcast offerings that can target the needs of small segments within any potential viewing audience.

Personal computers have long used digital compression techniques to reduce the amount of data storage capacity needed to save large computer files. For the pat decade, the global telephone industry also has been using compression techniques to reduce the bandwidth, and consequently the cost, required for establishing narrow-band telephone circuits. During the early 1990s, communications engineers began developing high-capacity very large scale integrated circuits (VLSICs) and sophisticated software routines that could compress broad-band telecommunications signals such as video.

From Analogue to Digital Television
A conventional PAL or SECAM video signal contains 625 lines per individual image or frame. Flashed at a rate of 25 frames per second, each frame has two interlaced fields, each consisting of 312 and 1/2 lines, with field '1' displaying the even numbered lines and field '2' displaying the odd numbered lines. The interlaced scanning of the two fields is so rapid that the eye perceives each complete image or frame rather than the two separate fields within any one frame.

A single frame of conventional analogue PAL or SECAM video is composed of more than 174,000 picture elements or pixels. Since video operates at a rate of 25 frames per second, more than 4 million pixels are being sent to the TV screen each second. Digital TV transmission systems convert the visual and audio information into streams of binary digits or bits: strings of zeros and ones that correspond to the 'off' and 'on' logic states of computer circuitry. A maximum of 32 bits per pixel, 8 bits per luminance element and 24 bits per chrominance element, is needed to digitize a single color video signal. Four million pixels times 32 bits per pixel equals a transmission rate of 128 Megabits per second.

A 128 Mb/s digital TV signal would fill an entire satellite transponder to capacity! To allow multiple digital TV signals to occupy the same transponder, some form of signal compression is needed to dramatically reduce the number of bits used by multiple TV services occupying the same satellite transponder.

The Moving Pictures Experts Group
In 1988, the International Standards Organization (ISO) of the International Telecommunication Union established the Moving Pictures Experts Group (MPEG) to agree on an internationally recognized world-wide standard for the compressed representation of video, film, graphic and text materials. The committee's goals were to develop a relatively simple, inexpensive, and flexible standard that put most of the complex functions at the transmitter rather than the receiver. Representatives from more than fifty corporations and governmental organizations world-wide took part in the MPEG committee's deliberations.

In 1991, the MPEG 1 standard was introduced to handle the compressed digital representation of non video sources of multimedia. However, some manufacturers soon discovered that MPEG 1 could be adapted for the transmission of video signals as long as the video material was first converted from the original interlaced mode to a progressively scanned format. A few TV programmers elected to use MPEG 1 to transmit via satellite while the MPEG committee developed a standard specifically for interlaced scanning applications known as MPEG 2. The MPEG committee selected its criteria for MPEG 2 in 1994.

MPEG Compression Techniques
MPEG compression is accomplished by four basic techniques: pre-processing, temporal prediction, motion compensation, and quantisation coding. Pre-processing filters out non essential visual information from the video signal, information that is difficult to encode but not an important component of human visual perception. Motion compensation takes advantage of the fact that video sequences are most often highly correlated in time: each frame in any given sequence may be similar to the preceding AND following frames.

The compressed digital video encoder scans subsections within each frame, called macro blocks, and identifies which ones will not change position from one frame to the next. The encoder also identifies predictor macro blocks while noting their position and direction of motion. Only the relatively small difference, called the motion compensated residual, between each predictor block and the affected current block is transmitted to the receiver. The receiver/decoder stores the information that does not change from frame to frame it's buffer memory and uses it periodically to fill in the blanks, so to speak.

A mathematical algorithm called the Discrete Cosine Transform (DCT) reorganizes the residual difference between frames from a spatial domain into an equivalent series of coefficient numbers in a frequency domain that can be more quickly transmitted. Quantization coding converts these sets of coefficient numbers into even more compact representative numbers. The encoder refers to an internal index or code book of possible representative numbers from which it selects the code word that best matches each set of coefficients. Quantization coding also rounds off all coefficient values, within a certain range of limits, to the same value. Although this results in an approximation of the original signal, it is close enough to the original to be acceptable for most viewing applications.

I, P, and B Frames
MPEG 2 provides for up to three types of frames called the I, P and B frames. The intra-frame, or 'I' frame, serves as a reference for predicting subsequent frames. 'I' frames, which occur on an average of one out of every ten to fifteen frames, only contains information presented within itself. 'P' Frames are predicted from information presented in the nearest preceding 'I' or 'P' frame. The bi-directional 'B' frames are coded using prediction data from the nearest preceding 'I' or 'P' frame AND the nearest following 'I' or 'P' frame. Not all MPEG 2 systems use 'B' frames. Although a more efficient level of compression is achieved by 'B' frames, compatible receiver/decoders must have an additional memory buffer, which increases the cost of the decoder.

MPEG 2 Satellite Transmission Rates
The transmission speed required for any MPEG 2 broadcast varies according to the nature of the video material. The MPEG 2 encoder located at the satellite uplink has a finite time to make encoding decisions. Pre-recorded movies and other taped material do not push the time constraints of the encoder to the limit; the encoder can select at its leisure the most efficient method for encoding at the lowest possible data rate. Live sports and other live action materials require a higher data rate because the encoder is forced to make immediate coding decisions and must also transmit complex, rapid motion changes without introducing high levels of distortion.

Representative MPEG-2 Transmission Rates

Type of Video Service Data Rate

MPEG DTH Transmissions
DTH satellite TV programmers use a transmission format called Multiple Channel Per Carrier (MCPC) to multiplex two or more program services into a single unified digital bit stream. With MCPC, a package of program services can use the same conditional access and forward error correction systems, thereby economizing on the overall bandwidth and transmission speed requirements. What's more, programmers can dynamically assign capacity within the digital bit stream of any multiplexed transmission, so that more bits are available to a live sport broadcast and fewer bits to a news report or interview program consisting of 'talking heads.' At the conclusion of a live basketball game, for example, the digital capacity previously used to relay a single sport event even could be used to transmit four separate movies.

Digital Satellite TV Installations
Satellite installers usually tune in to conventional satellite TV signals while viewing a TV monitor connected to the receiver and adjusting the antenna pointing. During the fine tuning process, extremely weak signals are detected first by the appearance of a sync bar across the TV screen, then black and white pictures, and finally, a color picture that is totally devoid of impulse noise or sparklies.

Tuning into an MPEG 2 compressed digital video signal, however, is totally different. Weak signals virtually appear to be random noise; the receiver will not display any picture at all until sufficient signal is reaching the antenna. Then once the digital threshold of the receiver/decoder has been exceed, a perfect picture will appear on the TV screen. MPEG 2 reception is like a light switch: it's either on or off. Furthermore, if the installer moves past the antenna's peak performance position, the receiver/decoder will 'freeze frame' on the last picture in its buffer memory and will not receive any further video until the antenna is brought back to it optimum position.

Rain fades, which occur whenever these is moisture in the atmosphere between the satellite and the receiving system on the ground, can degrade the incoming signal level. DTH installers are advised to use antenna tuning meters or portable spectrum analyzers to peak antenna performance, so that the DTH systems they install will have some signal margin above receiver/decoder threshold to compensate for any Ku-band signal degradation caused by rain fades.

The ABCS of Digital Receiver/Decoders
Coping in the new digital age will require a basic understanding of a few new terms that have grown up surrounding this new technology. The following is a brief explanation of some of the new digital jargon that you may come across.

Bit Rate. This is the amount of data information being transmitted in one second of time. The total bit stream passing through a single satellite transponder consists of as many as eight TV services with associated audio, auxiliary audio services, conditional access data, and auxiliary data services such as teletext. The informational bit rate for this transmission may be as high as 49 Mega (million) bits per second (Mb/s) over a 36-MHz-wide satellite transponder. A single video signal within this bit stream will have a lower bit rate. For example, a VHS quality movie can be transmitted at a bit rate of 1.152 Mb/s; a news or general entertainment TV program at 3.456 Mb/s; live sports at 4.608 Mb/s or studio-quality broadcasts at a rate of more than 8 Mb/s.

Bit Error Rate (BER). Measured in exponential notation, the Bit Error Rate (BER) expresses the performance level of the digital receiver. For example, a higher BER of 5.0 x 10-5 is superior to a lower BER of 9.0 x 10-4. The higher the BER, the greater the receiver/decoder's ability to perform well during marginal reception conditions, such as when there is heavy rainfall, snow or wind gusts.

Digital Receiver/Decoder Threshold. Unlike traditional analogue Receiver/decoders, where the unit continues to deliver a picture even when it is operating below the receiver/decoder's threshold rating, digital receiver/decoders stop dead in their tracks when the signal level falls below the digital threshold level of the unit. It's like a light switch: either on or off. Manufacturers define the threshold as a BER such as 3.0 x 10-3. Reception stops whenever the BER falls below receiver threshold.

Digital Video Broadcasting (DVB). A 'broadcast specific' version of MPEG 2 developed in Europe for DTH, cable and terrestrial TV applications which numerous satellite programmers world-wide have since adopted for their systems. However, just because a satellite receiver/decoder is MPEG 2 DVB compliant doesn't mean that it can pick up any MPEG 2 DVB compliant satellite TV signal. Major differences in encryption and conditional access methods, use or non-use of 'B' frames, and other technical parameters have prevented MPEG 2 from becoming a truly world-wide video standard. A few programmers have formed alliances so that their DTH subscribers will have access to the maximum number of services available from a given satellite. Other programmers, however, have elected to use unique systems that can only access a single DTH program package.

Forward Error Correction (FEC). The digital bit stream contains special codes that the receiver/decoder can use to check to ensure that all bits of information sent have been received. If bit errors due to thermal noise, for example, are detected, the receiver/decoder can use the convolutional encoding information that has been forwarded to it by the transmitting station to either correct each detected error or conceal it. Burst noise interference, which can be caused by nearby automobile ignition noise or microwave ovens, can be detected by the Reed Solomon block decoder.

All graphics presented are from The New Age of Digital Video Compression Produced by Shelburne Films © 1994. All Rights Reserved.