To the real word, MPEG is a generic means of compactly representing digital video and audio signals for consumer distribution. The basic idea is to transform a stream of discrete samples into a bitstream of tokens which takes less space, but is just as filling to the eye (卭r ear). This "transformation," or better representing, exploits perceptual and even some actual statistical redundancies. The orthogonal dimensions of Video and Audio streams can be further linked with the Systems layer---MPEG's own means of keeping the data types synchronized and multiplexed in a common serial bitstream.

The essence of MPEG is its syntax: the little tokens that make up the bitstream. MPEG's semantics then tell you (if you happen to be a decoder, that is) how to inverse represent the compact tokens back into something resembling the original stream of samples. These semantics are merely a collection of rules (which people like to called algorithms, but that would imply there is a mathematical coherency to a scheme cooked up by trial and error?). These rules are highly reactive to combinations of bitstream elements set in headers and so forth.

MPEG is an institution unto itself as seen from within its own universe. When (unadvisedly) placed in the same room, its inhabitants a blood-letting debate can spontaneously erupt among, triggered by mere anxiety over the most subtle juxtaposition of words buried in the most obscure documents. Such stimulus comes readily from transparencies flashed on an overhead projector. Yet at the same time, this gestalt will appear to remain totally indifferent to critical issues set before them for many months. It should therefore be no surprise that MPEG's dualistic chemistry reflects the extreme contrasts of its two founding fathers: the fiery Leonardo Chairiglione (CSELT, Italy) and the peaceful Hiroshi Yasuda (JVC, Japan). The excellent byproduct of the successful MPEG Processes became an International Standards document safely administered to the public in three parts: Systems (Part 1), Video (Part 2), and Audio (Part 3).

Pre MPEG:

Before providence gave us MPEG, there was the looming threat of world domination by proprietary standards cloaked in syntactic mystery. With lossy compression being such an inexact science (which always boils down to visual tweaking and implementation tradeoffs), you never know what's really behind any such scheme (other than a lot of marketing hype).

A respected method developed by the old Sarnoff Princeton NJ research group was purchased in 1988 by our friend Intel. (The August 1988 issue of Stereo Review discusses the early days of compact disc digital video). It then became known as DVI, or Digital Video Interactive.

Seeing this threat?that is, need for world interoperability, the Fathers of MPEG sought the help of their colleagues to form a committee to standardize a common means of representing video and audio (a la DVI) onto compact discs? and maybe it would be useful for other things too.

MPEG borrowed a significantly from JPEG and, more directly, H.261.

Seeing how this MPEG things was such a good deal, and not wanting to be left behind in the industry, participants amassed, reaching a peak of more than 200 people by 1992.

By the end of the third year (1990), a syntax emerged, which when applied to represent SIF-rate video and compact disc-rate audio at a combined bitrate of 1.5 Mbit/sec, approximated the pleasure-filled viewing experience offered by the standard VHS format.

After demonstrations proved that the syntax was generic enough to be applied to bit rates and sample rates far higher than the original primary target application ("Hey, it actually works!"), a second phase (MPEG-2) was initiated within the committee to define a syntax for efficient representation of broadcast video, or SDTV as it is now known (Standard Definition Television), not to mention the side benefits: frequent flier miles, impress friends, job security, obnoxious party conversations.

Yet efficient representation of interlaced (broadcast) video signals was more challenging than the progressive (non-interlaced) signals thrown at MPEG-1. Similarly, MPEG-1 audio was capable of only directly representing two channels of sound (although Dolby Surround Sound can be mixed into the two channels like any other two channel system).

MPEG-2 would therefore introduce a scheme to decorrelate mutlichannel discrete surround sound audio signals, exploiting the moderately higher redundancy factor in such a scenario. Of course, propriety schemes such as Dolby AC-3 have become more popular in practice.

Need for a third phase (MPEG-3) was anticipated way back in 1991 for High Definition Television, although it was later discovered by late 1992 and 1993 that the MPEG-2 syntax simply scaled with the bit rate, obviating the third phase. MPEG-4 was launched in late 1992 to explore the requirements of a more diverse set of applications (although originally its goal seemed very much like that of the ITU-T SG15 group, which produced the new low-birate videophone standard---H.263).

Today, MPEG (video and systems) is exclusive syntax of the United States Grand Alliance HDTV specification, the European Digital Video Broadcasting group, and the Digital Versital Disc (DVD).

What is MPEG video syntax ?

MPEG video syntax provides an efficient way to represent image sequences in the form of more compact coded data. The language of the coded bits is the "syntax." For example, a few tokens amounting to only, say, 100 bits can represent an entire block of 64 samples rather transparently ("you can't tell the difference") which otherwise normally consume (64*8), or, 512 bits. MPEG also describes a decoding (reconstruction) process where the coded bits are mapped from the compact representation into the original, "raw" format of the image sequence. For example, a flag in the coded bitstream signals whether the following bits are to be decoded with a DCT algorithm or with a prediction algorithm. The algorithms comprising the decoding process are regulated by the semantics defined by MPEG. This syntax can be applied to exploit common video characteristics such as spatial redundancy, temporal redundancy, uniform motion, spatial masking, etc.

MPEG Myths

Because it's new and sometimes hard to understand, many myths plague perception about MPEG.

1. Compression Ratios over 100:1

As discussed elsewere, articles in the press and marketing literature will often make the claim that MPEG can achieve high quality video with compression ratios over 100:1. These figures often include the oversampling factors in the source video. In reality, the coded sample rate specified in an MPEG image sequence is usually not much larger than 30 times the specified bit rate. Pre-compression through subsampling is chiefly responsible for 3 digit ratios for all video coding methods, including those of the non-MPEG variety ("yuck, blech!").

2. MPEG-1 是 352x240

Both MPEG-1 and MPEG-2 video syntax can be applied at a wide range of bitrates and sample rates. The MPEG-1 that most people are familiar with has parameters of 30 SIF pictures (352 pixels x 240 lines) per second and a coded bitrate less than 1.86 megabits/sec----a combination known as "Constrained Parameters Bitstreams". This popular interoperability point is promoted by Compact Disc Video (White Book).

In fact, it is syntactically possible to encode picture dimensions as high as 4095 x 4095 and a bitrates up to 100 Mbit/sec. This number would be orders of magnitude higher, maybe even infinite, if not for the need to conserve bits in the headers!

随 MPEG-2 规范，最常用的组合构成 "等级"，这点在本文后面叙述。两个常用的等级如下：