MPEG


Started in 1988, the MPEG project was developed by a group of hundreds of experts under the auspices of the ISO (International Standardization Organization) and the IEC (International Electrotechnical Committee). The name MPEG is an acronym for Moving Pictures Experts Group. MPEG is a method for video compression, which involves the compression of digital images and sound, as well as synchronization of the two. There currently are several MPEG standards. MPEG-1 is intended for intermediate data rates, on the order of 1.5 Mbit/s. MPEG-2 is intended for high data rates of at least 10 Mbit/s. MPEG-3 was intended for HDTV compression but was found to be redundant and was merged with MPEG-2. MPEG-4 is intended for very low data rates of less than 64 Kbit/s. A third international body, the ITU-T, has been involved in the design of both MPEG-2 and MPEG-4. This section concentrates on MPEG-1 and discusses only its image compression features.

The formal name of MPEG-1 is the international standard for moving picture video compression, IS11172-2. Like other standards developed by the ITU and ISO, the document describing MPEG-1 has normative and informative sections. A normative section is part of the standard specification. It is intended for implementers, is written in a precise language, and should be strictly followed in implementing the standard on actual computer platforms. An informative section, on the other hand, illustrates concepts discussed elsewhere, explains the reasons that led to certain choices and decisions, and contains background material. An example of a normative section is the various tables of variable codes used in MPEG. An example of an informative section is the algorithm used by MPEG to estimate motion and match blocks. MPEG does not require any particular algorithm, and an MPEG encoder can use any method to match blocks. The section itself simply describes various alternatives.

The discussion of MPEG in this section is informal. The first subsection (main components) describes all the important terms, principles, and codes used in MPEG-1. The subsections that follow go into more details, especially in the description and listing of the various parameters and variable-size codes. The importance of a widely accepted standard for video compression is apparent from the fact that many manufacturers (of computer games, CD-ROM movies, digital television, and digital recorders, among others) implemented and started using MPEG-1 even before it was finally approved by the MPEG committee. This also was one reason why MPEG-1 had to be frozen at an early stage and MPEG-2 had to be developed to accommodate video applications with high data rates.

There are many sources of information on MPEG. [Mitchell et al. 97] is one detailed source for MPEG-1, and the MPEG consortium [MPEG 98] contains lists of other resources. In addition, there are many web pages with descriptions, explanations, and answers to frequently asked questions about MPEG.To understand the meaning of the words “intermediate data rate” we consider a typical example of video with a resolution of 360×288, a depth of 24 bits per pixel,and a refresh rate of 24 frames per second. The image part of this video requires 360×288×24×24 = 59,719,680 bits/s. For the sound part, we assume two sound tracks (stereo sound), each sampled at 44 KHz with 16 bit samples. The data rate is 2×44,000×16 = 1,408,000 bits/s. The total is about 61.1 Mbit/s and this is supposed to be compressed by MPEG-1 to an intermediate data rate of about 1.5 Mbit/s (the size of the sound track alone), a compression factor of more than 40! Another aspect is the decoding speed. An MPEG-compressed movie may end up being stored on a CD-ROM or DVD and has to be decoded and played in real time.

MPEG uses its own vocabulary. An entire movie is considered a video sequence. It consists of pictures, each having three components, one luminance (Y ) and two chrominance (Cb and Cr). The luminance component (Section 4.1) contains the black-andwhite picture, and the chrominance components provide the color hue and saturation (see [Salomon 99] for a detailed discussion). Each component is a rectangular array of samples, and each row of the array is called a raster line. A pel is the set of three samples. The eye is sensitive to small spatial variations of luminance, but is less sensitive to similar changes in chrominance. As a result, MPEG-1 samples the chrominance components at half the resolution of the luminance component. The term intra is used, but inter and nonintra are used interchangeably.

The input to an MPEG encoder is called the source data, and the output of an MPEG decoder is the reconstructed data. The source data is organized in packs (Figure 6.16b), where each pack starts with a start code (32 bits) followed by a header, ends with a 32-bit end code, and contains a number of packets in between. A packet contains compressed data, either audio or video. The size of a packet is determined by the MPEG encoder according to the requirements of the storage or transmission medium, which is why a packet is not necessarily a complete video picture. It can be any part of a video picture or any part of the audio. The MPEG decoder has three main parts, called layers, to decode the audio, the video, and the system data. The system layer reads and interprets the various codes and headers in the source data, and routes the packets to either the audio or the video layers (Figure 6.16a) to be buffered and later decoded. Each of these two layers consists of several decoders that work simultaneously.