Finally, we come to the heart of the matter: the MPEG (Motion Picture Experts Group) standards. These are the main algorithms used to compress videos and have been international standards since 1993. Because movies contain both images and sound, MPEG can compress both audio and video. We have already examined audio compression and still image compression, so let us now examine video compression.
The first standard to be finalized was MPEG-1 (International Standard 11172). Its goal was to produce video-recorder-quality output (352 × 240 for NTSC) using a bit rate of 1.2 Mbps. A 352 × 240 image with 24 bits/pixel and 25 frames/sec requires 50.7 Mbps, so getting it down to 1.2 Mbps is not entirely trivial. A factor of 40 compression is needed. MPEG-1 can be transmitted over twisted pair transmission lines for modest distances. MPEG-1 is also used for storing movies on CD-ROM.
The next standard in the MPEG family was MPEG-2 (International Standard 13818), which was originally designed for compressing broadcast-quality video into 4 to 6 Mbps, so it could fit in a NTSC or PAL broadcast channel. Later, MPEG-2 was expanded to support higher resolutions, including HDTV. It is very common now, as it forms the basis for DVD and digital satellite television.
The basic principles of MPEG-1 and MPEG-2 are similar, but the details are different. To a first approximation, MPEG-2 is a superset of MPEG-1, with additional features, frame formats, and encoding options. We will first discuss MPEG-1, then MPEG-2. MPEG-1 has three parts: audio, video, and system, which integrates the other two, as shown in Fig. 7-20. The audio and video encoders work independently, which raises the issue of how the two streams get synchronized at the receiver.
