The MPEG includes multiple standards for encoding not only video but also the accompanying audio. Over the years it has progressed through several levels with increasing sophistication and quality.
The first MPEG standard, now usually called MPEG-1 but formally titled "Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 MBit/s" became an international standard in October 1992. It has four parts. The actual compression of video signals is covered under International Standard 11172-2. Related parts describe the compression of audio signals, synchronizing audio and video, and testing for compliance with the standard.
The reason MPEG-1 is used by CD-i (interactive Compact Discs) is that it achieves a data rate that is within the range of CD drives. To get down that low with the technology existing at the time the standard was developed, the system sacrifices resolution. At best, an MPEG-1 image on CD-i has about one-quarter the pixels of a standard TV picture. MPEG also requires hefty processing power to reconstruct the moving image stream, which is why CD-i players can display it directly to your TV or monitor, but only the most powerful PCs can process the information fast enough to get it to your display without dropping more frames than an art museum in an earthquake. If you're used to the stuff that pours out of a good VCR, this early MPEG looks marginal, indeed.
MPEG-2 was meant to rectify the shortcomings of MPEG-1, at least in regard to image quality. The most apparent difference appears on the screen. The most common form of MPEG-2 extends resolution to true TV quality, 720 pixels horizontally and 480 vertically, while allowing for both standard and wide screen formats (4:3 and 16:9 aspect ratios). Although MPEG-2 benefits from advances in compression technology, this higher quality also demands more data. The TV-quality image format requires a bandwidth of about 4 Mbps. Beyond that, the MPEG-2 standard supports resolutions into ionspheric levels. All MPEG-2 chips are also required to step back and process MPEG-1 formats.
In addition to high quality video, MPEG-2 allows for 5.1 audio channels-that is, left and right main channels (front), left and right rear channels (surround), and a special effects channel for gut-thumping rumbles limited to no higher than 100 Hz. (The ".1" in the channel description refers to the 100 Hz limit.) MPEG-1 only allows for a single stereo pair.
What was initially MPEG-3 has been incorporated into MPEG-2. The concept behind MPEG-3 was to make a separate system for High Definition TV for images with resolutions up to 1920 by 1080 pixels with a 30 Hz frame rate. Fine tuning the High levels of MPEG-2 worked well enough for HDTV images that there was insufficient need to support a separate standard.
MPEG-4 is slated to go the opposite direction, aimed at very low data rate applications that transfer information in the range of 4800 to 64,000 bits per second. Such low rates are suited to moving images through conventional modems for videophones or small screen video conferencing. The image itself would have low resolution (about 176 by 144 pixels) and a low frame rate, on the order of 10 Hz.
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