MPEG-1 was designed for coding progressive video at a transmission rate of about 1.5 million bits per second. It was designed specifically for Video-CD and CD-i media. MPEG-1 audio layer-3 (MP3) has also evolved from early MPEG work.
MPEG-2 was designed for coding interlaced images at transmission rates above 4 million bits per second. MPEG-2 is used for digital TV broadcast and DVD. An MPEG-2 player can handle MPEG-1 data as well.
MPEG-1 and -2 define techniques for compressing digital video by factors varying from 25:1 to 50:1. The compression is achieved using five different compression techniques:
- The use of a frequency-based transform called Discrete Cosine Transform (DCT).
- Quantization, a technique for losing selective information (sometimes known as lossy compression) that can be acceptably lost from visual information.
- Huffman coding, a technique of lossless compression that uses code tables based on statistics about the encoded data.
- Motion compensated predictive coding, in which the differences in what has changed between an image and its preceding image are calculated and only the differences are encoded.
- Bi-directional prediction, in which some images are predicted from the pictures immediately preceding and following the image.
The first three techniques are also used in JPEG file compression.
A proposed MPEG-3 standard, intended for High Definition TV (HDTV), was merged with the MPEG-2 standard when it became apparent that the MPEG-2 standard met the HDTV requirements.
MPEG-4 is a much more ambitious standard and addresses speech and video synthesis, fractal geometry, computer visualization, and an artificial intelligence (AI) approach to reconstructing images. MPEG-4 addresses a standard way for authors to create and define the media objects in a multimedia presentation, how these can be synchronized and related to each other in transmission, and how users are to be able to interact with the media objects.
MPEG-21 provides a larger, architectural framework for the creation and delivery of multimedia. It defines seven key elements:
- Digital item declaration
- Digital item identification and declaration
- Content handling and usage
- Intellectual property management and protection
- Terminals and networks
- Content representation
- Event reporting
The details of various parts of the MPEG-21 framework are in various draft stages.