ISO/IEC 11172-2 specifies a video codec which was originally designed for the application domain of video for CD storage. The intended picture resolution is CIF or SIF.
A number of requirements apply in the context of storage and replay of stored data, which mainly are related to random access:
- The video sequence must be replayable forward and backward;
- Fast forward/reverse modes have to be supported;
- Editing (e.g. extracting or replacement of frames) must be possible
2 Technical Solution
The basic principle of MPEG-1 Video is hybrid coding, a combination of block-wise motion-compensated prediction and scalar-quantized DCT-based coding of the residual. The same transform is applied when intraframe mode is selected for a whole picture or a macroblock. A number of tools and mechanisms were defined aiming in particular at good reconstruction quality for video sequences of general nature and more complex content, and to fulfill the requirements of random access:
- The basic access entity over the sequence is the Group of Pictures (GOP). Three picture (frame) types are defined, which are intraframe-encoded (type I) as well as the P (unidirectional) and B (bidirectional) motion compensated predictive types. Within a GOP, at least one picture must be an I picture which must be positioned such that the remaining pictures of the GOP can uniquely be decoded.
- Flexible prediction mode switching (forward, backward etc.) is enabled on a basis of macroblocks of size 16x16, with selectable modes depending on the picture type.
- Accuracy of motion compensation is half pixel.
- Weighting of DCT coefficients can be applied in quantization for frequency-specific perceptual customization of the quantization fidelity. A default quantization matrix is provided, but it is possible to define alternative quantization tables (e.g. adapted to properties of a sequence), which are then transmitted as side information in the header.
- DCT blocks are scanned in a zigzag order, by which coefficients quantized into the zero value are detected and their positions are converted into a run-length information. The remaining coefficients are compressed by a variable-length code which expresses both the run-length and the quantizer-level values.
- Picture sizes, color representations, pixel aspect ratio, picture rates and similar parameters are conveyed in the header, which makes MPEG-1 quite flexible for usage with different types of video sources.
- A decoder buffer and timing model is defined (called the Video Buffer Verifier, VBV), which allows to design encoders supporting normative decoder timing behavior. Necessary parameters such as expected buffer size are included in the video bitstream.
The bitstream defined in the MPEG-1 video part is structured into a number of syntactic hierarchy layers, which are video sequence layer, group of pictures layer, picture layer, slice layer, macroblock layer and block layer. The syntax of each layer contains the related characteristic information (e.g. length and structure of a GOP, specific prediction or intra modes invoked etc.). For the slice layer and above, start codes are defined which allow re-synchronization.
Subsequent to the standard text which was published in 1993, the following corrigenda are integral part of the MPEG-1 Video specification:
- ISO/IEC 11172-2:1993/Cor.1:1996
- ISO/IEC 11172-2:1993/Cor.2:1999
- ISO/IEC 11172-2:1993/Cor.3:2003
- ISO/IEC 11172-2:1993/Cor.4:200X (in preparation)
3 Application areas
MPEG-1 Video is a format that is now also widely used for video storage and replay on PCs, video file transfer over the Internet, etc.