The main purpose of digital audiovisual coding technologies is to compress the original digital information into a much smaller number of bits, without an unacceptable degradation of the signal quality. Regarding video, the current coding paradigm is mostly based on temporal prediction between video frames to exploit the temporal redundancy, transform coding (e.g. using DCT, Discrete Cosine Transform) to exploit the spatial redundancy, quantisation of the transform coefficients to exploit irrelevancy (related to the human visual system limitations), and entropy coding to exploit the statistical redundancy of the created coded symbols. The quality of the decoded video is mainly controlled by the quantisation process and may be adapted to the service needs or to the necessary compression factors. Because it considers both the temporal (prediction) and frequency (DCT) domains, this type of coding architecture is well known as hybrid coding. Because hybrid coding is the solution adopted in most available video coding standards, notably the ITU-T H.26X and ISO/IEC MPEG-X families of standards, this coding paradigm is nowadays used in millions of video encoders and decoders. Since this coding solution exploits the correlation between and within the video frames at the encoder, it typically leads to rather complex encoders and much simpler decoders without much flexibility in terms of complexity budget allocation besides making the encoder less complex and thus less efficient. This approach fits especially well some applications, e.g. broadcasting, using the so-called down-link model since few encoders typically provide coded content for millions of decoders, which makes the decoder complexity the real critical issue. Moreover the temporal prediction loop used to compute the residuals, responsible to exploit the temporal redundancy, requires the decoder to perform the same loop in perfect synchronisation with the encoder. This means that when there are channel errors, the temporal prediction synchronisation is lost and errors propagate in time, strongly affecting the video quality until some Intra coding refreshment is performed. is performed. Until 2000, the hybrid coding approach has been adopted by the following video coding standards:
1. ITU-T H.261 (1990) – mostly for videotelephony and videoconference
2. MPEG-1 Video (1991) – initially mostly targeting CD-ROM video storage
3. MPEG-2 Video, also ITU-T H.262 (1993) - digital TV, DVD, etc.
4. ITU-T H.263 (1995) – mostly for video over PSTN and mobile networks
5. MPEG-4 Visual or MPEG-4 Part 2 (1998) – most types of applications depending on the profile
6. ITU-T H.264, also MPEG-4 Advanced Video Coding (2003) - most types of applications depending on the profile with increased compression efficiency
From this list, the MPEG-2 Video (also ITU-T H.262) and the H.264/AVC standards, jointly developed by MPEG (Moving Picture Experts Group) and ITU-T VCEG (Video Coding Experts Group), are the clear winners in terms of market deployment.