Video transmission systems are characterised by relatively high bandwidth and processing power requirements, due to the large amounts of information involved. For example, if digitised TV-quality video was transmitted `as is', it would require a raw throughput of hundreds of Mbps.2.1 Although such scales are currently feasible in modern communication, storage and processing systems, they are completely uneconomical. Therefore, the need to compact the video information and keep the required bit rates at a reasonable level arises.
The main principle applied to video coding is that the limitations of the human visual system should be exploited, by throwing effectively away the information that cannot be perceived by the average viewer. This principle, which exploits the inherent subjective redundancy of video sequences, is applied universally, from the video capturing to the displaying systems, and has been the basis for considerable reductions to the bandwidth requirements of analogue broadcast TV systems, exploiting the limited human sensitivity to the spatial, temporal and colour resolution of the transmitted video sequences.
Furthermore, video information exhibits high correlation in the temporal and spatial direction. For the former, that is because when a moving scene is segmented into a number of still frames, they tend to be very similar to each other, over relatively long time intervals. The latter stands for regions of each of these still images that have similar brightness and colour intensities. Digital video can be further compressed by using numerical redundancy elimination techniques such as entropy and arithmetic coding.
Following the introductory paragraph on representation of video information, some techniques of video coding and compression are briefly described. In the literature, these are usually categorised to still and moving image coding techniques. However, an integrated presentation according to the underlying technology used, was considered more appropriate for this document. The applications of some of these techniques in standards are concluding the chapter.