TABLE OF CONTENTS In order to achieve better compression performance and channel error robustness levels than can be achieved by H.261, Version 1 of the international standard ITU-T H.263, entitled Video coding for low bitrate communications [6], provides better picture quality at low bitrates with little additional complexity. It also includes four optional modes aimed at improving compression performance. The H.263 standard has been adopted in several videophone-terminal standards, notably ITU-T H.324 (PSTN), H.320 (ISDN), and H.310 (B-ISDN). The H.263 Version 2 standard, also known as H.263+ in the standards community, was officially approved as a standard in January 1998 [7]. It is an extension of H.263 providing 12 new negotiable modes and additional features. These modes and features improve compression performance, allow the use of scalable bitstreams, enhance performance over packet-switched networks, support custom picture size and clock frequency, and provide supplemental display and external usage capabilities.
Figure 5.15 shows a block diagram of an H.263 baseline encoder, in which motion-compensated prediction is used first, to reduce temporal redundancies. Discrete-cosine-transform (DCT)-based algorithms are then used for encoding the motion-compensated prediction difference frames. The quantized DCT coefficients, motion vectors, and side information are entropy coded using variable-length codes (VLCs). The two switches represent intra or inter mode selection, which is not specified in the standard; this can be made at the macroblock level. The performance of the motion-estimation process, usually measured in terms of the associated SAD values, can be used to...
