Fixed-rate video codecs for mobile radio systems

A comparative study of arbitrarily programmable, but xed-rate videophone codecs using quarter common intermediate format (QCIF) video sequences scanned at 10 frames/s is ooered. In contrast to existing and forthcoming standard schemes, such as the H.261, H.263 and MPEG2, MPEG4 codecs, which rely on bandwidth-eecient but error-sensitive variable-length coding techniques combined with a complex self-descriptive bitstream structure, the proposed codecs exhibit a more robust, regular bit-stream and a constant bitrate. Clearly, their philosophy is diierent from the above error-sensitive and variable-rate standard schemes, since these constant-rate codecs were designed to allow direct replacement of mobile radio voice codecs in second generation wireless systems, such as the Pan-European GSM, the American IS-54 and IS-95 as well as the Japanese systems, operating at 13, 8, 9.6 and 6.7 kbps, respectively. This philosophy can, however, be adopted to higher-rate systems, such as the Digital European Cordless Telecommunications (DECT) and the Universal Mobile Telecommunications System (UMTS). The Type 1 codecs proposed beneet from invoking sophisticated compression techniques in order to achieve best video quality at a given bitrate. In contrast, the Type 2 schemes introduced maximise the codecs' error resilience at the cost of slightly reduced video quality under error-free conditions. Gain-cost quantised, xed but arbitrarily programmable rate discrete cosine transformed (DCT) video codecs, vector-quantised (VQ) and quad-tree (QT) coded algorithms are proposed and their video quality, complexity, compression ratio and error resilience trade-oos are comparatively analysed under identical conditions. Finally, our candidate codecs are compared to the standard H261, H.263 and MPEG2 benchmark codecs. 1. OVERVIEW OF VIDEO CODECS In parallel to the rapid proliferation of multimedia communications services over xed networks there is a growing demand for the added convenience of tetherless roaming. The earliest embodiment of the wireless multimedia communicator is expected to take the shape of a wireless videophone, which is likely to evolve from the family of the existing second generation mobile phones, such as the Pan-European GSM scheme 1], the North-American IS-54 2] and IS-95 3] communicators or the Japanese system 4]. Hence there is an urgent need for appropriate video codecs, which-in contrast to existing standard co-decs using bandwidth-eecient, but error-sensitive variable-length coding techniques, such as the MPEG1 and MPEG2 arrangements 5, 6] or the ITU H261 and H263 codecs-can dispense with variable-length coding and hence produce an error resilient, very low rate stream that can be accommodated in the speech channel of the systems mentioned and …