RFC 2190 (rfc2190) - Page 2 of 12
RTP Payload Format for H
Alternative Format: Original Text Document
RFC 2190 RTP Payload Format for H.263 Video Streams September 1997 4CIF: Picture source format with 704 x 576 pixels for luminance and 352 x 288 pixels for chrominance. 16CIF: Picture source format with 1408 x 1152 pixels for luminance and 704 x 576 pixels for chrominance. GOB: For H.263, a Group of Blocks (GOB) consists of k*16 lines, where k depends on the picture format (k=1 for QCIF, CIF and sub- QCIF; k=2 for 4CIF and k=4 for 16CIF). MB: A macroblock (MB) contains four blocks of luminance and the spatially corresponding two blocks of chrominance. Each block consists of 8x8 pixels. For example, there are eleven MBs in a GOB in QCIF format and twenty two MBs in a GOB in CIF format. 3. Design Issues for Packetizing H.263 Bitstreams H.263 is based on the ITU-T Recommendation H.261 [2] (referred to as H.261 in this document). Compared to H.261, H.263 employs similar techniques to reduce both temporal and spatial redundancy, but there are several major differences between the two algorithms that affect the design of packetization schemes significantly. This section summarizes those differences. 3.1 Optional Features of H.263 In addition to the basic source coding algorithms, H.263 supports four negotiable coding options to improve performance: Advanced Prediction, PB-frames, Syntax-based Arithmetic Coding, and Unrestricted Motion Vectors. They can be used in any combination. Advanced Prediction(AP): One or four motion vectors can be used for some macroblocks in a frame. This feature makes recovery from packet loss difficult, because more redundant information has to be preserved at the beginning of a packet when fragmenting at a macroblock boundary. PB-frames: Two frames (a P frame and a B frame) are coded into one bitstream with macroblocks from the two frames interleaved. From a packetization point of view, a MB from the P frame and a MB from the B frame must be treated together because each MB for the B frame is coded based on the corresponding MB for the P frame. A means must be provided to ensure proper rendering of two frames in the right order. Also, if part of this combined bitstream is lost, it will affect both frames, and possibly more. Zhu Standards Track
