RFC 2190 (rfc2190) - Page 2 of 12

RTP Payload Format for H

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