H.263 Video Coding

H.263 is another ITU-T video encoding standard. It was designed for low bitrate communication, less than 64 Kbits/s, however this limitation has now been removed, and it supports higher bit rates. The coding algorithm of H.263 is similar to that used by H.261, however there are some improvements and changes to improve performance and error recovery. The differences between the H.261 and H.263 coding algorithms are that H.263 uses half pixel precision for motion compensation rather than full pixel. H.263 can be configured for lower bit rates or alternatively for better error recovery. Like H.261, it has IPPPP...frames, however, some macroblocks of the P frames are encoded in Intra mode without motion prediction. This feature makes it very attractive to be used in real-time video transmission over the Internet, because the output stream is more resilient to packet loss. H.263 supports five resolutions. In addition to QCIF and CIF that were supported by H.261 there is SQCIF, 4CIF, and 16CIF. SQCIF is approximately half the resolution of QCIF. 4CIF and 16CIF are 4 and 16 times the resolution of CIF respectively.