There are two formats for video telephony, common intermediate format (CIF) and quarter common intermediate format (QCIF). All codecs must support QCIF. Compression is achieved by several methods. Inter-picture prediction eliminates picture information that has not changed between frames. A discrete cosine transform is also applied to individual frames. Audio is coded according to G.722 ITU-T recommendations and combined in the same channel. Chips are available to implement both H.261 and MPEG-1 in hardware.
The H.221 recommendation is a secure synchronous procedure which allows the control of several 64 kbps channels of audio and visual information and the setting up of multi-point calls. See the table below.
Audio Visual Services covered by ITU-T RecommendationsA number of video conferencing products based on the H.320 standard are available, largely for use on ISDN. A selection is described in section 11.0
INITIAL SET FUTURE SET
Narrowband videophone (1 and 2 x64kbps) Video Mail Broadband video phone Videotex with picures and sound Narrowband video conferencing Video Retrieval (m x384kbpsand n x 64 kbps) Broadband video conferencing High resoution image retrieval Audiographic Teleconferencing Video distribution services Telephony Telesurveilance
Microsoft have a standard called Audio Visual Interleaved which supports small video windows at up to 15 frames per second. Apple also have a software based system called Quicktime providing 160 x 120 pixel playback at 15 frames per second.
Phillips have developed a player for full motion video discs which can manage a data rate of 1.2 Mbps of video. MPEG based techniques are used.
Graphics Multimedia Virtual Environments Visualisation Contents