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4.3. ITU-T H.261 Compression

The ITU-T recommendation H.261 specifies a video codec for any number of 64 kbps channels between 1 and 30 combined. There are other H. series standards linked to video conferencing, and they are all subsumed under the general recommendation H.320. Video conferencing systems built to these recommendations should be able to inter-operate. (and most do !) The H.221 and H.222 standards describe the frame and signalling structure in 64 kbps channels. The channel can be shared dynamically between video, voice and data.

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 Recommendations
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
A 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

4.4. AVI, CDI and Quicktime

All these standards are proprietary. Digital Video Interactive (DVI) has been developed by Intel for the PC. A CD-ROM disc can store 20 minutes of motion video. It consists of two algorithms. The first plays back at 10 frames per second, the second at 30 frames per second. The faster algorithm requires specialist compression facilities. Hardware support is needed for playback of DVI.

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.

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