ber of factors, including how many other termi- Although the speech in a multipoint audiovisual connection is nals are trying to do likewise at the same time; the transmission is treated in exactly the same way, the video signals have obviously to therefore essentially asynchronous with an unspecified trans- be treated differently. 57/34 Video transmission In a 'switched' audiovisual MCU, the video signals are voice CCITT (1985) Guidelines for placement of microphones and loud- switched and forwarded from the MCU to each location according speakers in telephone conference room, Supplement No.25, p. to the following rules: 335, Fascicle III.1, CCITT Red Book, Geneva. CCITT (1988a) Characteristics of visual telephone systems, Recom- 1. The picture of the person currently speaking is forwarded to all mendation H.100, Melbourne. the other locations. CCITT (1988b) Infrastructure for audiovisual services, Recommen- 2. The person currently speaking is sent the picture of the previous dation H.200, Melbourne. speaker. CCITT (1988c) 7kHz audio coding within 64kbit/s, Recommenda- tion G.722, Melbourne. Such a system has the advantages of relative simplicity, operates CCITT (1988d) System aspects for the use of the 7kHz audio codec automatically and, apart from some additional control features, uses within 64kbit/s, Recommendation G.725, Melbourne. the same terminals and transmission capacity as point to point CCITT (1988e) Codecs for videoconferencing using primary digital connections. The CCITT is drafting a new Recommendation (cur- group transmission, Recommendation H.120, Melbourne. rent document number AV.231) for a multipoint control unit for use CCITT (1990a) Video codec for audio-visual services at p X with H.261 codecs at rates from 64kbit/s to 2048kbit/s. The multi- 64kbit/s, Recommendation H.261, Geneva. point control data is transmitted in the message channel in the H.221 CCITT (1990b) Broadband aspects of ISDN, Recommendation framing structure. 1.121, Matsuyama. If all the other participants are required to be 