seen all of the time, CCITT (1990c) Narrowband visual telephone systems and terminal SO called 'continuous presence' multipoint operation, a more com- equipment, Recommendation H.320, Geneva. plex solution is required. One way of achieving this would be by CCITT (1990d) Frame structure for a 64 to 1920kbit/s channel in increasing the number of receive channels and decoders at each audiovisual teleservices, Recommendation H.221, Geneva. location, and either increasing the number of displays or accepting CCITT (1990e) System for establishing communication between the fact that all the incoming pictures will be displayed at smaller audiovisual terminals using digital channels up to 2Mbit/s, Rec- sizes on the existing display screen. A second solution is to have a ommendation H.242, Geneva. single receive channel and decoder but a more complex MCU which CCITT (1990f) Frame-synchronous control and indication signals could generate a composite picture of all the other incoming pic- for audiovisual systems, Recommendation H.230, Geneva. tures, with the picture of the current speaker being of higher resol- ution than the others and therefore allocated a greater proportion of Clarke, R.J. (1985) Transform Coding of Images, Academic Press, London. the channel capacity. Further research and development in this area is continuing. Clark, W.J. et. al. (1990) Multipoint audiovisual telecommunica- tions, Br. Telecom Tech. Jn., 8 (3). ETSI (1991) ISDN and other telecom. networks - Audiovisual services - Narrowband visual telephone system, European Tele- 57.6 References comms. Standards Inst. provisional standard ETS 300 145. Gharavi, H. and Tabatabai, A. (1988) Sub-band coding of mono- Bermingham, A. et. al. (1990) The Video Studio 2nd edn, Focal chrome and colour images, IEEE Trans. Circuits and Systems, Press. 35 (2) pp. 207 - 204. CCIR (1986) Interfaces for digital component video signals in Griffiths, J.M. (1990) ISDN Explained, Wiley, Chichester, UK. 525-line and 625-line television systems, Recommendation 656, Huffman, D. 