rystal displays the 'split-screen' technique (CCITT, 1988a) illustrated in Figure (LCD) with the grey scale resolution required for television (White, 57.30. Effectively, the aspect ratio of the display has been changed 1988). At the time of writing, production LCDs for real time televi- to 16:3 which is close to the optimum value for 6 participants. sion applications are available in sizes up to 15cm diagonal (larger panels up to 35cm exist as development samples) and virtually all 57.5.1.2 Camera performance employ an active matrix of thin film transistors to control the transmittance of each cell. Colour is provided by having 3 cells per For both videophone and 'split-screen' videoconferencing termi- picture element and an overlay colour filter. A light source must be nals, the camera should have a fixed field of view of about 44 deg. provided behind the LCD, usually by means of a colour matched horizontally and 33 deg. vertically, which corresponds to lens focal cold cathode fluorescent tube. length of between 7.5mm and 8.5mm when used with an industry The main attraction of the LCD is that there is much more freedom standard 0.5 inch format CCD camera sensor (active area about to design an attractive, compact unit than would be the case with 6.4mm by 4.8mm). The width of the field of view will therefore be conventional cathode ray tube displays. One limitation of LCDs is about 54cm at a subject distance of 70cm and about 2m at a subject Coding and Source transmission Display 2 camera signals. One composite 2 conventional displays. Top and bottom 25% of picture comprising Blank lines introduced each picture discarded central 50% of as top and bottom 25% each source of each picture picture Figure 57.30 Split screen technique Videophony and videoconferencing 57/31 distance of 2.5m. One third inch CCD sensors, already used in mirror to achieve this apparent position do exist, although there is consumer video camcorders, are increasingly likely to be used for an inevitable consequential light loss to the camera a