s desk. In this case, where low user; for easy reach of controls which are integral with the terminal, bit rate coding is utilised, a reasonable implementation is to 'win- 70cm is the more appropriate. At this distance, the display size dow' the small videophone picture into one of the top corners of the should be about 20cm diagonal to preserve a viewing ratio of 6H. PC screen. For wideband videophone terminals, the whole PC In practice, designers of wideband terminals may opt for a larger screen can be used for the displayed picture if required. display (25cm to 35cm diagonal), preferring the greater impact of For single viewer use, the normal 4:3 aspect ratio is near optimum the bigger picture despite some visibility of the line structure. for head and shoulders pictures. Although there have been ex- Equally, where low bit rate coding algorithms are used, usually perimental videophone terminals in the past with aspect ratios of 1 accompanied by a deliberate reduction of the available picture and less, to better match the shape of a human face, these were less resolution, designers may prefer a smaller size (10cm to 15cm) to than satisfactory because the natural sideways movement of a seated mask the loss of picture quality incurred. Larger (30cm to 35cm user caused difficulty in remaining in the field of view of the diagonal) displays are normally used when positioned at the back of camera. the desk, some 1.5m from the user. For videoconferences involving groups of people at each location, The brightness of the displays should be adjustable up to at least a 4:3 aspect ratio display is non-optimum, resulting in the pictures 100cd/m², measured on a white rectangle occupying 50% of the of say 6 participants in a central band occupying less than 20% of screen area. the picture area. This situation can be significantly improved, at the The most important advance in display technology over the past expense of a second camera, display and some processing, by using few years has been the development of colour liquid c