ts in their compression should be familiar with the earlier section on digital video coding factors are likely to be small. The attentions of researchers are thus before proceeding further. being directed in other directions such as fractal based coding (Jacquin, 1990) and wavelet coding (Rioul, 1991). 57.5.1.1 Display performance An approach which shows promise for some applications such as videophone at very low bit rates is model based or knowledge based The optimum size of the display screen in a videophone terminal is coding (Pearson, 1989). From a limited number of still images and a function of a number of factors including the picture scanning knowledge of the generic shape of a human head and shoulders, the standard, picture resolution (signal bandwidth), viewing distance encoder and decoder construct a model of the subject. The encoder and, if a data compression scheme is implemented, the visibility of then transmits parameters to the receiver to move the model in some coding artefacts. The 625 line television scanning standard in Eu- way, e.g. to nod the head or open the mouth. An extension is to ally rope (525 line in the USA and Japan) was chosen SO that the line this to more conventional methods which are brought into action structure would be imperceptible to the majority of people when 57/30 Video transmission viewed at a distance greater than 6 times the picture height (a that they have a much more limited angle of view than CRTs, being viewing ratio of 6H). typically +20 deg. vertically and +40 deg. horizontally. For videoconferencing terminals in a meeting room equipped with Integrated videophone terminals tend to use LCDs up to 15cm large (65cm diagonal) CRT displays, a viewing distance of at least diagonal, but a personal computer (PC) is an attractive alternative 2.5m is recommended. Videophone or desk top videoconferencing videophone terminal implementation, particularly where the poten- terminals are usually positioned from 70cm to 1.5m from a single tial user already has such a unit on hi