hat is important, and the camera nals. sensitivity, auto iris lens performance and electronic automatic gain control must be such that satisfactory pictures are obtained over a range of illumination from 200 to 4000 lux. In addition, the colour 57.5.1.3 Audio performance temperature varies among different types of artificial lights and the colour temperature of daylight also varies with time of day and For people to be able to converse freely via a telecommunications prevailing weather conditions. Therefore the camera needs an auto- medium the transmission delay should be less than a few tens of matic or semi-automatic means of controlling the colour balance in milliseconds. However, with the video coding algorithm specified the television pictures SO that, for example, white objects always in CCITT Rec. H.261, the processing delay can vary from 200ms to appear white and even more importantly, facial flesh tones are well 500ms depending on the skill of the codec designer and the trans- reproduced. The range of colour temperature over which such mission bit rate available. To ensure acceptable lip synchronisation, colour balance must be maintained is typically from 3000 to 6000 the timing of the sound relative to the picture should be in the range degrees Kelvin. In the more stable environment of a custom de- -40ms to +20ms (CCIR, 1990f), and although the video delay can signed videoconference room, the need to automatically compen- vary with the amount of movement to be coded, a fixed compensat- sate for the range of lighting conditions described above is ing audio delay for a given bit rate, if carefully chosen, may be drastically reduced. acceptable. With compensating delay in each direction of trans- The signal/weighted noise ratio of the cameras should be at least mission, the ease of conducting an interactive conversation is adver- 45dB, measured in lighting conditions of at least 400 lux (and the sely affected, rather like telephony via one or two satellite hops, but appropriate aperture setting) and using 