the unified noise weighting the presence of a picture may help to avoid the tendency for both network specified in Rec. 567 (CCIR, 1990e). For videoconferenc- parties to speak at the same time as is often experienced by tele- ing, the face camera should have a limiting resolution of at least 300 phone users when the connection is via satellite. lines per picture height in the centre of the sensor area; for video- Virtually all videoconferencing terminals use an 'open' audio phone applications, this specification can be relaxed but graphics system in which monophonic sound is transmitted and received on cameras need to be of higher resolution. separate channels (4 wire operation). The positioning and relation- Another important aspect of the camera specification is that the ships between microphones and loudspeakers should be based on field rate of the camera should be the same as the local mains the guidelines given in CCITT Supplement No. 25 (CCITT, 1985). frequency; when cameras are operated in artificial lighting, particu- The echo return loss between loudspeakers and microphones should larly from fluorescent tubes, the video signal is modulated by the be greater than 6dB over the frequency range 0.1kHz to 8kHz to illumination frequency (twice mains frequency). If, for example, a preserve stability. However, even in well designed studios with 60 field/s camera is operated in 50Hz lighting, there is 20Hz modu- suitable acoustic treatment, it is difficult to avoid completely the lation flicker running through the displayed picture and 20Hz is possibility of the received sound being reflected into the micro- close to the most visible temporal input frequency for the human phones and being re-transmitted as a delayed echo (the total delay eye-brain system. is room delay plus the processing compensation delay mentioned Another disadvantage of such a system is that the CCITT Rec. above) and the use of echo cancellation is advisable. The effect of H.261 algorithm encodes differences between the current and pre- echo 