cancellation is to enhance the echo return loss to a value 40dB. vious pictures. The modulation flicker is interpreted as a difference For videophone and desk top videoconferencing terminals having signal and valuable transmission capacity will be used unnecessar- hands free operation, echo cancellation may not at present be a ily to encode the flicker. Means for removing or at least reducing the viable economical solution, and voice switched echo suppression, flicker exist (for example, use of an electronic shutter) but the although inferior, may be the preferred solution. Further study is expense and complexity involved may outweigh the advantage of required to confirm satisfactory voice switched suppression when having a universal camera and display standard. the audio signal has been delayed by a few hundred milliseconds. To provide the most natural conditions for a videophone conver- Where a telephone handset is used, the problem of room echo sation, good eye contact with the displayed image of the remote user disappears and the only requirement is that the side tone attenuation is essential and this implies that the ideal position for the camera is should be at least 45dB, a figure that is achievable with a well near the centre of the display screen. Systems using a semi-silvered designed handset. 57/32 Video transmission 57.5.2 Standards for audiovisual the service channel. Each frame has an 8 bit Frame Alignment communications Signal (FAS) and an 8 bit Bit Allocation Signal (BAS) occupying the first 16 bits of the service channel and corresponding to a As interest has grown in a whole range of audiovisual services framing overhead of 1.6kbit/s; all the remaining bits of the service supported by terminals with different capabilities, the CCITT antici- channel and the remainder of the frame can be used for a mix of pated the need for a framework of interrelated standards to ensure video, audio and ancillary data. Hence, in a 64kbit/s videophone satisfactory interworking at the highest level of compatibilty 