GB1173918A - Improvements in or relating to Video Telephone Systems - Google Patents

Abstract

1,173,918. Video-telephonesystem. BRITISH TELECOMMUNICATIONS RESEARCH Ltd. 4 Dec., 1967 [5 Dec., 1966], No. 54415/66. Heading H4F. In a video-telephone system arranged to establish simultaneous connection, i.e. video conference facilities, between more than two subscribers, e.g. four subscribers A, B, C and D, Fig. 4, each subscriber has a television camera (transmitter) and a television receiver connected to the exchange, and each receiver is arranged to display the pictures generated by the cameras of all four subscribers, e.g. each subscriber is pictured in a different corner of the display screen of each reciever. To achieve this display the speed of both the line scan and field scan is doubled for each camera as compared with the scanning speeds of the receivers, and the resulting waveforms are suitable blanked so that only a quarter of the total output from each camera is effective, the blanking being such that the four waveforms may be combined without interference, Fig. 3 (not shown). In addition to synchronizing the four cameras, correction must be applied for phase differences due to differences in the lengths of the transmission paths between the cameras and the exchange where the combination of the picture signals takes place. In the exchange, common timing pulses from a generator G are fed via a summing point S to the scanning circuits of the receivers A, B, C and D. Regenerated pulses from these receivers are used to operate the scanning circuits of their associated transmitters, the picture signals from which are fed to the summing point S. To correct the resulting phase errors due to the different transmission paths, a variable pulse delay arrangement is added between each receiver and transmitter, which is arranged to delay the scanning pulses so that the picture signals from the transmitters arrive at the summing point S in phase with the original pulses from generator G. To achieve this, each delay is equal to the time of one line scanning period minus the time delay due to the respective transmission path in both directions. The timing pulses may be generated at the transmitter of one of the subscribers, e.g. the originator of the conference. The delays may be arranged to be equal to a complete number of line-scanning periods in a permanently set-up system. A separate signal which provides the delay correction information may be transmitted from each subscriber to the exchange during the vertical flyback period. Two pairs of subscribers respectively associated with local exchanges may be connected by a long trunk route, Fig. 5 (not shown), in which case the signal time delay in the trunk route is arranged to be equal to a whole number of line-scanning periods.