The Picturephone System: A Digital Transmission System for TD-2 Radio

The TD-2 Radio Relay System is the major trunk carrier system in the Bell transcontinental network, interconnecting every major metropolitan center in the United States. 1 Therefore, as digital transmission grows, it is natural to consider how TD-2 might be used to distribute these signals. A terminal that applies the digital bit stream to TD-2 has been designed to transmit at a rate of 20.2 megabits per second (Mb/s) using a four-level, 10.1-megabaud pulse format to modulate standard TD-2 terminal equipment. The input is arranged to accept up to three 6.312-Mb/s bipolar streams from such sources as Picturephone coders and data sets. Design features include the use of pulse stuffing for multiplex synchronization, dedicated pulses for timing recovery, a transversal filter for optimizing the received eye pattern and the use of self-synchronizing scramblers for testing.2,3 The signal is regenerated at baseband about every 400 miles to limit the accumulation of unequalizable pulse distortion. A number of digital terminals have been constructed and tested in conjunction with the New York-Pittsburgh Picturephone product trial. Test results on this 450-mile trial route, using one regenerator station, have indicated that performance of better than one error in 1010 bits, 481