Video Transmission over Telephone Cable Pairs by Pulse Code Modulation

An experimental seven digit pulse code modulation (PCM) system has been built for the transmission of monochrome and color television signals over seven pairs of 22-gauge exchange area telephone cable, installed in the laboratory. A beam coding tube converts the signal to seven parallel digits of a binary Gray code at a 10 mc rate. All circuits except the coding tube deflection amplifier are transistorized. The coded digits are sent over the cable in parallel formn with alternate groups converted to complements of the coded signal, thus substantially removing the low frequency component. This makes it possible to use simple repeaters without special compensation for duty factor variation. A repeater group is used after every 3000 foot section of cable. One ten megabit repeater, consisting of an amplifier and blocking oscillator, is required for each digit. All seven digit repeaters are retimed with a common timing wave. At the decoding terminal, transmitted complements are restored to Gray code before translation to natural binary. A binary weighted resistance network decoder converts the signals to a quantized reproduction of the video signal. Good quality composite color and monochrome pictures are obtained with six digits. Seven digits are believed to be necessary for broadcast quality with some margins. Waveform photographs illustrate the various functions, and photographs of decoded pictures are shown.