The London-Birmingham television-cable system

The London-Birmingham television-cable system provides channels for relaying television signals between central points in the two cities, in both directions simultaneously, with extensions to the television transmitters at Alexandra Palace and Sutton Coldfield, a total of five links being involved. The vision-frequency signals modulate a carrier of 6.12 Mc/s at the sending-end, the lower-sideband, the carrier and part of the upper-sideband being transmitted over the cable. The transmission paths are provided by coaxial pairs approximately one inch in diameter, equipped with repeaters nominally at 12-mile intervals, the gain and delay being substantially constant from 3 to 7 Mc/s. In the event of repeater failure, change-over from working to standby repeaters is automatic, and power for some of the intermediate repeater stations is supplied over the coaxial pairs themselves. At the receiving end the signals are passed through an asymmetric-sideband-shaping filter before the vision-frequency signal is recovered by envelope detection. The overall vision-frequency characteristics of the London-to-Birmingham channel, in tandem with the Alexandra Palace and Sutton Coldfield extensions, show a total variation in gain of 0.8 db, and in phase delay of 0.04 microsec for frequencies up to 3 Mc/s. Above this frequency the cut-off is rapid. The performance in respect of linearity, signal/noise ratio and time response is adequate, and photographs are reproduced showing the effect on a television test-pattern of sending signals from London to Birmingham and back.