Permanent point-to-point links for relaying television

Abstract

In the point-to-point relaying of 405-line television signals, the objective is to provide a transmission system having a video-frequency bandwidth of 3 Mc/s over which the gain and delay are substantially constant, and satisfying certain requirements with respect to noise, linearity and gain-stability; in addition, a high degree of reliability is essential. Experience up to the present time in the United Kingdom has shown that excellent results can be obtained either by cable or by radio transmission, and the choice between the two systems must therefore be largely a question of economics.Two coaxial cables are already in operation; one is between London and Birmingham and uses 1-in coaxial pairs transmitting frequencies in the band 3-7 Mc/s with a carrier of 6-12 Mc/s, whilst the other is between Birmingham and Manchester and uses 1-in coaxial pairs transmitting frequencies between 0-5 and 4 Mc/s with a carrier of 1 Mc/s, asymmetrical-sideband methods being used in both cases. A radio link between London and Birmingham, operating at about 900 Mc/s, has also been in service for an extended period, and a 4000-Mc/s microwave link has been installed between Manchester and Edinburgh, frequency-modulation being used in both systems.The cost of television transmission by cable depends largely on how the provision of a system fits in with the development of the trunktelephone network. Where a television-relay system and a large number of new telephone channels are required over the same route, composite cables can be laid to carry both services, thereby spreading the cost of the cable-sheath, ductwork, repeater buildings and local services between the two systems, with considerable economy. On routes where there is no requirement for telephony, however, a radio link is generally the cheaper system. When radio links for the simultaneous transmission of both television and multi-channel telephony have been developed, the present economic advantages of cable links on combined television and telephone routes may be nullified.