Cable Carrier Telephone Terminals

A BOUT twenty years ago the first commercial carrier telephone * system was installed between Baltimore and Pittsburgh. Until recently, telephone circuits were obtained by carrier methods largely on open-wire lines. The notable exceptions were on short deep sea submarine cables. 1,2 Ten years ago, experiments were initiated which have now resulted in the design of a carrier system which can be applied with substantial economy to existing long distance toll cables on land. Its general features are described in another paper. 3 The present paper describes in detail the circuits and performance of the carrier terminals of this system. GENERAL FEATURES The carrier system for existing cables, designated type " K , " is designed to provide twelve telephone channels in the frequency range between 12 and 60 kilocycles, using one non-loaded 19-gauge paper insulated cable pair in each direction. Previous carrier systems employed for open-wire lines used vacuum tubes for the modulating or translating circuits and electrical filters composed of coil and condenser networks for separating the frequency bands associated with the respective channels. The terminals of the new type " K " system are simpler and yet provide improved performance by using copper oxide bridges for the modulation function and quartz crystal filters 4 for the separation of the individual channel bands. The quartz crystal filter is economical only in a comparatively highfrequency range, necessitating the use of high intermediate frequencies.