An Improved Three-Channel Carrier Telephone System

H E R E are now in service in the Bell System approximately 750,000 miles of telephone circuit which are furnished by carrier systems. Of this total, almost 90 per cent is provided by some 600 Type C systems, ranging from about 75 miles to over 2000 miles in length. Basically designed to add three carrier channels to the normal voice channel on open-wire lines, the Type C system has also been used in special cases to provide additional circuits over deep sea cables of moderate lengths. The system was first described in this Journal in the July 1928 issue.1 Improved designs and the application of new circuit elements have recently permitted a very extensive revision of the terminal and repeater equipment which results not only in striking reductions in size and cost as compared with the older equipment, but also gives a considerable improvement in transmission performance. A new type of automatic regulating equipment has been provided for both the terminal and repeater. The improved system employs heater type pentode tubes, copperoxide modulators and demodulators and makes use of the negative feedback type of amplifier at both terminal and repeater points. The terminal band filters are newly designed to give improved transmission frequency characteristics on all channels. Each channel is arranged to terminate on a four-wire basis in the same manner as the Type K system for cables.2 An outstanding feature of the modified design is the large saving in space in comparison with the previous equipment.