Trunk circuit with loop length gain equalization

Abstract

A key telephone system comprised of a key service unit connected to a plurality of local sets via four-wire conductors for providing full duplex signal communication between the key service unit and sets. Signals are carried by a first pair of each of the conductors for unidirectional transmission of signals from each set to the key service unit, and the remaining pair of conductors carries signals transmitted by the key service unit for reception by one or more of the sets. Four-to-two wire signal conversion and line impedance balancing is effected by one or more trunk circuits of the key service unit in the event one or more of the local sets are connected via the key service unit for communication between the sets and the outside telephone lines. As a result of separate undirectional communication paths being provided between individual ones of the subscriber sets and the key service unit, via the four-wire conductors, the impedance balancing function of the trunk circuits is independent of the length and quality of the four-wire conductors. Each of the trunk circuits preferably incorporates an electronic telephone circuit for performing the four-to-two wire signal conversion as well as AC and DC termination and loop length gain equalization for signals carried by the outside telephone lines. The key service unit and subscriber sets of the telephone system are inexpensive, compact and highly efficient as a result of minimizing the number of four-to-two wire conversions therein.