A class of lossless, reciprocal anti-sidetone networks for telephone sets

Anti-sidetone networks are used in telephone sets to interconnect the transmitter, receiver, and telephone line. Most passive networks used in modern telephone instruments belong to the class of lossless, reciprocal networks known as maximum output networks. This paper defines a larger class of lossless, reciprocal networks suitable for use as anti-sidetone networks in telephone sets and describes the properties of the networks belonging to this class. The class includes maximum output networks, such as those conventionally used for this application, as well as certain nonmaximum output networks. The latter, although they do not exhibit maximum power transfer characteristics at all network ports, have the same transmit and receive power transfer ratios as the maximum output networks. The network presently used in the Trimline® telephone set is an example of a nonmaximum output network belonging to this class. The use of such a network provides a convenient method of biasing the LEDs which are used to illuminate the dial in this telephone set. However, the power transfer characteristics between the transmitter, receiver, and telephone line ports of this network are the same as those of a conventional maximum output network.