A Class of Lossless, Reciprocal Anti-Sidetone Networks for Telephone Sets

In a classic paper,1 Campbell and Foster defined a class of networks suitable for use as anti-sidetone networks in telephone sets. Campbell and Foster called these networks maximum output networks. Such networks have been used as anti-sidetone networks in telephone sets for many years.2 1 As we show in this paper, however, the properties which make maximum output networks suitable for this application are properties of certain nonmaximum output networks as well. Therefore, maximum output networks are but a subset of a larger class of anti-sidetone networks. 1483 The network presently used in the Trimline® telephone set is an example of a nonmaximum output network belonging to this larger class. In this telephone set, LEDS (light-emitting diodes) illuminate the dial. The LEDS are powered from the telephone line through an additional winding on the network transformer. This eliminates the need for ac isolation of the LED dc biasing circuit, as would have been required if a conventional maximum output network had been used. Instead, the dc biasing circuit, being one of the windings on the network transformer, is an integral part of the network itself. If a conventional maximum output network had been used in this application, an isolated dc biasing circuit for the LEDS would have been required. The impedance of this circuit would have had to be high enough to prevent its loading the network. Although this could have been accomplished using an inductor, an inductor large enough to be effective at voice frequencies would have been undesirable in terms of both cost and physical size.