Answer supervision circuit for paystation telephone with non-muted

Abstract

A call progress circuit for both a customer owned, coin operated telephone and a regulated telephone is provided that does not mute the paystation's transmitter, so that voice signals from the calling party may be heard by the called party, thereby preventing an answering party from hanging up prematurely. The call progress circuit employs a transmitter-sourced audio signal discriminator which monitors audio signals placed on the line from the called station, the calling paystation and a central office, and controllably removes or subtracts out those audio signals that originate at the paystation transmitter. The discriminator includes a pair of tunable bandpass filters respectively associated with a telephone line receiver and the paystation transmitter circuit. Coupled to these filters is an audio spectrum scanner which sequentially adjusts the sensitivity characteristics of the filters. Q-adjustment of each filter is effected by controlling the value of a time constant setting resistor that determines the transfer function of each of a set of filter stages, with tuning of the filter's center frequency being a function of the duty cycle of a variation in each stage's resistor value. The output of the line receiver filter is coupled through a transmitter-modifier representative of the effects of the circuit components and the line, so as to adjust transmitter-sourced audio signals to a level that will permit them to be correctly excised from the receiver filter's output.