Patent ID: 6744889
Filing Date: 2004-06-01
Classification: H04M

Abstract:
A subscriber circuit for a public telephone set, comprising:a loop current supply circuit for supplying loop current between two subscriber lines; a line voltage detection circuit for detecting a voltage variation between the two subscriber lines; a high-pass filter for principally removing a high dc voltage from an output signal of said line voltage detection circuit; a channel filter for setting a voice signal band; variation signal generation means for generating a variation signal corresponding to a voltage variation applied between the two subscriber lines upon and caused by acceptance of a coin; and subtraction means for subtracting the variation signal from a signal of the voltage variation between the two subscriber lines detected by said line voltage detection circuit, wherein said variation signal generation means includes a voltage holding circuit for holding a voltage detected by said line voltage detection circuit prior to a reversal of the polarity of loop current, a reversal circuit for reversing the polarity of the voltage held by said voltage holding circuit, and a low-pass filter for buffering the voltage of the reversed polarity from said reversal reversal circuit with a characteristic same as that of a low-pass filter which buffers the polarity reversal in said loop current supply circuit.