Interface circuit and method for telephone extension lines

Abstract

A semiconductor interface circuit and method of operation for transmitting and receiving information between a telephone set and a four wire switching system. The interface circuit includes DC current sources which may be modulated by a voice signal from the switching system. The current sources provide both DC power and the voice signal to the telephone set through a two wire cable. A high impedance amplifier amplifies the signal on the cable, which cable signal includes a DC voltage, a microphone signal from the telephone set, and a component due to the voice signal from the switching system, all three being dependent on the cable length. Cable termination resistors are optional. The signal from the switching system and the DC component of the output signal of the amplifier are multiplied by an inverting multiplier circuit. The output signal of the multiplier circuit is proportional to cable length and is utilized to cancel the amplifier output signal component which is related to the voice signal from the switching system. Thus, echo return is eliminated or minimized regardless of cable length. Differential optical coupling circuits are utilized to couple the signal from the switching system to the cable and to couple the cable signal to the switching system relatively independently of the efficiency of the light emitting diodes of the differential optical coupling circuits. A DC ring insertion signal is coupled from the switching system to an output of the interface circuit via one of the differential optical coupling circuits. Status signals from the telephone set are conducted via another of the differential optical coupling circuits. The semiconductor interface circuit is housed in a package having two optically isolated cavities therein.