US 4,142,075 A · 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.

Claims21

Dependent claims

An interface circuit for transmitting telephone signals between an electronic switching system and a two wire cable connected to a telephone set, said telephone set having a telephone set resistance coupled across the two wires of said two wire cable, said two wire cable having a cable resistance, said telephone set producing a microphone voltage signal transmitted along said two wire cable, said electronic switching system producing an audio frequency signal, said interface circuit comprising in combination:

(a) means for producing a substantially constant DC reference current;
(b) current source means connected to said two wire cable for producing a substantially constant DC current component in said two wire cable in response to said DC reference current and an AC current component in said two wire cable in response to said audio frequency signal, said AC current component producing a corresponding AC voltage component between the two wires of said two wire cable;
(c) amplifying means coupled to said two wire cable for amplifying said microphone voltage signal, said amplifying means having a sufficiently high input impedance to prevent substantial reduction of the said microphone voltage signal due to voltage division of said microphone voltage signal across said input impedance, said cable resistance, and said telephone set resistance;
and
(d) there being no termination resistance substantially reducing said microphone voltage signal and said AC voltage component due to voltage division of said microphone voltage signal and said AC voltage component across said telephone set resistance, said cable resistance, and said termination resistance;
whereby, said AC voltage component and said microphone voltage signal are substantially independent of the distance between the points of said two wire cable to which said amplifying means and said telephone set are respectively coupled.

3 more elements

An interface circuit for coupling a two wire cable connected to a telephone set to an electronic switching system, the interface circuit comprising in combination:

(a) means for producing a first AC signal on the two wire cable proportional to the length of the two wire cable in response to a first signal transmitted to the interface circuit by the electronic switching system;
(b) means for producing a DC voltage on the two wire cable proportional to the length of the two wire cable;
(c) means for receiving a microphone signal from the telephone set on the two wire cable;
(d) means responsive to the first signal coupled to the two wire cable for producing an AC cancellation signal proportional to the DC voltage, the cancellation signal being out of phase with the first AC signal;
(e) means responsive to said cancellation means for summing the cancellation signal, the first AC signal, and the microphone signal, thereby effecting cancellation of the first AC signal from the two wire cable and producing a signal which includes the microphone signal, but is substantially free of any components due to the first signal;
and
(f) means responsive to said summing means for transmitting the microphone signal to the electronic switching system, wherein the echo return loss of the first signal is maximized.

4 more elements

Dependencies are inferred from the claim text.

Interface circuit and method for telephone extension lines

Life of this patent

Interface circuit and method for telephone extension lines

Life of this patent

Abstract

Claims21