Telephone headset interface circuit

• US 5,544,243 A
• Filed: 08/10/1994
• Issued: 08/06/1996
• Est. Priority Date: 05/17/1993
• Status: Expired due to Term

• Current Assignee: VXI Corporation (GN Store Nord A/S)
• Original Assignee: VXI Corporation (GN Store Nord A/S)
• Inventors: Papadopoulos, Costas
• Primary Examiner(s): Hofsass, Jeffery
• Assistant Examiner(s): Kumar, Devendra
• Application Number:
  US08/288,941
• Time in Patent Office: 727 Days
• Field of Search: 379/413, 379/348, 379/156, 379/395, 379/93, 379/98, 379/387, 379/165, 379/399, 379/412
• US Class Current: 379/413
• CPC Class Codes: H04M 1/60 including speech amplifiers
• CPC Class Codes: H04M 19/08 with current supply sources at the substations generating ringing current H04M19/04
• CPC Class Codes: H04M 3/51 Centralised call answering arrangements requiring operator intervention , e.g. call or contact centers for telemarketing

Abstract

A telephone headset interface circuit is coupled to a telephone line to extract power for use by a headset amplifier and is particularly adapted for use with telephone handsets having dynamic or electret microphones. The telephone line connects a telephone to a service system, such as a Central Office, a PBX system, or a Key system. The interface circuit includes a power extraction circuit having a plurality of circuit portions, each one coupled to a corresponding one of the telephone line wires for providing a predetermined output voltage, so that substantially identical electrical impedance characteristics are introduced to each such wire. A power compensation circuit is coupled to both a first and second pair of wires of the system for controlling the power extraction circuit to provide the predetermined output voltage in response to the presence of AC voltage on the second pair of wires, such that the power compensation circuit controls the power extraction circuit to maintain substantially the same electrical impedance characteristics to the first pair of wires regardless of the presence or absence of AC voltage on the second pair of wires. An isolation circuit is coupled to the outputs of the power extraction circuit portions for preventing shunt loading of the first and second pairs of wires and for providing electrical impedance characteristics to the first and second pairs of wires representative of an open circuit.

Claims

1. A telephone interface for use with a telephone, said telephone having a first connector for connection to a telephone line and a second connector for connection to a telephone voice accessory, wherein said telephone line includes at least a first and second pair of wires, said telephone interface comprising:

a power extraction circuit coupled to said first pair of wires for providing a predetermined output voltage in response to a specified DC voltage provided on said first pair of wires and in the absence of AC voltage on said second pair of wires,

a power compensation circuit coupled to both said first and second pairs of wires for controlling said power extraction circuit to provide said predetermined output voltage in response to the presence of AC voltage on said second pair of wires, wherein said power compensation circuit controls said power extraction circuit to maintain substantially the same electrical impedance characteristics to said first pair of wires regardless of the presence or absence of AC voltage on said second pair of wires; and

an amplifier coupled to said second connector and to said power extraction circuit, said amplifier being powered by said output voltage.

2. The telephone interface of claim 1, wherein said power extraction circuit comprises a plurality of power extraction circuit portions, each one adapted for coupling to a corresponding wire of said first pair of wires.

3. The telephone interface of claim 2, wherein each one of said power extraction circuit portions comprises:

a resistor coupled in series with said corresponding one of said wires in said first pair, said resistor having a first terminal coupled to said corresponding wire of said first pair and a second terminal coupled to said first connector; and

a rectifier operable in response to a predetermined voltage drop thereacross, said rectifier providing a rectified current.

4. The telephone interface of claim 3, wherein said rectifier comprises a complementary pair of transistors, each one having a first electrode coupled to said first resistor terminal, a second electrode at which said rectified current is provided, and a control electrode coupled to said second resistor terminal, wherein a first one of said complementary pair of transistors conducts when said predetermined voltage drop is of a first polarity and said second one of said complementary pair of transistors conducts when said predetermined voltage drop is of a second, opposite polarity.

5. The telephone interface of claim 4, wherein said power compensation circuit comprises a rectifier circuit coupled between said second pair of wires and said first connector for providing a DC shunt current across said first pair of wires so that each of said power extraction circuit portions conducts said rectified current.

6. The telephone interface of claim 5, wherein said rectifier circuit comprises a complementary pair of transistors, each one having a first electrode coupled to a first wire of said first pair through a first resistor and a first wire of said second pair, a second electrode coupled to a second wire of said first pair through a second resistor, and a control electrode coupled to a second wire of said second pair for receiving said AC voltage, each of said transistors further comprises a capacitor coupled across said first and second electrodes.

7. The telephone interface of claim 6, wherein in response to the presence of said AC voltage of a first polarity on said second wire of said second pair and said DC voltage of the same polarity on said second wire of said first pair, a first one of said complementary pair of transistors conducts and forces a discharge of the associated capacitor, thus said DC shunt current is allowed to flow across said first pair.

8. The telephone interface of claim 7, wherein in response to the presence of said AC voltage of a first polarity on said second wire of said second pair and said DC voltage of the same polarity on said first wire of said first pair, the second one of said complementary pair of transistors conducts and forces a discharge of the associated capacitor, thus said DC shunt current is allowed to flow across said first pair.

9. A telephone interface for use with a telephone, said telephone having a first connector for connection to a telephone line and a second connector for connection to a telephone voice accessory, wherein said telephone line includes at least a first and second pair of wires, said telephone being powered by a DC voltage of one polarity provided on said first pair and the DC voltage of the opposite polarity on said second pair, said telephone interface comprising:

a power extraction circuit coupled to said first and second pairs of wires and having first and second output terminal sets for providing portions of a predetermined output voltage of one polarity on said first output terminal set and of the opposite polarity on said second output terminal set;

an isolation circuit coupled to both said first and second output terminal sets for preventing shunt loading of said first and second pairs of wires and for providing electrical impedance characteristics to said first and second pairs of wires representative of an open circuit, said isolation circuit further operable for combining said portions into said output voltage; and

an amplifier coupled to said second connector and to said isolation circuit, said amplifier being powered by said output voltage.

10. The telephone interface of claim 9, wherein said power extraction circuit comprises a plurality of power extraction circuit portions, each one adapted for coupling to a corresponding wire of said first and second pair of wires, each of said portions having a first output associated with said first output terminal set and a second output associated with said second output terminal set.

11. The telephone interface of claim 10, wherein said isolation circuit comprises first and second center tapped to center tapped transformers.

12. The telephone interface of claim 11, wherein said first transformer comprises a first winding coupled at opposite ends to said first outputs of the power extraction circuit portions coupled to said first pair of wires, and a second winding coupled at opposite ends to said first outputs of the power extraction circuit portions coupled to said second pair of wires, each winding having a center tap coupled to said first output terminal set.

13. The telephone interface of claim 12, wherein said second transformer comprises a first winding coupled at opposite ends to said second outputs of the power extraction circuit portions coupled to said first pair of wires, and a second winding coupled at opposite ends to said second outputs of the power extraction circuit portions coupled to said second pair of wires, each winding having a center tap coupled to said second output terminal set.