Echo cancellation in a full-duplex speakerphone

• US 5,323,458 A
• Filed: 10/25/1993
• Issued: 06/21/1994
• Est. Priority Date: 10/25/1993
• Status: Expired due to Fees

• Current Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Hillman, Garth D., Park, Sangil
• Primary Examiner(s): Dwyer, James L.
• Assistant Examiner(s): SAINT SURIN, JACQUES M
• Application Number:
  US08/140,927
• Time in Patent Office: 239 Days
• Field of Search: 379/390, 379/389, 379/391, 379/410, 379/409, 381/83
• US Class Current: 379/406.08
• CPC Class Codes: H04M 9/082 using echo cancellers echo cancellers per se H04B3/23

Abstract

A direct-path echo cancellation circuit includes a loudspeaker (12), first and second microphones (14, 16), and an in-phase signal cancellation circuit (21, 43). The first and second microphones (14, 16) are positioned adjacent the loudspeaker (12) in order to receive in-phase direct-path echo components (31, 32) of an output signal from the loudspeaker (12). The in-phase signal cancellation circuit (21 or 43) combines the in-phase direct-path echo components of the output signal in order to cancel the direct-path echo components. Direct-path echo cancellation helps to reduce howling and/or oscillation in a full-duplex speakerphone system. Also, direct-path echo cancellation provides more dynamic range to an input signal which is provided to a speakerphone signal processing system (26).

Claims

1. In a full-duplex speakerphone system, an apparatus for direct-path echo cancellation, comprising:

a loudspeaker for providing an output signal;

first and second microphones located adjacent to the loudspeaker, the second microphone positioned on a side of the loudspeaker opposite the first microphone, the first and second microphones positioned for receiving in-phase direct path echo components of the output signal; and

an in-phase signal cancellation circuit having input terminals coupled to the first and second microphones, for combining with opposite polarity the in-phase direct-path echo components of the output signal in order to cancel the in-phase direct-path echo components.

2. The apparatus of claim 1, wherein the in-phase signal cancellation circuit combines the in-phase direct path echo components in an analog-domain.

3. The apparatus of claim 1, wherein the in-phase signal cancellation circuit comprises:

a summation circuit having a first input terminal coupled to the first microphone, a second input terminal coupled to the second microphone, and an output terminal; and

an analog-to-digital converter having an input terminal coupled to the output terminal of the summation circuit, and an output terminal.

4. The apparatus of claim 3, wherein the summation circuit comprises:

a first resistive element having a first terminal coupled to the first input terminal, and a second terminal;

a second resistive element having a first terminal coupled to the second input terminal, and a second terminal;

an operational amplifier having an inverting input terminal coupled to the second terminal of the first resistive element, a noninverting input terminal coupled to the second terminal of the second resistive element, and an output terminal;

a third resistive element having a first terminal coupled to the noninverting input terminal of the operational amplifier, and a second terminal coupled to a power supply voltage terminal; and

a fourth resistive element having a first terminal coupled to the inverting input terminal of the operational amplifier, and a second terminal coupled to the output terminal of the operational amplifier.

5. The apparatus of claim 4, wherein the power supply voltage terminal is for receiving a ground potential.

6. The apparatus of claim 4, further comprising:

a first capacitive element coupled between the first input terminal and the first resistive element; and

a second capacitive element coupled between the second input terminal and the second resistive element.

7. The apparatus of claim 4, wherein a resistance value of the first resistive element is substantially equal to a resistance value of the second resistive element, and a resistance value of the third resistive element is substantially equal to a resistance value of the fourth resistive element.

8. The apparatus of claim 1, wherein the in-phase signal cancellation circuit combines the in-phase direct path echo components in a digital-domain.

9. The apparatus of claim 1, wherein the in-phase signal cancellation circuit comprises:

a first analog-to-digital converter having an input terminal coupled to the first microphone, and an output terminal;

a second analog-to-digital converter having an input terminal coupled to the second microphone, and an output terminal; and

a summation circuit having a first input terminal coupled to the output terminal of the first analog-to-digital converter, and a second input terminal coupled to the output terminal of the second analog-to-digital converter, and an output terminal.

10. An apparatus for direct-path echo cancellation in a full-duplex speakerphone, comprising:

a loudspeaker for providing an output signal;

a first microphone positioned adjacent to the loudspeaker and having an output terminal, the microphone for receiving a first input signal;

a first analog-to-digital converter having an input terminal coupled to the output terminal of the first microphone, and an output terminal;

a second microphone positioned adjacent to the loudspeaker, on a side opposite the first microphone and having an output terminal, the second microphone for receiving a second input signal;

a second analog-to-digital converter having an input terminal coupled to the output terminal of the second microphone, and an output terminal; and

a summation circuit having a first input terminal coupled to the output terminal of the first analog-to-digital converter, and a second input terminal coupled to the output terminal of the second analog-to-digital converter, the summation circuit for combining in-phase direct path echo components of the first and second input signals in order to cancel the in-phase direct path echo components.

11. A circuit for direct-path echo cancellation in a full-duplex speakerphone, comprising:

a loudspeaker for providing an output signal;

a first microphone positioned adjacent to the loudspeaker and having an output terminal, the first microphone for receiving a first input signal;

a second microphone positioned adjacent to the loudspeaker, on a side opposite the first microphone and having an output terminal, the second microphone for receiving a second input signal;

a summation circuit having a first input terminal coupled to the output terminal of the first microphone, and a second input terminal coupled to the output terminal of the second microphone, the summation circuit for combining in-phase direct-path echo components of the first and second input signals in order to cancel the in-phase direct path echo components, the summation circuit providing an intermediate signal at an output terminal; and

an analog-to-digital converter having an input terminal coupled to the output terminal of the summation circuit for receiving the intermediate signal, and an output terminal coupled to a digital signal processing system.

12. The circuit of claim 11, wherein the summation circuit comprises:

a first resistor having a first terminal coupled to the first input terminal, and a second terminal;

a second resistor having a first terminal coupled to the second input terminal, and a second terminal;

an operational amplifier having an inverting input terminal coupled to the second terminal of the first resistor, a noninverting input terminal coupled to the second terminal of the second resistor, and an output terminal;

a third resistor having a first terminal coupled to the noninverting input terminal of the operational amplifier, and a second terminal coupled to a power supply voltage terminal; and

a fourth resistor having a first terminal coupled to the inverting input terminal of the operational amplifier, and a second terminal coupled to the output terminal of the operational amplifier.

13. The circuit of claim 12, further comprising:

a first capacitor coupled between the first input terminal and the first resistor; and

a second capacitor coupled between the second input terminal and the second resistor.

14. The circuit of claim 12, wherein the power supply voltage terminal is for receiving a ground potential.