Method and apparatus for canceling noise in a microphone communications path using an electrical equivalence reference signal

• US 6,480,824 B2
• Filed: 07/12/1999
• Issued: 11/12/2002
• Est. Priority Date: 06/04/1999
• Status: Expired due to Fees

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Christensson, Nils, Feltström, Alberto Jimenez
• Primary Examiner(s): Banks-Harold, Marsha D.
• Assistant Examiner(s): Azad, Abul K.
• Application Number:
  US09/351,267
• Publication Number:
  US 20020065650A1
• Time in Patent Office: 1,219 Days
• Field of Search: 704/221, 704/233, 704/243, 704/244, 704/226, 704/234, 379/93.09, 379/406, 379/410, 379/406.03, 379/406.15, 379/3, 379/22.02, 381/94.7, 381/71.9, 381/92, 381/94.1, 370/286, 370/289
• US Class Current: 704/233
• CPC Class Codes: H04R 3/00 Circuits for transducers , loudspeakers or microphones

Abstract

In a technique for canceling noise from a microphone communications path, an electrical equivalence circuit which is positioned close to and electrically matched to the microphone produces a signal free reference signal. The microphone converts speech to a voice signal. An analog multiplexer alternately switches from the microphone to the electrical equivalence circuit to produce a multiplexed analog signal composed of the voice signal from the microphone and the signal free reference signal from the electrical equivalence circuit. A wire connects the output of the analog multiplexer switch to an A/D converter. The wire picks up noise from the surrounding environment. The A/D converter converts the multiplexed signal having the noise to a plurality of voice samples taken from the voice signal portion of the multiplexed signal and a plurality of noise samples taken from the signal free reference signal portion of the multiplexed signal. A noise cancellation unit applies a noise suppression algorithm (e.g., spectral subtraction) to the plurality of noise samples and the plurality of voice samples to reproduce the voice signal with the noise suppressed.

Claims

1. An apparatus for canceling electromagnetic noise in a microphone communications path, comprising:

a microphone configured to convert speech to an electrical voice signal;

an electrical equivalence circuit, in close proximity to and electrically matching the microphone, configured to produce a signal free reference signal;

an analog multiplexer configured to alternately switch between the microphone and the electrical equivalence circuit to produce a multiplexed signal comprising the voice signal and the signal free reference signal;

a communications path connected to the analog multiplexer for carrying the multiplexed signal through an electromagnetic noise intensive environment such that the multiplexed signal acquires an electromagnetic noise component;

an A/D converter connected to the communications path for converting the multiplexed signal having the noise component to a plurality of voice samples and a plurality of noise samples; and

a noise cancellation unit applying a noise suppression procedure to the plurality of noise samples and the plurality of voice samples to reproduce the voice signal with the electromagnetic noise component suppressed.

2. The apparatus of claim 1, wherein:

the plurality of noise samples are taken from the signal free reference signal of the multiplexed signal having the acquired electromagnetic noise component.

3. The apparatus of claim 1, wherein:

the plurality of voice samples are taken from the voice signal of the multiplexed signal having the acquired electromagnetic noise component.

4. The apparatus of claim 1, wherein:

the communications path is a wire.

5. The apparatus of claim 1, wherein:

the analog multiplexer switches from the voice signal to the signal free reference signal at a rate of 16 kHz.

6. The apparatus of claim 1, wherein:

the A/D converter samples the multiplexed signal at a rate of 16 kHz.

7. The apparatus of claim 1, wherein:

the voice signal from the microphone is sampled by the A/D converter at an 8 kHz rate.

8. The apparatus of claim 1, wherein:

the signal free reference signal is sampled by the A/D converter at an 8 kHz rate.

9. The apparatus of claim 1, wherein:

the electromagnetic noise component includes bumblebee noise centered at approximately 217 Hz.

10. The apparatus of claim 1, wherein:

the noise suppression procedure is a spectral subtraction procedure.

11. The apparatus of claim 1, further comprising:

a transmitter; and

wherein;

the electromagnetic noise component is a result of electromagnetic energy generated by the transmitter radiating the electromagnetic energy centered at a predetermined frequency.

12. The apparatus of claim 11, wherein:

the predetermined frequency is approximately 217 Hz.

13. The apparatus of claim 1, wherein the electrical equivalence circuit electrically matches the microphone by having the same electrical characteristics as the microphone.

14. The apparatus of claim 13, wherein the electrical equivalence circuit has the same impedance as the microphone.

15. The apparatus of claim 1, wherein the A/D converter and the multiplexer are synchronized at the same sampling rate.

16. A method for canceling electromagnetic noise in a microphone communications path, comprising:

converting speech from the microphone to an electrical voice signal;

producing a signal free reference signal from an electrical equivalence circuit in close proximity to and electrically matching the microphone;

switching alternately from the microphone to the electrical equivalence circuit to produce a multiplexed signal comprising the voice signal and the signal free reference signal;

carrying the multiplexed signal through an electromagnetic noise intensive environment such that the multiplexed signal acquires an electromagnetic noise component;

digitizing the multiplexed signal having the noise component to produce a plurality of voice samples and a plurality of noise samples; and

applying a noise suppression procedure to the plurality of noise samples and the plurality of voice samples to reproduce the voice signal with the electromagnetic noise component suppressed.

17. The method of claim 16, wherein:

the plurality of noise samples are taken from a signal free reference signal part of the multiplexed signal having the acquired electromagnetic noise component.

18. The method of claim 16, wherein:

the plurality of signal samples are taken from a voice signal part of the multiplexed signal having the acquired electromagnetic noise component.

19. The method of claim 16, wherein:

the switching step includes the analog multiplexer switching from the voice signal to the signal free reference signal at a rate of 16 kHz.

20. The method of claim 16, wherein:

the digitizing step includes sampling the multiplexed signal at a sampling rate of 16 kHz to produce the plurality of voice samples and the plurality of noise samples.

21. The method of claim 16, wherein:

the digitizing step includes sampling the voice signal at an 8 kHz rate to produce the plurality of voice samples.

22. The method of claim 16, wherein:

the digitizing step includes sampling the signal free reference signal at an 8 kHz sampling rate to produce the plurality of noise samples.

23. The method of claim 16, wherein:

the electromagnetic noise component includes bumblebee noise centered at approximately 217 Hz.

24. The method of claim 16, wherein:

the noise suppression procedure is a spectral subtraction procedure.

25. The method of claim 16, further comprising:

a transmitter; and

wherein;

the noise component is a result of electromagnetic energy generated by the transmitter radiating the electromagnetic energy centered at a predetermined frequency.

26. The method of claim 25, wherein:

the predetermined frequency is approximately 217 Hz.

27. An apparatus for canceling electromagnetic noise in a microphone system, comprising:

a microphone configured to convert speech to an electrical voice signal;

an electrical equivalence circuit, in close proximity to and electrically matching the microphone, configured to produce a signal free reference signal;

a first communications path for carrying the voice signal from the microphone to an analog multiplexer,

a second communications path for carrying the signal free reference signal from the electrical equivalence circuit to the analog multiplexer, the first and second communications paths being carried through an electromagnetic noise intensive environment such that the voice signal and the signal free reference signal both acquire an electromagnetic noise component;

said analog multiplexer, connected to the first and second communications paths, configured to alternately switch between the microphone and the electrical equivalence circuit to produce a multiplexed signal comprising the voice-laden component voice signal and the noise-laden component signal free reference signal;

an A/D converter coupled to the analog multiplexer for converting the multiplexed signal having the noise component to a plurality of voice samples and a plurality of noise samples; and

a noise cancellation unit applying a noise suppression procedure to the plurality of noise samples and the plurality of voice samples to reproduce the voice signal with the electromagnetic noise component suppressed.