Feedback cancellation improvements

• US 6,498,858 B2
• Filed: 12/21/2000
• Issued: 12/24/2002
• Est. Priority Date: 11/18/1997
• Status: Expired due to Term

• Current Assignee: GN Resound A/S (GN Store Nord A/S)
• Original Assignee: GN Resound A/S (GN Store Nord A/S)
• Inventors: Kates, James Mitchell
• Primary Examiner(s): LE, HUYEN D
• Application Number:
  US09/745,497
• Publication Number:
  US 20010002930A1
• Time in Patent Office: 733 Days
• Field of Search: 381/312, 381/318, 381/320, 381/71.8, 381/71.11, 381/71.12, 381/92, 381/93, 381/66, 381/313, 381/317, 381/83, 381/321, 381/23.1, 381/FOR.127, 381/FOR.129, 381/FOR.131
• US Class Current: 381/318
• CPC Class Codes: H04R 25/453 electronically

Abstract

Feedback cancellation apparatus uses a cascade of two filters along with a short bulk delay. The first filter is adapted when the hearing aid is turned on in the ear. This filter adapts quickly using a white noise probe signal, and then the filter coefficients are frozen. The first filter models parts of the hearing-aid feedback path that are essentially constant over the course of the day. The second filter adapts while the hearing aid is in use and does not use a separate probe signal. This filter provides a rapid correction to the feedback path model when the hearing aid goes unstable, and more slowly tracks perturbations in the feedback path that occur in daily use. The delay shifts the filter response to make the most effective use of the limited number of filter coefficients.

Claims

1. A hearing aid comprising:

a microphone for converting sound into an audio signal;

feedback cancellation means including means for estimating a physical feedback signal of the hearing aid, and means for modeling a signal processing feedback signal to compensate for the estimated physical feedback signal;

subtracting means, connected to the output of the microphone and the output of the feedback cancellation means, for subtracting the signal processing feedback signal from the audio signal to form a compensated audio signal;

hearing aid processing means, connected to the output of the subtracting means, for processing the compensated audio signal; and

speaker means, connected to the output of the hearing aid processing means, for converting the processed compensated audio signal into a sound signal;

wherein said feedback cancellation means forms a feedback path from the output of the hearing aid processing means to the input of the subtracting means and comprises

a first filter for modeling at least one near constant factor in a physical feedback path, and

a second, adaptive, filter for modeling variable factors in the physical feedback path.

2. The hearing aid of claim 1, wherein the first filter is a fixed filter.

4. The hearing aid of claim 1, wherein the near constant factor is selected from the group consisting of a frequency response of the microphone, a frequency response of the speaker means, a frequency response of the processing means, and a frequency response of a vent.

5. The hearing aid of claim 1, wherein the first filter models at least two near constant factors.

6. The hearing aid of claim 5, wherein the near constant factors are selected from the group consisting of a frequency response of the microphone, a frequency response of the speaker means, a frequency response of the processing means, and a frequency response of a vent.

3. A hearing aid comprising:

a microphone for converting sound into an audio signal;

feedback cancellation means including means for estimating a physical feedback signal of the hearing aid, means for modeling a signal processing feedback signal to compensate for the estimated physical feedback signal;

subtracting means, connected to the output of the microphone and the output of the feedback cancellation means, for subtracting the signal processing feedback signal from the audio signal to form a compensated audio signal;

hearing aid processing means, connected to the output of the subtracting means, for processing the compensated audio signal; and

speaker means, connected to the output of the hearing aid processing means, for converting the processed compensated audio signal into a sound signal;

wherein said feedback cancellation means forms a feedback path from the output of the hearing aid processing means to the input of the subtracting means and comprises

a first filter for modeling at least one near constant factor in a physical feedback path, and

a second, adaptive, filter for modeling variable factors in the physical feedback path, wherein the first filter is an adaptive filter having an adaptation rate substantially slower than an adaptation rate of the second filter.

7. A hearing aid comprising:

a first microphone for converting sound into a first audio signal;

a second microphone for converting sound into a second audio signal;

feedback cancellation means including means for estimating physical feedback signals to each microphone of the hearing aid, and means for modeling a first signal processing feedback signal to compensate for the estimated physical feedback signal to the first microphone and a second signal processing feedback signal to compensate for the estimated physical feedback signal to the second microphone;

means for subtracting the first signal processing feedback signal from the first audio signal to form a first compensated audio signal;

means for subtracting the second signal processing feedback signal from the second audio signal to form a second compensated audio signal;

beamforming means, connected to each subtracting means, to combine the compensated audio signals into a beamformed signal;

hearing aid processing means, connected to the beamforming means, for processing the beamformed signal; and

speaker means, connected to the output of the hearing aid processing means, for converting the processed beamformed signal into a sound signal;

wherein said feedback cancellation means includes

a first filter, connected to the output of the hearing aid processing means, for modeling at least one near constant factor in one of the physical feedback paths;

a second, adaptive, filter, connected to the output of the first filter and providing an input to the first subtraction means, for modeling variable factors in the first feedback path; and

a third, adaptive, filter, connected to the output of the first filter and providing an input to the second subtraction means, for modeling variable factors in the second feedback path.

8. The hearing aid of claim 7, wherein the first filter is a fixed filter.

9. The hearing aid of claim 7, wherein the first filter is an adaptive filter having an adaptation rate substantially slower than an adaptation rate of the second or third filters.

10. The hearing aid of claim 7, wherein the near constant factor is selected from the group consisting of a frequency response of the first microphone, a frequency response of the second microphone, a frequency response of the speaker means, a frequency response of a first vent;

and a frequency response of a second vent.

11. The hearing aid of claim 7, wherein the first filter models at least two near constant factors.

12. The hearing aid of claim 11, wherein the near constant factors are selected from the group consisting of a frequency response of the first microphone, a frequency response of the second microphone, a frequency response of the speaker means, a frequency response of a first vent;

and a frequency response of a second vent.

13. A method of compensating feedback signals in a hearing aid comprising the steps of:

turning on the hearing aid;

configuring the hearing aid to operate in an open loop manner;

inserting a test signal into a hearing aid output;

estimating a physical feedback path of the hearing aid based on the test signal;

designing a first filter modeling at least one near constant factor in the estimated physical feedback path;

designing a second, adaptive, filter modeling variable factors in the estimated physical feedback path;

configuring the hearing aid to operate in a closed loop manner, and

adapting at least the second filter to compensate for changes in the physical feedback path.

14. The method of claim 13, further comprising the step of fixing the first filter after it is designed.