Feedback cancellation in a hearing aid with reduced sensitivity to low-frequency tonal inputs

• US 6,831,986 B2
• Filed: 08/20/2002
• Issued: 12/14/2004
• Est. Priority Date: 12/21/2000
• 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 M.
• Primary Examiner(s): LE, HUYEN D
• Application Number:
  US10/224,519
• Publication Number:
  US 20030053647A1
• Time in Patent Office: 847 Days
• Field of Search: 381/312, 381/318, 381/320, 381/321, 381/71.11, 381/71.12, 381/83, 381/93, 381/94.1, 381/94.2
• US Class Current: 381/312
• CPC Class Codes: H04R 25/453 electronically
• CPC Class Codes: H04R 25/505 using digital signal processing
• CPC Class Codes: H04R 3/002 Damping circuit arrangements for transducers, e.g. motional feedback circuits

Abstract

A feedback cancellation system with reduced sensitivity to low-frequency tonal inputs is provided. Such a system can be used, for example, in a hearing aid to prevent cancellation of the desired tonal inputs to the hearing aid, thus improving the gain at high frequencies of the hearing aid while simultaneously preserving the desired tonal inputs at low frequencies. The feedback cancellation system comprises a first adaptive filter block for adaptively filtering an error signal to remove the low-frequency tonal components from the error signal. The first adaptive filter block is constrained so that only low-frequency tones in the error signal are cancelled, thus enabling the feedback cancellation system to still cancel “whistling” at high frequencies due to the temporary instability of the hearing aid. A second adaptive filter block adaptively filters a feedback path signal to produce an adaptively filtered feedback path signal. The first and second adaptive filter blocks are identical and filter coefficients of the first adaptive filter block are copied to those of the second adaptive filter block. Using an LMS adaptation algorithm, filter coefficients of an adaptive filer of the feedback cancellation system are controlled by the adaptively filtered error signal and the adaptively filtered feedback path signal respectively inputted from the first and second adaptive filter blocks. The adaptive filter then produces an adaptively filtered modeled feedback signal to be subtracted from an electrical audio signal input for updating the error signal of the hearing aid. The hearing aid processes the updated error signal with a digital signal processor to generate an audio output.

Claims

1. A hearing aid, comprising:

a signal path capable of receiving an audio input signal and an acoustic feedback signal from an acoustic feedback path and of generating an audio output signal, said signal path having subtracting means for generating an error signal; and

feedback cancellation means adapted to adaptively model the acoustic feedback path for canceling the acoustic feedback signal, wherein said feedback cancellation means comprises a first adaptive filter means for adaptively filtering the error signal from said signal path to remove low-frequency tonal components of the error signal during coefficient adaptation of the acoustic feedback path model, said first adaptive filter means removing the low-frequency tonal components from the error signal for preserving the low-frequency tonal components in the audio output signal.

2. The hearing aid of claim 1, wherein said first adaptive filter means comprises at least one adaptive notch filter.

3. The hearing aid of claim 2, wherein said at least one adaptive notch filter includes two or more adaptive notch filters connected in cascade to each other.

4. The hearing aid of claim 2, wherein said first adaptive filter means further comprises a bandpass filter filtering the error signal and connected in series to said at least one adaptive notch filter.

5. The hearing aid of claim 2, wherein said first adaptive filter means further comprises a highpass filter filtering the error signal and connected in series to said at least one adaptive notch filter.

6. The hearing aid of claim 2, wherein a center frequency of each adaptive notch filter of said at least one adaptive notch filter is constrained to between 0 Hz and a predetermined maximum allowable frequency.

7. The hearing aid of claim 1, wherein said first adaptive filter means includes a plurality of adaptive notch filters arranged in parallel combination, said first adaptive filter means further comprising:

a plurality of bandpass filters arranged in parallel combination, each of said plurality of bandpass filters filtering the error signal and being coupled to one of said plurality of adaptive notch filters; and

adder means for summing outputs of said plurality of adaptive notch filters.

8. The hearing aid of claim 1, wherein said first adaptive filter means comprises an adaptive FIR filter for canceling the low-frequency tonal components of the error signal.

9. The hearing aid of claim 8, wherein said first adaptive filter means further comprises:

a highpass filter, said highpass filter filtering the error signal from said signal path to generate a highpass filtered error signal; and

a lowpass filter, said lowpass filter filtering the error signal from said signal path to generate a lowpass filtered error signal, wherein said adaptive FIR filter causes low-frequency tonal components to be removed from the lowpass filtered error signal.

10. The hearing aid of claim 9, wherein said first adaptive filter means further comprises:

a delay unit, said delay unit being coupled between said lowpass filter and said adaptive FIR filter for delaying the lowpass filtered error signal inputted into said adaptive FIR filter;

first subtracting means, coupled to said lowpass filter and said adaptive FIR filter, for subtracting an output of said adaptive FIR filter from the lowpass filtered error signal; and

first adder means, coupled to said highpass filter and said first subtracting means, for summing the highpass filtered error signal with an output of said first subtracting means.

11. The hearing aid of claim 1, wherein said feedback cancellation means further comprises second adaptive filter means adaptively filtering a feedback path signal for coefficient adaptation of the acoustic feedback path model, said second adaptive filter means being identical to said first adaptive filter means.

12. The hearing aid of claim 11, wherein said feedback cancellation means further comprises:

an adaptive filter, said adaptive filter generating an adaptively filtered feedback signal in accordance with the feedback path signal to the subtracting means; and

LMS adaptation means, coupled to said adaptive filter and said first and second adaptive filter means, for controlling adaptation of filter coefficients of said adaptive filter.

13. The hearing aid of claim 12, wherein said feedback cancellation means further comprises:

a feedback delay unit coupled to said signal path; and

frozen filter means, coupled to said feedback delay unit, for generating the feedback path signal to said second adaptive filter means and said adaptive filter.

14. The hearing aid of claim 13, wherein said signal path further comprises hearing aid processing means for processing the error signal, said feedback delay unit of said feedback cancellation means coupled to said hearing aid processing means for receiving an output therefrom.

15. The hearing aid of claim 11, wherein filter coefficients of said second adaptive filter means are copied from filter coefficients of said first adaptive filter means.

16. A hearing aid, comprising:

a microphone, said microphone being adapted to receive an input audio signal and an acoustic feedback signal and generate an electrical audio signal;

feedback cancellation means for canceling the acoustic feedback signal, said feedback cancellation means generating a signal processing feedback signal in response to a feedback path signal;

subtracting means, coupled to said microphone and said feedback cancellation means, for subtracting the signal processing feedback signal from the electrical audio signal to form a compensated electrical audio signal;

hearing aid processing means, coupled to said subtracting means, for processing the compensated electrical audio signal; and

receiver means, coupled to said hearing aid processing means, for converting the processed compensated electrical audio signal into a sound signal,

wherein said feedback cancellation means adaptively models an acoustic feedback path and includes first adaptive filter means for adaptively filtering the compensated electrical audio signal to remove low-frequency tonal components of the compensated electrical audio signal for coefficient adaptation of the acoustic feedback path model.

17. The hearing aid of claim 16, wherein said first adaptive filter means comprises at least one adaptive notch filter.

18. The hearing aid of claim 17, wherein said at least one adaptive notch filter includes two or more adaptive notch filters connected in cascade to each other.

19. The hearing aid of claim 17, wherein said first adaptive filter means further comprises a bandpass filter filtering the compensated electrical audio signal and connected in series to said at least one adaptive notch filter.

20. The hearing aid of claim 17, wherein said first adaptive filter means further comprises a highpass filter filtering the compensated electrical audio signal and connected in series to said at least one adaptive notch filter.

21. The hearing aid of claim 17, wherein a center frequency of each adaptive notch filter of said at least one adaptive notch filter is constrained to between 0 Hz and a predetermined maximum allowable frequency.

22. The hearing aid of claim 16, wherein said first adaptive filter means comprises a plurality of adaptive notch filters arranged in parallel combination, said first adaptive filter means further comprising:

a plurality of bandpass filters arranged in parallel combination, each of said plurality of bandpass filters filtering the compensated electrical audio signal and being coupled to one of said plurality of adaptive notch filters; and

adder means for summing outputs of said plurality of adaptive notch filters.

23. The hearing aid of claim 16, wherein said first adaptive filter means comprises:

an adaptive FIR filter;

a highpass filter, said highpass filter filtering the compensated electrical audio signal to generate a highpass filtered error signal;

a lowpass filter, said lowpass filter filtering the compensated electrical audio signal to generate a lowpass filtered error signal,

a delay unit, said delay unit being coupled between said lowpass filter and said adaptive FIR filter for delaying the lowpass filtered error signal inputted into said adaptive FIR filter;

first subtracting means, coupled to said lowpass filter and said adaptive FIR filter, for subtracting an output of said adaptive FIR filter from the lowpass filtered error signal; and

first adder means, coupled to said highpass filter and said first subtracting means, for summing the highpass filtered error signal with an output of said first subtracting means.

24. The hearing aid of claim 16, wherein said feedback cancellation means further comprises second adaptive filter means adaptively filtering the feedback path signal for coefficient adaptation of the acoustic feedback path model, said second adaptive filter means being identical to said first adaptive filter means.

25. The hearing aid of claim 24, wherein said feedback cancellation means further comprises:

an adaptive filter, said adaptive filter generating the signal processing feedback signal to said subtracting means;

LMS adaptive means, coupled to said adaptive filter and said first and second adaptive filter means, for controlling adaptation of filter coefficients of said adaptive filter;

a feedback delay unit coupled to an output of said hearing aid processing means; and

frozen filter means, coupled to said feedback delay unit, for generating the feedback path signal to said second adaptive filter means and said adaptive filter.

26. The hearing aid of claim 24, wherein filter coefficients of said second adaptive filer means are copied from filter coefficients of said first adaptive filter means.

27. A method for compensating feedback noise in an audio system, comprising the steps of:

receiving an input signal;

generating an electrical audio signal in accordance with the input signal;

processing the electrical audio signal by a digital signal processor to produce an electrical output signal;

estimating a modeled feedback signal in accordance with the electrical output signal;

generating an error signal by subtracting the modeled feedback signal from the electrical audio signal;

adaptively filtering the error signal to remove low-frequency tonal components of the error signal with a first adaptive filter block;

adaptively controlling filter coefficients of an adaptive filter in accordance with the adaptively filtered error signal;

updating the modeled feedback signal by the adaptive filter;

updating the error signal by subtracting the updated modeled feedback signal from the electrical audio signal; and

processing the updated error signal by the digital signal processor to update the electrical output signal.

28. The method according to claim 27, wherein the step of adaptively filtering the error signal is accomplished by adaptively filtering the error signal with at least one adaptive notch filter of the first adaptive filter block.

29. The method according to claim 28, wherein the at least one adaptive notch filter includes two or more adaptive notch filters arranged in cascade to each other.

30. The method according to claim 28, wherein the step of adaptively filtering the error signal further comprises a step of filtering the error signal with a bandpass filter prior to the at least one adaptive notch filter.

31. The method according to claim 28, wherein the step of adaptively filtering the error signal further comprises a step of filtering the error signal with a highpass filter prior to the at least one adaptive notch filter.

32. The method according to claim 27, wherein the step of adaptively filtering the error signal comprises the steps of:

filtering the error signal by a plurality of bandpass filters arranged in parallel combination, each of the plurality of bandpass filters being adapted to filter a specific range of the error signal frequency;

filtering outputs of the plurality of bandpass filters by a plurality of adaptive notch filters, each of the plurality of adaptive notch filters being coupled to one of the plurality of bandpass filters; and

generating the adaptively filtered error signal by summing outputs of the plurality of adaptive notch filters.

33. The method according to claim 27, wherein the step of adaptively filtering the error signal comprises the steps of:

generating a highpass error signal by filtering the error signal with a highpass filter;

generating a lowpass filtered error signal by filtering the error signal with a lowpass filter;

delaying the lowpass filtered error signal;

generating an adaptively filtered lowpass error signal by filtering the delayed lowpass filtered error signal with an adaptive FIR filter;

generating a lowpass error signal by subtracting the adaptively filtered lowpass error signal from the lowpass filtered error signal; and

generating the adaptively filtered error signal by summing the lowpass error signal and the highpass error signal.

34. The method according to claim 27, further comprising the steps of:

delaying the electrical output signal of the digital signal processor with a delay unit;

generating a feedback path signal by filtering an output of the delay unit with a frozen filter;

generating an adaptive feedback path signal by filtering the feedback path signal with a second adaptive filter block; and

adaptively controlling filter coefficients of the adaptive filter in accordance with the adaptive feedback path signal and the adaptively filtered error signal.

35. The method according to claim 34, further comprising a step of copying filter coefficients of the second adaptive filter block from the filter coefficients of the first adaptive filter block.

36. The method according to claim 27, wherein the step of adaptively controlling filter coefficients of an adaptive filter is accomplished by using an LMS adaptation algorithm.

37. A hearing aid, comprising:

a microphone, said microphone being adapted to receive an input audio signal and an acoustic feedback signal and generate an electrical audio signal;

feedback cancellation means for canceling the acoustic feedback signal, said feedback cancellation means generating a signal processing feedback signal in response to a feedback path signal;

subtracting means, coupled to said microphone and said feedback cancellation means, for subtracting the signal processing feedback signal from the electrical audio signal to form a compensated electrical audio signal;

a signal process unit coupled to said subtracting means, said signal process unit processing the compensated electrical audio signal; and

receiver means, coupled to said signal process unit, for converting the processed compensated electrical audio signal into a sound signal,

wherein said feedback cancellation means adaptively models an acoustic feedback path, said feedback cancellation means comprising;

a frozen filter block coupled to said signal process unit, said frozen filter block generating the feedback path signal;

first adaptive filter means, coupled to said subtracting means, for adaptively filtering the compensated electrical audio signal to remove low-frequency tonal components of the compensated electrical audio signal;

second adaptive filter means, coupled to said frozen filter block, for adaptively filtering the feedback path signal, said second adaptive filter means being identical to said first adaptive filtering means;

an adaptive filter coupled to said subtracting means, said adaptive filter generating the signal processing feedback signal to said subtracting means; and

LMS adaptive means, coupled to said adaptive filter and said first and second adaptive filter means, for controlling adaptive adaptation of filter coefficients of said adaptive filter in accordance with outputs from said first and second adaptive filter means.

38. The hearing aid of claim 37, wherein said frozen filter block includes a feedback delay unit coupled to said signal process unit and a frozen filter coupled to said feedback delay unit, said frozen filter generating the feedback path signal to said second adaptive filter means and said adaptive filter.

39. The hearing aid of claim 37, wherein said first adaptive filter means comprises at least one adaptive notch filter.

40. The hearing aid of claim 39, wherein said at least one adaptive notch filter includes two or more adaptive notch filters connected in series to each other.

41. The hearing aid of claim 39, wherein said first adaptive filter means further comprises a bandpass filter coupled to said subtracting means to filter the compensating electrical audio signal, said bandpass filter being connected in series to said at least one adaptive notch filter.

42. The hearing aid of claim 39, wherein said first adaptive filter means further comprises a highpass filter coupled to said subtracting means to filter the compensating electrical audio signal, said highpass filter being connected in series to said at least one adaptive notch filter.

43. The hearing aid of claim 37, wherein said first adaptive filter means comprises a plurality of adaptive notch filters arranged in parallel combination, said first adaptive filter means further comprising:

a plurality of bandpass filters arranged in parallel combination, each of said plurality of bandpass filters filtering the compensated electrical audio signal and being coupled to one of said plurality of adaptive notch filters; and

adder means for summing outputs of said plurality of adaptive notch filters.

44. The hearing aid of claim 37, wherein said first adaptive filter means comprises:

an adaptive FIR filter;

a highpass filter, said highpass filter filtering the compensated electrical audio signal to generate a highpass filtered error signal;

a lowpass filter, said lowpass filter filtering the compensated electrical audio signal to generate a lowpass filtered error signal,

a delay unit, said delay unit being coupled between said lowpass filter and said adaptive FIR filter for delaying the lowpass filtered error signal inputted into said adaptive FIR filter;

first subtracting means, coupled to said lowpass filter and said adaptive FIR filter, for subtracting an output of said adaptive FIR filter from the lowpass filtered error signal; and

first adder means, coupled to said highpass filter and said first subtracting means, for summing the highpass filtered error signal with an output of said first subtracting means.

45. The hearing aid of claim 37, wherein filter coefficients of said second adaptive filer means are copied from filter coefficients of said first adaptive filter means.