Method and apparatus for feedback reduction in acoustic systems, particularly in hearing aids

• US 6,347,148 B1
• Filed: 04/16/1998
• Issued: 02/12/2002
• Est. Priority Date: 04/16/1998
• Status: Expired due to Term

• Current Assignee: K/S HIMPP
• Original Assignee: ON Semiconductor Corporation
• Inventors: Brennan, Robert, Schneider, Anthony Todd
• Primary Examiner(s): LE, HUYEN D
• Application Number:
  US09/060,822
• Time in Patent Office: 1,398 Days
• Field of Search: 381/231.1, 381/312, 381/318, 381/320, 381/321, 381/83, 381/93, 381/94.2, 381/71.11, 381/71.12, 381/71.13, 381/FOR.127, 381/FOR.129, 381/FOR.131
• US Class Current: 381/318
• CPC Class Codes: H04R 25/453 electronically
• CPC Class Codes: H04R 25/505 using digital signal processing
• CPC Class Codes: H04R 27/00 Public address systems circuits for preventing acoustic reaction H04R3/02; circuits for distributing signals to loudspeakers H04R3/12; monitoring or testing arrangements for public address systems H04R29/007; amplifiers H03F
• CPC Class Codes: H04R 3/02 for preventing acoustic reaction , i.e. acoustic oscillatory feedback specially adapted for hearing aids H04R25/453

Abstract

There is provided a method of controlling feedback in an acoustic system, for example a digital hearing aid, in which there is a potential feedback path between the output and the input. The method comprises making a spectral estimate of the input signal spectrum, and then subjecting the spectral estimate to a psycho-acoustic model to generate a control signal. A noise source is passed through a shaping filter, which is controlled with the control signal, to generate frequency-shaped noise, which is inaudible to someone hearing the output. The frequency-shaped noise is then added to the input signal to form a combined signal, which is processed in a forward path, to generate a first output signal. The first output signal and the frequency-shaped noise signal are analyzed, to determine the presence of feedback at difference frequencies, and the characteristics of the forward path are modified to reduce the gain thereof at frequencies where feedback is detected.

Claims

1. A method of controlling feedback in an acoustic system having an input for an acoustic input signal and an output for an acoustic output signal that generates a potential feedback path between the output and the input, the method comprising the steps of:

(1) generating a first input signal from the acoustic input signal and making a spectral estimate of the first input signal;

(2) subjecting the spectral estimate to a psycho-acoustic model to generate a control signal;

(3) passing a noise signal through a shaping filter and controlling the shaping filter with the control signal, to generate frequency-shaped noise, which is inaudible to someone hearing the acoustic output signal;

(4) adding the frequency-shaped noise to the first input signal to form a combined signal;

(5) processing the combined signal in a forward signal path having a transfer function, to generate a first output signal;

(6) analyzing the first output signal and the frequency-shaped noise signal, to determine the presence of feedback at different frequencies;

(7) using the first output signal to generate the acoustic output signal; and

(8) modifying the transfer function of the forward signal path, to reduce the gain thereof at frequencies where feedback is detected.

2. A method as claimed in claim 1, wherein, in step (2), the psycho-acoustic model is selected from one of a normative psycho-acoustic model and a specific psycho-acoustic model representative of the hearing characteristics of an individual user.

3. A method as claimed in claim 1, wherein step (6) comprises forming a cross-spectral estimate between the first output signal and the frequency-shaped noise and an auto-spectral estimate for the frequency-shaped noise, dividing the cross-spectral estimate by the auto-spectral estimate to obtain a spectral ratio, and determining when the frequency response of the spectral ratio varies from the frequency response of the forward path, indicative of feedback.

4. A method as claimed in claim 1, when applied to one of a digital hearing aid, and a public address system.

5. A method as claimed in claim 1, wherein steps (3) and (6) are based on maximum length sequence methods, such that step (3) comprises taking the fast Hadamard transform of the control signal to generate the frequency-shaped noise, and step (6) comprises taking the fast Hadamard transform of the first output signal from the forward path, generating the power spectrum of the fast Hadamard transform of the first output signal and the power spectrum of the fast Hadamard transform of the control signal, and dividing the two power spectrums to obtain a spectral ratio from which feedback can be detected.

6. A method as claimed in claim 1, wherein in step (1), following generation of the input signal, the input signal is passed through one path of a stereo N channel analysis filterbank, which provides the spectral estimate of the first input signal spectrum so that the psycho-acoustic model generates N channel gains as an output, and wherein step (3) comprises passing the noise signal through the other path of the stereo analysis filterbank to provide output noise channel signals, and multiplying the output noise channel signals by the N channel gains of the psycho-acoustic model to provide the frequency-shaped noise.

7. An apparatus for processing an acoustic signal and generating an acoustic output, the apparatus comprising:

an input means for receiving an acoustic input signal and for generating a first input signal;

an output transducer for generating an output acoustic signal;

a forward signal path within the apparatus connecting the input means to the receiver and having a main transfer function for generating a first output signal;

a feedback path between the receiver and the input means enabling at least a portion of the output acoustic signal to be received at the input means;

a spectral estimation means connected to the input means for receiving the first input signal and for generating a spectral estimate of the acoustic input signal;

a psycho-acoustic model means connected to the spectral estimation means for forming a control signal from the spectral estimate;

a noise generation means connected to the psycho-acoustic model means for generating a noise signal in dependence upon the control signal;

means for adding the noise signal to the first input signal to form a combined signal, for processing in the forward signal path; and

means for analyzing the noise signal and the combined signal after processing in the forward signal path to determine the presence of feedback and for modifying the main transfer function of the forward path to eliminate any substantial feedback.

8. An apparatus as claimed in claim 7, wherein the means for analyzing the noise signal comprises:

a first correlation means for forming a cross correlation between the noise signal and the first output signal; and

a second correlation means for forming an auto correlation of the noise signal; and

means for dividing the cross correlation signal by the auto correlation signal, to generate a ratio of the cross correlation spectrum to the auto correlation spectrum, which is indicative of the forward path transfer function.

9. An apparatus as claimed in claim 7 or 8, wherein the noise generation means comprises a primary noise source for generating a primary noise signal and a shaping filter connected thereto and to the psycho-acoustic model means for generating a noise signal shaped in dependence upon the control signal.

10. An apparatus as claimed in claim 7, wherein the noise generation means comprises means for performing a fast Hadamard transform on the control signal from the psycho-acoustic model means, and wherein the means for analyzing the noise signal comprises:

second fast Hadamard transform means connected to a forward path for generating the fast Hadamard transform of the first output signal;

first power spectrum generating means for generating a first power spectrum of the fast Hadamard transform of the control signal;

second power spectrum generating means for generating a second power spectrum of the fast Hadamard transform of the first output signal;

means for dividing the second power spectrum by the first power spectrum to obtain a signal indicative of the forward path transfer function.

11. An apparatus as claimed in claim 7 or 10, which includes an analysis filterbank connected between the input means and the forward path, the analysis filterbank means providing the spectral estimation means, wherein the noise generation means is connected through the analysis filterbank means and the psycho-acoustic model means is connected to the analysis filterbank means for control thereof, for generating a noise signal in dependence upon said control signal.