WO2008003135A1

WO2008003135A1 - Method and apparatus for feedback cancellation in the presence of tonal signals

Info

Publication number: WO2008003135A1
Application number: PCT/AU2007/000927
Authority: WO
Inventors: Brenton Robert Steele; Michael John Grant
Original assignee: Dynamic Hearing Pty Ltd
Priority date: 2006-07-03
Filing date: 2007-07-03
Publication date: 2008-01-10

Abstract

A system and technique for feedback cancellation. An off-line feedback estimation filter is adaptive in response to an error signal. An on-line feedback cancellation filter produces a feedback cancellation signal. The on-line feedback cancellation filter is updated by deriving on-line filter taps from taps of the off-line filter, with the update occurring when a tonality measure determined from the off-line filter taps meets a pre-defined criterion. The tonality measure may include reference to non-causal filter taps.

Description

"Method and apparatus for feedback cancellation in the presence of tonal signals"

Cross-Reference to Related Applications

The present application claims priority from United States of America Provisional Patent Application No 60/818410 filed on 3 July 2006, the content of which is incorporated herein by reference.

Technical Field

The present invention relates to hearing aid devices in which a sound input is processed and converted to a sound output, and in particular relates to the cancellation of acoustic feedback in such a device when the sound input may include tonal and other periodic components.

Background of the Invention

Typical hearing aids comprise a microphone or other input transducer to pick up sounds and convert them into an electrical signal, an electronic amplifier to increase the power of the electrical signal, and a speaker or other output transducer to convert the amplified electrical signal back into sound. If the input and output transducers are close enough, the output acoustic signal may be picked up by the input transducer and fed back into the amplifier with a delay, the delay being the time taken for the sound to travel from the output transducer to the input transducer (plus any delay due to the electrical processing of the signal). This is 'acoustic feedback'. Electrical feedback can also occur if the electrical signal at the output is coupled back to the input, for example by inductive or capacitive coupling. Further, mechanical feedback can also occur if vibrations are transmitted from the output transducer to the input transducer via the body or case of the amplification system.

Under feedback conditions the loop gain is greater than 1, such that the feedback signal self-reinforces and increases in intensity to drive the components into saturation, reaching an equilibrium when the loop gain reduces to unity. At this equilibrium level the hearing aid device usually emits a continuous and unpleasant high pitched whistle or squeal. Further, oscillation and instability in the processing path are undesirable because they can distort the signal processing performance. This can lead to problems both for the hearing aid user and for those around.

One approach for increasing the stability of a hearing aid is to reduce the gain at high frequencies, as suggested in, for example, US Patent 4,689,818. In multi-band processing this may be done by setting a maximum gain value for each band which reduces with increasing frequency, or automatic high frequency (HF) gain roll-off may be used. However, this means that the desired high-frequency response of the instrument must be sacrificed in order to maintain stability, which is particularly undesirable given that human hearing loss often occurs to a greater extent in the higher audible frequencies than in the lower frequencies.

Efforts have been undertaken to reduce the susceptibility of hearing aids to feedback oscillation by: attenuation and notch filtering, as disclosed in US Patent No. 4,088, 835; estimation and subtraction of the feedback signal (feedback cancellation), as disclosed in US Patent No. 5,016, 280; and frequency shifting or delaying the signal, as disclosed in US Patent No. 5,091, 952.

A further difficulty in feedback cancellation arises where an input sound signal comprises tonal and other periodic signals which ideally should not be cancelled, such as music, beeps, dial tones and the like. The phrase 'tonal signal' is used herein to refer to tonal and other periodic input signals which should ideally not be cancelled, and is intended to exclude oscillatory feedback signals. Such tonal signals can be difficult for signal processing techniques to distinguish from oscillatory feedback which should be cancelled. For example, some feedback cancellation techniques assess an autocorrelation of an input signal, and attempt to filter out signals with a high correlation, oscillatory feedback being one such signal with high correlation. However, tonal signals such as music also have a strong auto-correlation, with the result that feedback cancellation is inappropriately applied to the music signal in such techniques. This can result in a decreased efficacy of cancellation of actual feedback signals occurring simultaneously with the tonal input, and/or the production of audible artifacts such as 'warbling'. Such artifacts can also arise if adaptive feedback cancellation techniques cause a feedback estimation filter response to alter at a rate or by such an amount as to be perceptible to the user.

To provide a feedback estimation filter which responds appropriately to both tonal input signals and oscillatory feedback signals, respectively, some solutions utilise training to set a fixed filter response. However, such filter training necessitates an extra step in hearing aid fitting or implementation. Further, such fixed filters tend to have a limited range of situations in which feedback cancellation is adequately provided.

Other solutions utilise complicated tone detectors to detect situations where signals are present which include tones which could cause artefacts, for example by corrupting the filter taps. However, not all tones lead to corruption of filter taps, and such systems can thus detect a tone and reach a false positive determination that a filter has been corrupted, even when the filter has not been corrupted or has not been unacceptably corrupted. Conversely, tonal signals which may not be detected by a tone detector can nevertheless cause filter corruption, leading to a false negative determination. A further problem is that tone detectors indicate either the presence or absence of a tone, but provide no graduated measure of the extent of tonality in a signal.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. Summary of the Invention

According to a first aspect the present invention provides a system for feedback cancellation comprising: an off-line feedback estimation filter which is adaptive in response to an error signal; and an on-line feedback cancellation filter for producing a feedback cancellation signal; wherein the on-line feedback cancellation filter is updated by deriving on-line filter taps from taps of the off-line filter, such update occurring when a tonality measure determined from the off-line filter taps meets a pre-defined criterion.

According to a second aspect the present invention provides a feedback cancellation module comprising: an off-line feedback estimation filter which is adaptive in response to an error signal; and an on-line feedback cancellation filter for producing a feedback cancellation signal; wherein the on-line feedback cancellation filter is updated by deriving on-line filter taps from taps of the off-line filter, such update occurring when a tonality measure determined from the off-line filter taps meets a pre-defined criterion. ,

According to a third aspect the present invention provides a method for feedback cancellation comprising: adapting an off-line feedback estimation filter in response to an error signal; producing a feedback cancellation signal with an on-line feedback cancellation filter; and when a tonality measure determined from the off-line filter taps meets a predefined criterion, updating the on-line feedback cancellation filter by deriving on-line filter taps from taps of the off-line filter. According to a fourth aspect the present invention provides a computer program for feedback cancellation comprising: code for adapting an off-line feedback estimation filter in response to an error signal; and code for updating the on-line feedback cancellation filter by deriving on-line filter taps from taps of the off-line filter, such update occurring when a tonality measure determined from the off-line filter taps meets a pre-defined criterion.

The present invention recognises that techniques utilising a regularly or continuously updating on-line filter are easily corrupted when the input signal includes a tonal signal with strong auto-ccrrelation, such corruption usually leading to artifacts when listening to tones or music. By providing an on-line filter which only updates when the predefined criterion is met by the tonality measure of the off-line filter, the present invention provides for an on-line feedback cancellation filter which does not adapt continuously, thus reducing or delaying the onset of artefacts when a tonal input signal occurs, but nevertheless providing for an adaptive nature of the on-line filter.

Further, by updating ihe on-line filter by derivation from the off-line filter, the present invention avoids the need for pre-use training of the on-line filter. Updating of the on- line filter may comprise copying the tap values of the off-line filter, unchanged, to become the tap values of the on-line filter. Alternatively, the off-line filter tap values may be modified for use as the on-line filter tap values. For example the on-line filter may be updated by replacing existing on-line filter taps with new taps which are a weighted sum of the off-line filter taps and the existing on-line filter taps. Alternatively new on-line filter tap values may be derived from the off-line filter so as to remain within bounds set by a maximum adaption rate of the on-line filter. Additionally or alternatively, before being used to update the on-line filter, the off-line filter taps may be altered in a manner which cleanses tap values corrupted by a tonal input sound signal. Still further, by obtaining a tonality measure directly from the off-line filter taps avoids the problems of false positives and false negatives which come with use of a separate tone detector. Such a tonality measure in accordance with the present invention, being obtained from the filter taps, provides a more direct indication of the quality of the filter taps.

In preferred embodiments of the invention, a second tonality measure is obtained from the on-line filter taps, such a tonality measure being referred to herein as an on-line tonality measure. In such embodiments, assessment of whether the pre-defined criterion is met may include comparison of the tonality measure of the off-line filter (referred to herein as the off-line tonality measure) to the on-line tonality measure. For example, the pre-defined criterion may be determined by, or may rely upon, a ratio of the on-line tonality measure to the off-line tonality measure, and if the criterion is so met updating of the on-line feedback cancellation filter from the off-line filter taps may then be initiated.

In preferred embodiments of the invention, the on-line tonality measure and the off-line tonality measure are each a graduated measure of filter quality in the presence of a tonal input sound signal. In such embodiments, the on-line tonality measure and the off-line tonality measure may each be determined by comparing the values of end-taps of the respective filter to values of centre-taps of the filter. Each tonality measure may additionally or alternatively be determined by reference to a number of peaks or troughs in the filter tap profile, a DC bias in the filter tap profile, or a number of slope changes in the filter tap profile.

In preferred embodiments, the on-line filter and/or the off-line filter may be implemented in a manner such that one or more of the leading taps of that filter are non-causal. In such embodiments, the tonality measure for a non-causal filter may comprise a comparison of the values of one or more of the non-causal taps to the values of one or more of the causal taps. Such embodiments of the invention exploit the recognition that non-causal FIR filter taps exhibit behaviour in the presence of tonal signals which can be distinguished from the behaviour of the non-causal taps in the presence of oscillatory feedback signals, even when both tonal signals and oscillatory feedback signals are present. In such embodiments the tonality measure may comprise a ratio of the mean squared values of the non-causal taps to the mean squared values of the causal taps. Such embodiments of the present invention recognise that such a tonality measure is small when the filter is correctly modelling a feedback path, but increases when a tonal signal corrupts the feedback cancellation filter taps.

In further preferred embodiments of the invention a first error signal of the on-line filter and a second error signal of the off-line filter are included in the comparison. For example, the on-line tonality measure may be multiplied by a power of the first error signal to produce an acceptability measure of the on-line filter, referred to as an on-line acceptability measure. Similarly, the off-line tonality measure may be multiplied by a power of the second error signal to produce an acceptability measure of the off-line filter, referred to as an off-line acceptability measure. Each such acceptability measure gives a measure of filter performance under current signal conditions, with increasing value of such an acceptability measure indicating decreasing performance. In such embodiments the comparison may involve calculating a ratio of the on-line acceptability measure to the off-line acceptability measure.

Embodiments of the type set out in the preceding paragraph thus may effectively compare which filter is 'best', and update the on-line filter if the off-line filter is better. Preferably, the off-line filter should be better than the on-line filter by some margin to avoid excessive on-line filter updates. For example, in such embodiments the pre- defined result may be defined by reference to whether the ratio exceeds a pre-defined threshold. The pre-defined threshold could be of the order of one, or may be greater than one by a small margin, for example the threshold may have a value of around 1.25. Such a margin may be appropriate so as to avoid a large number of updates of the online filter and the accompanying risk of artifacts. When the ratio exceeds the threshold, this indicates that the on-line filter performance is worse than the off-line filter performance, and that the on-line filter should be updated. Embodiments which involve comparison of the on-line and off-line tonality measures thus provides a metric representing whether the on-line filter or the off-line filter is performing best in response to signal conditions at that time. Thus, in preferred such embodiments, no update of the on-line filter occurs while the on-line filter performance is better than the off-line filter performance, or is not substantially worse.

Preferably, a maximum tonality limit is applied to the off-line tonality measure, such that updating of the online taps from the offline taps is prevented if the off-line tonality measure exceeds the maximum tonality limit. Such embodiments provide a level of protection against corruption of the online filter taps.

The off-line filter may be made adaptive by implementation of a normalised least mean squares (NLMS) analysis of a filter error signal.

In some embodiments of the invention, feedback cancellation may be limited to operation within a sub-band, for example to greater than 1 kHz. Such band-limited feedback cancellation may be implemented in accordance with the disclosure of International Patent Publication No. WO 00/19605, the content of which is incorporated herein by reference.

Updating of the on-line filter may further be controlled by reference to a time since a previous update, a total number of updates within a given time, a user setting, an adaption rate of the off-line filter, or other factors. To minimise the influence of initial conditions encountered upon activation, the on-line filter is preferably updated substantially continuously for a pre-defined period of time after start-up, such as 1 second.

According to a fifth aspect the present invention provides a method for distinguishing tonal signals from feedback signals, the method comprising: providing a niter having one or more non-causal taps; adapting the filter in a manner which minimises an error signal; and monitoring a behaviour of at least one non-causal tap of the feedback cancellation filter.

According to a sixth aspect the present invention provides a method for determining an acceptability measure of a feedback filter, the method comprising: obtaining a tonality measure of the feedback filter; and combining the tonality measure with an error signal of the feedback filter to produce the acceptability measure.

The feedback filter may be a feedback cancellation filter or a feedback estimation filter.

Brief Description of the Drawings

An example of the invention will now be described with reference to the accompanying drawings, in which:

Figures Ia and Ib are system block diagrams of a system comprising feedback cancellation in accordance with the present invention;

Figure 2 is a flowchart of a process for updating the on-line filter of Figure 1 ; and Figures 3a and 3b are illustrations of feedback estimation filter profile when uncorrupted, and when corrupted by a tonal input signal, respectively.

Description of the Preferred Embodiments

Figure Ia illustrates a system 100 for sound signal processing. An input signal 110 derived from an input sound signal is passed to a summing node 112. A feedback cancellation module 120 provides a level of cancellation of a feedback signal arising from feedback of the output signal 116 back to the system input. The feedback cancellation signal 122 produced by the feedback cancellation module 120 is subtracted from the input signal 110 to produce a feedback cancelled signal 113. Signal 113 is processed in accordance with a signal processor 114 implementing a processing algorithm, which could be any suitable hearing aid signal processing algorithm, one example of which being the ADRO technique set out in US Patent No. 6,731,767, the content of which is incorporated herein by reference.

Following processing by processor 114, the output signal 116 is output for conversion back to audio by a speaker and/or for further processing. The output signal 116 is also passed to the feedback cancellation module 120. A finite impulse response (FIR) filter

124 has a filter response which approximates the response of the feedback path, and filters the output signal 116 to produce feedback cancellation signal 122. Filter 124 is thus an 'on-line' filter. On-line filter 124 is only updated at spaced apart times and thus can be considered to be a relatively static filter.

Feedback cancellation module 120 further comprises an adaptive FIR filter 126, which is an adaptive FIR filter in that it is substantially continuously updated by a normalised least means squares (NLMS) algorithm applied by processor 128. Processor 128 has inputs comprising the output signal 116 and an off-line error signal 130. Off-line error signal 130 is produced by the subtraction at 132 of the adaptive FIR output signal 134 from the input signal 110. Filter 126 is an off-line filter as the output of off-line filter 126 is not directly used apart from within feedback cancellation module 120. Unlike the on-line filter 124. the off-line FIR 126 is updated substantially continuously by the adaptive NLMS algorithm.

In order to make a determination of whether the off-line filter 126 is functioning 'better' than the on-line filter 124 in the presence of a tonal input sound signal, the present invention provides for a tonality measure of the off-line filter 126 to be obtained. In the present invention, the tonality measure is calculated as being the ratio of the mean squared value of non-causal taps of the off-line filter 126 to the mean squared value of central causal taps of the off-line filter 126. The off-line filter 126 is provided with non-causal taps by ensuring that a reduced or zero bulk delay exists in the feedback path. This is in contrast to normal feedback techniques which utilise a bulk delay in order to ensure that no filter taps are non-causal, such non-causal taps generally being considered to be of no value in real filter implementations. While the off-line tonality measure obtained from the off-line filter 126 could of itself convey valuable information to assist a determination of whether the on-line filter should be updated, the present embodiment further provides for the on-line tonality measure to be determined in a corresponding manner.

Further, the on-line filter error signal at 113 is passed to node 140. Referring to Figure IB, it can be seen that the on-line filter error signal is squared at 142 and smoothed at 144 before being multiplied at 146 by the on-line tonality measure to produce an on- line acceptability measure b, which is then multiplied by a factor of 0.8 at 148. Similarly, the off-line filter error 130 is passed to node 150. Referring to Figure IB it can be seen that the off-line error signal 130 is squared at 152 and smoothed at 154 before being multiplied at 156 by the off-line tonality measure, to produce an off-line acceptability measure a.

At 160, it is determined whether a is less than 0.8 b. If a is less than 0.8 b, this indicates that, when considering tonality measure and filter error, the performance of the off-line filter 126 is better than the performance of the on-line filter 124, by a margin of 0.2. Accordingly, this indicates that it is appropriate to update the on-line filter 124 from the taps of the off-line filter 126, as indicated at 125.

The process 200 by which it is decided whether or not to update the on-line filter 124 is further illustrated in Figure 2. Upon beginning the process at 210, at 220 the off-line tonality measure is obtained and multiplied by the off-line filter error to obtain element a. At 230 the on-line tonality measure is obtained and multiplied by the on-line filter error to obtain element b. At 240, it is determined whether a is less than 0.8b. If so the on-line filter 124 is updated as indicated at 125, and the process 200 returns to block 220. If a is not less than 0.86, then the process returns to block 220 without updating the on-line filter 124. Figures 3a and 3b are illustrations of feedback estimation filter profile when uncorrupted, and when corrupted by a tonal input signal, respectively. In Figure 3 a, the filter profile 300 has 8 leading non-causal filter taps, indicated at 310, which have tap values which are close to zero while the filter is not corrupted by any input tonal signal. On the other hand, the causal filter taps numbered 9 to 32, indicated at 320, take values which cause the filter profile 300 to have a complicated shape, as defined by the NLMS algorithm functioning to estimate the feedback path. The filter profile 300 is representative of one state of a feedback estimation filter.

Figure 3b illustrates a filter profile 330 which arises when the filter's feedback estimation process is corrupted by a tonal input signal. As indicated at 340, the 8 leading non-causal taps now have a significant non-zero magnitude. On the other hand, the causal taps 9 to 32, indicated at 350, have a profile which can be seen by eye to be somewhat different from the profile 300 of Figure 3a, but an extent of this difference is difficult to quantify. Thus, a measure of the non-causal tap magnitudes or powers provides a substantially more quantifiable measure of an extent to which a filter has been corrupted.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A method for feedback cancellation comprising: adapting an off-line feedback estimation filter in response to an error signal; producing a feedback cancellation signal with an on-line feedback cancellation filter; and when a tonality measure determined from the off-line filter taps meets a predefined criterion, updating the on-line feedback cancellation filter by deriving on-line filter taps from taps of the off-line filter.

2. The method of claim 1 wherein the on-line filter is updated by replacing existing on-line filter taps with new taps which are a weighted sum of the off-line filter taps and the existing on-line filter taps.

3. The method of claim 1 wherein updating of the on-line filter comprises copying the tap values of the off-line filter to become the tap values of the on-line filter.

4. The method of claim 1 or claim 2 wherein updating of the on-line filter comprises deriving new on-line filter tap values from the off-line filter so as to remain within bounds set by a maximum adaption rate of the on-line filter.

5. The method of any one of claims 1 to 4 wherein updating of the on-line filter further comprises cleansing of tap values corrupted by a tonal input sound signal.

6. The method of any one of claims 1 to 5 further comprising determining an on- line tonality measure from the on-line filter taps.

7. The method of claim 6 wherein assessment of whether the pre-defined criterion is met includes comparison of the off-line tonality measure to the on-line tonality measure.

8. The method of claim 7 wherein comparison of the off-line tonality measure to the on-line tonality measure relies upon a ratio of the on-line tonality measure to the off-line tonality measure.

9. The method of any one of claims 1 to 8, wherein at least one of the off-line tonality measure and the on-line tonality measure is determined by at least one of: comparing the values of end-taps of the respective filter to values of centre-taps of the filter; determining a number of peaks or troughs in the filter tap profile; determining a DC bias in the filter tap profile; and determining a number of slope changes in the filter tap profile.

10. The method of any one of claims 1 to 9, wherein at least one of the off-line tonality measure and the on-line tonality measure is determined by comparison of the value of at least one non-causal tap of the respective filter to the value of at least one causal tap.

11. The method of any one of claims 1 to 10 further comprising, for at least one of said filters determining an acceptability measure from a product of that filter's tonality measure and a power of an error signal of that filter.

12. The method of claim 11 wherein the pre-defined criterion relies upon a ratio of an acceptability measure of the on-line filter to an acceptability measure of the off-line filter.

13. The method of any one of claims 1 to 12 further comprising applying a maximum tonality, limit to the off-line tonality measure, such that updating of the online taps from the offline taps is prevented if the off-line tonality measure exceeds the maximum tonality limit.

14. A system for feedback cancellation comprising: an off-line feedback estimation filter which is adaptive in response to an error signal; and an on-line feedback cancellation filter for producing a feedback cancellation signal; wherein the on-line feedback cancellation filter is updated by deriving on-line filter taps from taps of the off-line filter, such update occurring when a tonality measure determined from the off-line filter taps meets a pre-defined criterion.

15. The system of claim 14 wherein the on-line filter is updated by replacing existing on-line filter taps with new taps which are a weighted sum of the off-line filter taps and the existing on-line filter taps.

16. The system of claim 14 wherein updating of the on-line filter comprises copying the tap values of the off-line filter to become the tap values of the on-line filter.

17. The system of claim 14 or claim 15 wherein updating of the on-line filter comprises deriving new on-line filter tap values from the off-line filter so as to remain within bounds set by a maximum adaption rate of the on-line filter.

18. The system of any one of claims 14 to 17 wherein updating of the on-line filter further comprises cleansing of tap values corrupted by a tonal input sound signal.

19. The system of any one of claims 14 to 18 wherein an on-line tonality measure is determined from the on-line filter taps.

20. The system of claim 19 wherein assessment of whether the pre-defined criterion is met includes comparison of the off-line tonality measure to the on-line tonality measure.

21. The system of claim 20 wherein comparison of the off-line tonality measure to the on-line tonality measure relies upon a ratio of the on-line tonality measure to the off-line tonality measure.

22. The system of any one of claims 14 to 21, wherein at least one of the off-line tonality measure and the on-line tonality measure is determined by at least one of: comparing the values of end-taps of the respective filter to values of centre-taps of the filter; determining a number of peaks or troughs in the filter tap profile; determining a DC bias in the filter tap profile; and determining a number of slope changes in the filter tap profile.

23. The system of any one of claims 14 to 22, wherein at least one of the off-line tonality measure and the on-line tonality measure is determined by comparison of the value of at least one non-causal tap of the respective filter to the value of at least one causal tap.

24. The system of any one of claims 14 to 23 wherein, for at least one of said filters an acceptability measure is determined from a product of that filter's tonality measure and a power of an error signal of that filter.

25. The system of claim 24 wherein the pre-defined criterion relies upon a ratio of an acceptability measure of the on-line filter to an acceptability measure of the off-line filter.

26. The system of any one of claims 14 to 25 wherein a maximum tonality limit is applied to the off-line tonality measure, such that updating of the online taps from the offline taps is prevented if the off-line tonality measure exceeds the maximum tonality limit.

27. A feedback cancellation module comprising: an off-line feedback estimation filter which is adaptive in response to an error signal; and an on-line feedback cancellation filter for producing a feedback cancellation signal; wherein the on-line feedback cancellation filter is updated by deriving on-line filter taps from taps of the off-line filter, such update occurring when a tonality measure determined from the off-line filter taps meets a pre-defined criterion.

28. A computer program for feedback cancellation comprising: code for adapting an off-line feedback estimation filter in response to an error signal; and code for updating the on-line feedback cancellation filter by deriving on-line filter taps from taps of the off-line filter, such update occurring when a tonality measure determined from the off-line filter taps meets a pre-defined criterion.

29. A method for distinguishing tonal signals from feedback signals, the method comprising: providing a filter having one or more non-causal taps; adapting the filter in a manner which minimises an error signal; and monitoring a behaviour of at least one non-causal tap of the feedback cancellation filter.

30. A method for determining an acceptability measure of a feedback filter, the method comprising: obtaining a tonality measure of the feedback filter; and combining the tonality measure with an error signal of the feedback filter to produce the acceptability measure.