FIELD OF THE INVENTION

This disclosure relates to detection of the onset of feedback in feedback cancellation subsystems of acoustic systems, and more particularly to such for feedback cancellation subsystems in hearing devices.

BACKGROUND OF THE INVENTION

Acoustic feedback occurs because the output speaker signal from an audio system providing amplification of a signal picked up by a microphone is partly returned to the microphone via an acoustic coupling through the air or other media. The part of the output signal returned to the microphone is then re-amplified by the system before it is re-presented at the speaker, and again returned to the microphone. As this cycle continues, the effect of acoustic feedback becomes audible as artefacts, squealing, etc. when the system becomes unstable. The problem appears typically when the microphone and the loudspeaker are placed closely together, as e.g. in hearing aids or other audio systems. Some other classic situations with feedback problems are telephony, public address systems, headsets, audio conference systems, etc. Unstable systems due to acoustic feedback tend to significantly contaminate the desired audio input signal with narrow band frequency components, which are often perceived as squeal or whistle.

A variety of feedback cancellation methods have been described to increase the stability of an audio processing system. The acoustic feedback path of an audio processing device, e.g. a listening device, e.g. a hearing aid, may vary over time. Adaptive feedback cancellation has the ability to track acoustic feedback path changes over time and is e.g. based on an adaptive filter comprising a finite impulse response filter (variable filter part of the adaptive filter) to estimate the feedback path. The filter weights are updated over time (e.g. calculated in an update (algorithm) part of the adaptive filter). The filter update may be calculated using stochastic gradient algorithms, including some form of the Least Mean Square (LMS) or the Normalized LMS (NLMS) algorithms A drawback of these methods is that the estimate of the acoustic feedback path (provided by the adaptive filter) will be biased, if the input signal to the system is not white (i.e. if there is autocorrelation) because the estimate is made in a ‘closed loop’. This means that the anti-feedback system may introduce artefacts when there is autocorrelation (e.g. tones) in the input.

Feedback in acoustic systems, especially hearing aids, is prevalent because of air conduction paths that exist between the output of the system and its input. Other physical paths can also cause feedback; for example a loose package may be subject to transmitting vibrations from output back to the input. When the system provides some gain to the acoustic input signal, the corresponding output then can loop back into the input via air conduction or other acoustic or physical means, creating a reinforcing recursion known as feedback. To the hearing aid user, this can be audible as a loud squeal which persists unless cancelled by some method.

Most hearing aids use electronic means to provide an anti-phase signal that adapts to the feedback, attempting to cancel it. Rather than manipulating the gain, feedback cancellation (FBC) algorithms introduce an additional signal to cancel out the acoustic leakage. When feedback is detected at the output of the hearing aid, a cancellation signal is generated to mimic the feedback. The feedback is then cancelled by subtracting the cancellation signal from the input.

A negative side-effect of some adaptive FBC algorithms is the occurrence of entrainment artifacts. Entrainment occurs when the feedback canceller mistakenly attempts to cancel a tonal input to the hearing aid. This results in the addition of a tone to the original by the hearing aid itself when the input tonal signal changes or stops. The hearing aid user may report hearing the additional tone, a tone or squeal or whistling after the original sound has stopped, or a modulation-type distortion of the sound.

There is a need in the art for improved feedback cancellation to avoid unwanted artifacts generated in existing feedback cancellation systems at the time the feedback path changes, while differentiating between tonal inputs and feedback signals in order to reduce entrainment.

There is a need in the art for improved feedback cancellation that can better distinguish feedback path changes from changes due to tonal sounds such as music, tonal parts of speech, pure tones (beeps or alarms) and impulsive sounds like clicks and pops.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method of signal processing an audio signal in a hearing device is provided to detect when feedback path change occurs and control an adaptive feedback canceler to remove the feedback. The hearing device includes a receiver and a microphone. This method includes: detecting whether a tonal signal is due to feedback onset by estimating a product of a subband error signal and a subband output signal generated in response to a subband audio input signal; estimating a fast metric based on the estimated product and estimating a slow metric; and applying or maintaining an adaptation rate to the adaptive feedback canceler of the hearing device, wherein the adaptation rate applied or maintained is based upon a value of the difference between the fast and slow metrics.

In at least one embodiment, the adaptation rate applied or maintained is selected based upon a value of the difference between the fast and slow metrics compared to a threshold value.

In at least one embodiment, it is detected that feedback path change occurs when the difference between the fast metric and the slow metric is greater than or equal to the threshold value.

In at least one embodiment, the adaptation rate applied when it is detected that feedback path change occurs is a relatively fast adaptation rate.

In at least one embodiment, the adaptation rate applied when it is detected that feedback path change occurs is proportional to the value of the difference between the fast and slow metrics.

In at least one embodiment, it is detected that feedback path change is considered to not occur when the difference between the fast metric and the slow metric is less than the threshold value.

In at least one embodiment, the adaptation rate applied when it is detected that feedback path change does not occur is a relatively slow adaptation rate.

In at least one embodiment, the product of a subband error signal and a subband output signal generated in response to a subband audio input signal is estimated for each subband of the audio signal having been split into a plurality of subbands.

In at least one embodiment, the product of a subband error signal and a subband output signal generated in response to a subband audio input signal is estimated for less than a total number of a plurality of subbands of the audio signal having been split into the plurality of subbands.

In at least one embodiment, the method further includes reducing gain applied to at least one of the audio signal or at least one of the subband audio signals to reduce squeal associated with the onset of feedback path change.

In at least one embodiment, the adaptation rate is applied for a predetermined time after detection of feedback path change to ensure complete adaptation of the adaptive feedback filter of the adaptive feedback canceler to cancel the feedback.

According to an aspect of the present invention, a method of signal processing an audio signal in a hearing device to detect when feedback path change occurs and controlling an adaptive feedback canceler to remove the feedback is provided, wherein the hearing device includes a receiver and a microphone, and the method includes: detecting whether a tonal signal is caused by feedback path change by estimating a product of a subband error signal and a subband output signal generated in response to a subband audio input signal; estimating a metric based upon the estimated product; and applying or maintaining an adaptation rate to the adaptive feedback canceler of the hearing device, wherein the adaptation rate applied or maintained is based on a value of the metric.

In at least one embodiment, the metric is used to gradate a value of the adaptation rate by scaling the value of the adaptive rate according to a value of the metric.

In at least one embodiment, the metric is estimated by estimating a fast metric based on the estimated product and estimating a slow metric, and taking the difference between the fast metric and the slow metric.

In at least one embodiment, estimation of the slow metric is not performed when the absolute value of the fast metric exceeds a predetermined value.

In at least one embodiment, an adaptation rate is applied to the adaptive feedback canceler that is faster than a currently applied adaptation rate, when it is detected that the tonal signal is caused by a feedback path change, and wherein the faster adaptation rate is applied for a predetermined time after it is detected that the tonal signal is caused by the feedback path change to ensure complete adaptation of an adaptive feedback filter of the adaptive feedback canceler to cancel the feedback.

According to an aspect of the present invention, a hearing device is provided that includes: a microphone; a receiver; and a processor connected to the microphone and said receiver, the processor configured to receive audio signals from the microphone and process the audio signals and the receiver configured to provide output signals to a user, the processor is further configured to: detect whether a tonal signal is caused by a feedback path change by estimating a product of a subband error signal and a subband output signal generated in response to a subband audio input signal split from the audio signals; estimate a metric based upon the estimated product; and apply or maintain an adaptation rate to an adaptive feedback canceler of the hearing device, wherein the adaptation rate applied or maintained is based on a value of the metric.

In at least one embodiment, the hearing device is a hearing aid.

In at least one embodiment, the hearing aid is a completely-in-the-canal (CIC) hearing aid.

In at least one embodiment, the metric is estimated by estimating a fast metric based on the estimated product and estimating a slow metric, and taking the difference between the fast metric and the slow metric.

In at least one embodiment, the metric is used to gradate a value of the adaptation rate by scaling the value of the adaptation rate according to a value of the metric.

In at least one embodiment, it is detected that feedback path change occurs when the difference between the fast metric and the slow metric is greater than or equal to the threshold value.

In at least one embodiment, the adaptation rate applied when it is detected that feedback path change occurs is a relatively fast adaptation rate.

In at least one embodiment, it is detected that feedback path change occurs when the difference between the fast metric and the slow metric is greater than or equal to a predetermined threshold value; and wherein the adaptation rate applied when it is detected that feedback path change occurs is a relatively fast adaptation rate relative to an adaptation rate that is currently being applied.

In at least one embodiment, the relatively fast adaptation rate is applied for a predetermined time after detection of feedback path change.

These and other advantages and features of the invention will become apparent to those persons skilled in the art upon reading the details of the methods, systems and devices as more fully described below.

