Five techniques for reducing acoustic feedback in hearing aids were investigated: an adaptive notch filter, three previously described methods for adaptive feedback cancellation, and a novel method for feedback cancellation with adaptation during quiet intervals. Through real-time implementations, these techniques were assessed for added stable gain and sound quality. Test results showed the novel system to provide the best overall performance. >

Reducing acoustic feedback in hearing aids

Feedback cancellation in hearing aids involves estimating the feedback signal and subtracting it from the microphone input signal. The feedback-cancellation system described updates the estimated feedback path whenever changes are detected in the feedback behavior. When a change is detected, the normal hearing-aid processing is interrupted, a pseudorandom probe signal is injected into the system, and a set of filter coefficients is adjusted to give an estimate of the feedback path. The hearing aid is then returned to normal operation with the feedback-cancellation filter as part of the system. Two approaches are investigated for computing the filter coefficients: a least-mean square (LMS) adaptive filter and a Wiener filter. Test results are presented for a computer simulation of an in-the-ear (ITE) hearing aid. The simulation results indicate that more than 10 dB of cancellation can be obtained and that the Wiener filter is more effective in the presence of strong interference. >

Feedback cancellation in hearing aids: results from a computer simulation

The authors describe a technique for making in situ measurements of the acoustic feedback path transfer function and present results demonstrating the effect of changes in the acoustic environment on this transfer function. Three methods of feedback suppression are explored: time-varying delay, adaptive inverse filtering, and adaptive feedback cancellation. It is concluded that the latter approach is the most successful, increasing the maximum gain the hearing aid can deliver without acoustic feedback by 6-10 dB. This increased gain can provide a benefit for moderately-impaired listeners. >

Measurement and adaptive suppression of acoustic feedback in hearing aids

In feedback cancellation in hearing aids, an adaptive filter is used to model the feedback path. The output of the adaptive filter is subtracted from the microphone signal to cancel the acoustic and mechanical feedback picked up by the microphone, thus allowing more gain in the hearing aid. In general, the feedback-cancellation filter adapts on the hearing-aid input signal, and signal cancellation and coloration artifacts can occur for a narrow-band input. In this paper, two procedures for LMS adaptation with a constraint on the magnitude of the adaptive weight vector are derived. The constraints greatly reduce the probability that the adaptive filter will cancel a narrow-band input. Simulation results are used to demonstrate the efficacy of the constrained adaptation.

Constrained adaptation for feedback cancellation in hearing aids.

A mobile radiotelephone (120) includes a microcomputer (220), speakerphone control circuitry (106) and mobile radio (112). Mobile radio (112) includes a transmitter (242), receiver (244) and a duplexer (243) for coupling the transmitter (242) and receiver (243) to the mobile antenna. In the transmit voice path, speakerphone control circuitry (106) includes switch (201), variable gain amplifier (202), compressor amplifier (203), switch (204), variable gain amplifier (205), and two registers (212-213). In the receive voice path, speakerphone control circuitry (106) includes variable gain amplifier (232), expander amplifier (233), switch (234), variable gain amplifier (235), switch (236) and two registers (216-217). Speakerphone control circuitry (106) also includes rectifier/filter (224), a companding analog-to-digital (A/D) converter (225), two latches (218-219), rectifier/filter (226) and two registers (214-215). Microcomputer (220) accesses registers (212-217) by means of a data bus (222) including address, control and data lines. Microcomputer (220) utilizes digital samples from A/D converter (225) to continuously generate and store in its memory an envelope signal and a valley signal for the transmit and receive voice paths. In order to determine when to switch gain between the voice paths, microcomputer (220) compares the envelope signal to the valley signal to detect the presence of voice signals and, if the voice signals are detected for a pre-selected time, the gain of the path in which voice signals have been detected is increased and the gain of the other path is decreased.

Speakerphone using digitally compressed audio to control voice path gain

An acoustic feedback control to be installed in every extension provided with a loud speaker and a microphone in a digital telephone system. Each extension is provided with a coder and a decoder of the syllabic compression type which therefore have quantizing steps varying with the voice level at the input of the transmission channels between two extensions. A comparator receives the step information from the coder and decoder and prevents the decoder step from increasing when the coder step exceeds the decoder step.

Jacques A. Besseyre

Papers

Acoustic feedback control