Electrical audio signal processing systems and devices – Hearing aids – electrical
Reexamination Certificate
2002-08-20
2004-12-14
Le, Huyen (Department: 2643)
Electrical audio signal processing systems and devices
Hearing aids, electrical
C381S318000
Reexamination Certificate
active
06831986
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to apparatus and methods for adaptive feedback cancellation in an audio system such as a hearing aid and, more specifically, to a feedback cancellation system of the hearing aid with reduced sensitivity to low-frequency tonal inputs.
BACKGROUND OF THE INVENTION
An audio system, such as a hearing aid, almost invariably incurs some sort of mechanical and/or acoustic feedback during operation of the audio system. The mechanical and/or acoustic feedback often limits the maximum gain that can be achieved in the hearing aid. Moreover, system instability caused by the feedback, whether mechanical and/or acoustic, is sometimes audible as a continuous high-frequency tone or whistle emanating from the hearing aid. The mechanical feedback of the hearing aid is usually caused by mechanical vibrations from a component thereof such as a receiver. Mechanical vibrations from the receiver of a high-power hearing aid can be reduced by combining outputs of two receiver units mounted back-to-back so as to cancel the net mechanical movement of the receiver units. As such, as much as 10 dB additional gain can be achieved for the high-power hearing aid before the onset of oscillation by the hearing aid. Many hearing aids also provide a venting capability to reduce unpleasant occlusion experienced by users of the hearing aids. But venting an earmold of a behind-the-ear (BTE) type hearing aid or a shell of an in-the-ear (ITE) type hearing aid establishes an acoustic feedback path that would limit the maximum possible gain to approximately less than 40 dB for a small vent and even less for a large vent. The acoustic feedback path includes effects from many of the hearing aid components such as the amplifier, receiver, and microphone as well as vent acoustics.
As mentioned, the acoustic feedback of the hearing aid tends to cause system instability of the hearing aid, particularly at high frequencies. A traditional approach for increasing the stability of a hearing aid is to reduce the gain at high frequencies. Reducing the gain of the hearing aid only at high frequencies modifies the overall system frequency response of the hearing aid. Therefore, controlling feedback by modifying the system frequency response to avoid instability means that a desired high-frequency response of the hearing aid will be sacrificed. Phase shifters and notch filters have also been suggested to control feedback, but have not proven to be very effective.
A more effective technique to control feedback is by feedback cancellation. For instance, an internal feedback signal is estimated and subtracted from a microphone signal of the hearing aid. Feedback cancellation typically uses an adaptive filter that models the dynamically changing feedback path of the hearing aid. Such an adaptive feedback cancellation system, however, can generate a large mismatch between an actual feedback path and an adaptive filter modeled feedback path when the input signal of the hearing aid is either narrowband or sinusoidal. One example of such a system has been disclosed by U.S. Pat. No. 5,091,952 to Williamson et al., as is illustrated in FIG.
1
.
FIG. 1
shows a hearing aid
100
having the adaptive feedback cancellation system incorporated therein. As shown in
FIG. 1
, an adaptive filter
101
is used to model the feedback path of the hearing aid, and a Least Mean Square (LMS) adaptation algorithm
103
is used to control filter coefficients adaptation of adaptive filter
101
. A delay
105
is placed in the feedback path model to decorrelate the hearing aid output from the input. The delay
105
improves the system convergence of the hearing aid for signals such as speech. However, for tonal inputs at low frequencies such as music, sinusoids, or audiological test signals commonly used to measure hearing loss of a patient, this system tends to cancel the tonal inputs instead of accurately modeling the actual feedback path of the hearing aid for feedback cancellation.
An improved effective feedback cancellation scheme used in a hearing aid is disclosed by the present inventor in U.S. Pat. No. 6,072,884, entitled “Feedback Cancellation Apparatus and Methods”, the contents of which are incorporated herein by reference. This improved system is illustrated in FIG.
2
. The feedback path of such improved system is modeled by the combination of an adaptive filter
201
and a delay
205
plus a slowly-varying or non-varying (frozen) filter
219
. The frozen filter
219
can be a frozen IIR filter or a frozen all pole filter, and the adaptive filter
201
can be an adaptive (all zero) FIR filter. Specifically, when the hearing aid is first turned on, filter (pole) coefficients of the frozen filter
219
are adapted to model those aspects of the feedback path that can have high-Q resonance but which stay relatively constant during normal hearing aid operation. Thus, pole coefficients of the feedback path, once determined, are modified and then frozen or, at least, changed vary slowly. Once the pole coefficients are determined, filter (zero) coefficients of the adaptive filter
201
are adapted to correspond to the modified poles. The objective of this adaptation is to minimize an error signal e(n) produced at the output of adder
209
. Unlike the filter coefficients of the frozen filter
219
, the adaptive filter
201
continues to adapt its filter coefficients in response to changes in the feedback path. Therefore, the adaptive filter
201
models those portions of the feedback path that are changing, and the frozen filter
219
models those portions of the feedback path that remain essentially constant while the hearing aid is in use. This improved system will, however, also attempt to cancel a tonal input signal. Nonetheless, adaptive filter coefficients of this improved system are constrained to prevent excessive deviation from an initial setting thereof. In the presence of a tonal input, the degree of input signal cancellation resulting from the adaptive filter is greatly reduced, but it is still not completely eliminated.
The feedback cancellation systems shown in
FIGS. 1 and 2
use the LMS algorithm for adaptation of the adaptive filter coefficients. As shown in
FIGS. 1 and 2
, the hearing aid receives an input signal x(n), a transfer function of a hearing aid processing unit is given by h(n), and the hearing aid output is y(n), where n is a sample index. The LMS algorithm adaptation in both the above-mentioned feedback cancellation systems uses the cross-correlation of an error signal e(n) and a feedback path signal d(n) that is inputted to the adaptive filter (i.e., the adaptive filter
101
or the adaptive filer
201
). The objective of the adaptive filter is to minimize the power of the error signal e(n). Let the adaptive filter be a K-tap finite impulse response (FIR) filter having adaptive coefficients b
l
(n) through b
k
(n), a power-normalized adaptive filter update for input sample index n is then given by
b
k
(
n
+
1
)
=
b
k
(
n
)
+
2
μ
σ
d
2
(
n
)
e
(
n
)
d
(
n
-
k
)
(
1
)
where &mgr; controls the rate of adaptation and &sgr;
d
2
(n) is the average power in the feedback path signal d(n). If the input signal x(n) is white noise, the adaptive filter will normally converge to a model of its feedback path. If the input x(n) is a pure tone, however, the adaptive feedback cancellation system will minimize the error signal e(n) by adjusting the filter coefficients b
l
(n) through b
k
(n) so that v(n), which is an adaptively filtered version of d(n), has the same amplitude and phase as of the input x(n) and thus will cancel the tone. Slowing the rate of adaptation by making &mgr; smaller will reduce the tendency to cancel short-duration tonal inputs, but will also reduce the ability of the adaptive system to rapidly adapt to large changes to the acoustic feedback path.
A further improvement in feedback cancellation for hearing aids is disclosed by Gao et al. in an international patent application WO 00/019605 A2. This system is
Bingham & McCutchen LLP
Bolan Michael J.
GN ReSound A/S
Le Huyen
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