Electrical audio signal processing systems and devices – Sound effects – Reverberators
Patent
1995-04-13
1997-04-08
Kuntz, Curtis
Electrical audio signal processing systems and devices
Sound effects
Reverberators
381 68, 381 684, H04R 2500
Patent
active
056195800
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention concerns a digital hearing aid as disclosed in more detail in the preamble to claim 1.
A hearing aid of this kind with digital suppression of or compensation for acoustic feedback is known from the applicant's earlier European patent application no. 90309342.5 (publication no. EP-A2-0415677).
Such a hearing aid has in practice proved to function as intended. In order for the hearing aid not to oscillate, the compensation, which is carried out by updating the coefficients in a digital filter in a feedback circuit, is effected by means of an algorithm which takes into account the error in the filter, i.e. the difference between the filter's actual setting and the desired setting. Such a hearing aid will not always be quick enough to adapt to sudden changes in the acoustic feedback path, even though it is still able to compensate for the acoustic feedback which arises. The lack of speed in the adaptation function can result in undesired acoustic signals which can be heard by the user of the hearing aid.
Hearing aid designs of the kind disclosed in the preamble to claim 1 are known from U.S. Pat. Nos. 4,453,039 and 5,091,952, wherein the amplification in the hearing aid is regulated depending on the loop gain, so that the amplification is reduced so much that the hearing aid does not start to oscillate. The disadvantage of this is that in some cases the amplification is regulated downwards to such a degree that this becomes inexpedient for the user.
In order to increase the adaptation speed without the hearing aid beginning to oscillate, the algorithm which takes care of the updating of the coefficients in the digital filter in the compensation circuit must take into consideration that the filter error depends on a number of coefficients, signal
oise ratio, input level, volume, and on the degree of peak clipping in the limiter circuit. Such an embracing algorithm will not be particularly fast in adapting itself to changes in the acoustic feedback path, but on the other hand it will provide a reliable and precise adjustment of the filter under stationary conditions in the feedback path.
When it has been ascertained that an important change is in progress, i.e. that a significant change has occurred in the acoustic feedback path, the circuit automatically effects a changeover of the algorithm in order to increase the speed of adaptation, e.g. by adding more noise and/or increasing the speed of adaptation in excess of what is prescribed by the basic algorithm. The quick condition lasts until the circuit ascertains that the filter coefficients are stable again, after which the circuit automatically switches back to the basic algorithm for continuous adjustment of the electronic compensation.
Such an apparatus is disclosed in Danish patent application no. 432/92 filed on Mar. 31, 1992 (=PCT/DK93/00106).
In a hearing aid with digital compensation for acoustic feedback, it will be possible to achieve an increased maximum amplification. If the hearing aid has already been adjusted to provide a given amplification, e.g. by the user, the extra amplification which the hearing aid can provide, because it has compensation for acoustic feedback, can perhaps be so great that the regulation system cannot compensate for a sudden increased level in the feedback path, and the apparatus will oscillate until it is screwed down or until the amplication in the feedback path is reduced. This can be of inconvenience for the user.
ADVANTAGES OF THE INVENTION
The object of the present invention is to avoid that a hearing aid with compensation for acoustic feedback, and of the kind disclosed in the preample to claim 1, can start to oscillate, in that the apparatus is arranged in such a manner that it automatically reduces the amplification if a sudden increase of the level in the feedback path arises. As soon as the condition with increased level in the feedback path ceases, the hearing aid's amplification will automatically be adjusted back to the level which has been selected by the user.
This is achi
REFERENCES:
patent: 4453039 (1984-06-01), Ferrieu
patent: 5091952 (1992-02-01), Williamson et al.
patent: 5206911 (1993-04-01), Eriksson et al.
patent: 5259033 (1993-11-01), Goodings et al.
GN Danovox A/S
Kuntz Curtis
Nguyen Duc
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