HEARING AID COMPENSATING FOR ACOUSTIC FEEDBACK

Abstract: A hearing aid with digital, electronic compensation for acoustic feedback comprises a microphone (5), a preamplifier (7), a digital compensation circuit (3), an output amplifier (9) and a transducer (11). The digital circuit (3) comprises a noise generator (33) for the insertion of noise, and an adjustable, digital filter (27) for the adaptation of the feedback signal. The adaptation takes place using a correlation circuit (31). The circuit further comprises a digital circuit (210) which monitors the loop gain and regulates the hearing aid amplification via a digital summing circuit (211), so that the loop gain is less than a constant K. The circuit further comprises a digital circuit (79) which carries out a statistical evaluation of the filter coefficients in the correlation circuit, and changes the feedback function in accordance with this evaluation.

Description: HERRING AID COMPENSATING FOR ACOUSTIC FEEDBACK 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 ap- plicant'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 co¬ efficients 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 USA patents 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 ampli¬ fication 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 di¬ gital filter in the compensation circuit must take into consideration that the filter error depends on a number of coefficients, signal/noise 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 pre- cise 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 ef¬ fects 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 coeffi¬ cients are stable again, after which the circuit automatic¬ ally 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 March 31st 1992 (= PCT/DK93/00106) . In a hearing aid with digital compensation for acoustic feedback, it will be possible to achieve an increased max- imum amplification. If the hearing aid has already been ad¬ justed 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 per¬ haps be so great that the regulation system cannot compens- ate 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 man- ner 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 feed¬ back path ceases, the hearing aid's amplification will au¬ tomatically be adjusted back to the level which has been selected by the user. This is achieved by configuring the hearing aid according to the invention as characterized in claim 1. The circuit carries out the control by continuously calcu¬ lating the amplification in the adaptive filter at differ¬ ent frequencies, and at the same time herewith the circuit monitors the setting of the volume control, and on this ba¬ sis regulates the hearing aid's loop gain so that it is al- ways less than a constant K, where K _> 1. K is a constant or a function of the frequency. The hearing aid's FIR fil¬ ter is able to provide extra amplification at high fre¬ quencies. If the total loop gain is greater than or equal to K, the amplification is reduced, possibly down to a lower level than that set by the user. This form of regulation can be used with great advantage in connection with a hearing aid which is arranged as dis¬ closed in Danish patent application no. 432/92 (PCT/DK93/00106) , and as disclosed in the preamble to claim 2, so that an optimized compensation for acoustic feedback is achieved. Consequently, the resulting hearing aid is one which always gives the user the optimum possible amplifica¬ tion, while at the same time strongly reducing the hearing aid's tendency to oscillate. Claim 3 discloses an advantageous embodiment of the inven¬ tion. The drawing The invention will now be described in more detail with reference to the drawing, in that fig. 1 shows a block diagram of a hearing aid according to Danish patent application, no. 432/92, and fig. 2 shows the hearing aid in fig. 1, but further pro¬ vided with the regulation circuit according to the invention. Description of the preferred embodiment The following description of the preferred embodiment of the invention, with reference to figs. 1 and 2 of the draw- ing, is only an example of how the invention can be util¬ ized in practice. In all of the figures of the drawing, the same reference designations are used for identical compon¬ ents or circuits etc. Fig. 1 shows the hearing aid which is disclosed and de¬ scribed as the preferred embodiment in Danish patent ap¬ plication no. 432/92, and for this reason a -number of the part-circuits are not explained more fully in the present application. In fig. 1 is shown a hearing aid comprising a sound re- ceiver, for example in the form of a microphone 5, a pre¬ amplifier 7, a digital adaptation circuit 3, an output amp¬ lifier 9 and a sound reproducer 11, for example a miniature electro-acoustic transducer. The preamplifier 7 is of a commonly-known type, for example of the type known from the applicant's earlier European application no. 90309342.5, and the output amplifier 9 is similarly of a commonly-known type, for example correspond- ing to the output amplifier which is used in the hearing aid in the applicant's earlier European application no. 90309342.5. The digital adaptation circuit 3 is shown within the stippled frame in the connection between the preamplifier 7 and the output amplifier 9. However, there is nothing to prevent the circuit 3 from being a mixed analogue and/or digital circuit, but in the preferred embodiment a purely digital circuit is used. The input to the digital adaptive circuit 3 comprises an A/D converter 17 and the output from the circuit comprises a D/A converter 19. In the circuit path c, d, i, e and f between the input 17 and the output 19 there is a digital limiter circuit 15 of a known kind, for example as known from the applicant's earlier European application no. 90309342.5. The function of the limiter circuit 15 is to prevent the electrical signal from reaching a level of am¬ plitude which exceeds the linearity limits of the output amplifier 9 and the transducer 11, and as explained in said European application. A digital summing circuit 21 is inserted in the path be¬ tween the limiter circuit 15 and the D/A converter 19. The summing circuit 21 serves as a place for the introduction of a noise signal N as explained later. A digital subtrac- tion circuit 23 is inserted in the path between the A/D converter 17 and the limiter circuit 15. The subtraction circuit 23 comprises means for the introduction of elec¬ trical feedback, as will also be described later. The normal signal path for a desired signal from the micro¬ phone 5 to the transducer 11 is the direct circuit path a- b-c-d-i-e-f-g-h as shown in fig. 1. It should be noted that the electrical path a, b, g and h is arranged for analogue signals and thus normally comprises only a single con¬ ductor, while the electrical signal path c, d, i, e and f is arranged for digital signals and will thus comprise a number of parallel conductors, for example 8 or 12 con¬ ductors, depending on the bit number from the A/D converter 17. Electrical feedback is derived from a tap 25 in section f in the digital signal path between the summing circuit 21 and the D/A converter 19, which means that the electrical, digital feedback signal comprises a noise-level component. The feedback signal is led through an adaptive filter 27 which is shown as a "limited impulse response filter", a so-called FIR filter (Finite - Impulse - Response filter), and after passing through this filter, the feedback signal is fed to the digital subtraction circuit 23 via a digital signal path m. Preferably, the digital signal from the tap 25 is fed via a delay circuit 29 before being fed to the FIR filter 27 as a digital signal 41 via the digital lead k. The delay in the delay circuit 29 is of the same order as the minimum acoustic path length between the transducer 11 and the microphone 5, and must introduce a delay which corresponds hereto. It is not necessary to introduce such a delay by means of the delay circuit 29, but significant redundancy in filters and correlation circuits is hereby avoided, so that the overall circuit is simplified. The impulse response from the filter 27 is continuously adjust- ed, controlled by coefficients from a correlation circuit 31. The correlation circuit constantly seeks for correla¬ tion between the inserted digital noise and any noise com¬ ponent in the residual signal in the connection d after the digital subtraction circuit 23. The inserted noise signal N is generated from a noise source 33 and is introduced via the digital summing circuit 21 after level adjustment in the regulation circuit 35. The noise signal is also coupled to a reference input on the correlation circuit 31 via a second delay circuit 37, which also introduces a de¬ lay of the same order as the minimum acoustic path length between the transducer 11 and the microphone 5 via the sig¬ nal path n. The residual signal on the lead d constitutes the input signal on the correlation circuit 31, in that the signal is fed hereto from a point 39 on the lead d and by means of a digital lead. In addition to the above, there is inserted a circuit 79 in the form of an algorithm control circuit which determines the algorithm in accordance with which the correlation cir¬ cuit 31 must send coefficients further to the filter 27, in that the algorithm control circuit 79, via the digital con¬ nections 80, 81, constantly monitors and controls the cor¬ relation circuit 31. The algorithm control circuit 79 also controls the supply of digital noise from the noise gener¬ ator 33 by regulating the level in the circuit 35 via the leads 82 and a digital calculation unit 65. Moreover, the residual signal is fetched from the tap 39 via the lead 84, the amplitude of the noise signal is fetched via the lead 83, and the volume signal is fetched via the lead 86, which is explained later. The electrical output signal from point 25 is thus fed via the delay circuit 29 to the adaptive filter 27 (FIR), and to the subtraction circuit 23 as the final feedback signal, where the subtraction from the input signal is carried out. In an optimum situation, the feedback signal will corres¬ pond completely to an undesired acoustic feedback signal which, via a feedback path w, is conducted from the trans¬ ducer 11 to the microphone 5. If the feedback signal and the signal from the acoustic feedback are completely ident¬ ical, there will be no residual signal from the acoustic feedback on the lead d, the reason being that the digital feedback signal from the lead m will completely cancel out the acoustic feedback signal. In order for the filter 27 to be able to be set correctly, the noise signal N is added to the output signal via the summing circuit 21 after level regulation in the circuit 35. The noise signal will thus exist in both the inner feedback circuit 3 and the outer acoustic feedback path w. The noise signal will thus pass the D/A converter 19 and, via the amplifier 9, reach the transducer 11 and be con¬ verted to an acoustic signal which is superimposed on the desired signal. The level of the noise signal is set in such a manner that it is of no inconvenience to the user of the hearing aid. In practice, the two said signals do not cancel each other out completely, and a small amount of noise and other feed- back signals are to be found in the residual signal on the digital lead d, and these are detected by the correlation circuit 31 which constantly looks for correlation between the residual signal and the delayed version of the noise signal n. The output signal from the correlation circuit 31 is an expression for the residual signal, and is used for controlling the filter 27 by changing the filter coeffi¬ cients. The adaptation is thus arranged that the filter 27 is constantly adjusted so that the feedback system seeks towards a situation in which the noise is cancelled. Phys- ical changes in the environment for the hearing aid and its user, and limitations in the algorithm which controls the system, give rise to the result that complete cancellation cannot always be achieved, which is why the algorithm con¬ trol circuit 79 is inserted. Further details of a hearing aid according to the invention shown in fig. 1 of the drawing, and comprising a user- operated volume control 73 and a similarly user-operated adjustment rheostsat 75 for the setting of the level in the limiter circuit 15. In a hearing aid there is normally a volume control which can be operated by the user. This can be placed in the microphone amplifier or in front of the output amplifier, but in both cases the adaptive filter 27 must change its coefficients when the setting of the volume control is changed. In fig. 1 is shown a multiplication amplifier 77 between the tap 39 and the amplitude limiting circuit 15. The amplifier 77 is coupled to the volume control 73 via an A/D converter 67, and from the input to the amplifier 77 there is a digital lead 86 for the algorithm control cir¬ cuit 79 so that this circuit can scan the volume setting. The amplitude limiter 15 can also be user-operated, in that the potentiometer 75 is coupled to the amplifier 15 via an A/D converter 69. It is desirable that the limiter 15 is user-operated, since the limiting circuit determines the maximum sound-pressure level which can be applied to the user's ear. The output level can be reduced without reduc¬ ing the gain of the amplifier, which is of significance. The maximum positive and negative sound pressure is thus regulated by the user with the potentiometer 75. Fig. 1 also shows that the two potentiometers 73 and 75 are con¬ nected to a common source of reference voltage 71. As mentioned above, the level of the inserted noise can be regulated to obtain optimum adaptation. In fig. 1 it is seen that the amplifier 35 after the noise generator 33 is controlled by a computation unit 65, for example in the form of a single-stage recursive filter. The unit 65 is coupled via the two-way connection 82, 83 to the algorithm control unit 79, so that the unit 79 can fetch the noise amplitude from the unit 65, and such that the signal/noise ratio can be regulated by the algorithm control unit 79. In order to be sure that the hearing aid with built-in di- gital feedback does not begin to oscillate of its own ac¬ cord, it must be ensured that the updating in the correla¬ tion circuit 31 is effected on the basis of an algorithm which takes into consideration that errors in the filter depend upon: The number of coefficients, signal/noise ratio, input level, the volume and the extent to which the signal is peak clipped, which is explained in more detail in the applicant's earlier application no. 432/92. Figure 2 shows the same hearing aid as figure 1, but the circuit comprises a further digital circuit 210, the func¬ tion of which is to measure and calculate the loop gain, and to regulate the hearing aid's amplification if this is greater than or equal to K. A digital multiplication cir¬ cuit 211 for the regulation of the hearing aid's amplifica- tion is introduced before the amplification limiting cir¬ cuit 15 and after the digital multiplication circuit 77. The circuit 210 receives information concerning the filter coefficients from the correlation circuit 31, and informa- tion concerning the setting of the user-operated volume control 73, in that the digital output signal from the A/D converter 67 is led to the additional digital circuit 210 via the digital lead 203. At a number of frequencies, the digital circuit 210 carries out a calculation of the loop gain, and controls the algo- rithm control circuit 79 by means of the digital lead 202, and also increases or reduces the amplification by multi¬ plying digital values via the multiplication circuit 211. If it is possible, due to the digital feedback circuit in figure 1, to achieve an increased maximum amplification of 15 dB, the situation during use can be that the user has already increased the amplification by means of the volume control 73, so that the system, for example, is further capable of providing 10 dB extra amplification. If a sudden change in the undesired feedback path w increases the feed¬ back by, e.g., 6 dB, the digital compensation circuit will perhaps not be able to neutralise this increase in the level in the feedback path, and the hearing aid will start to oscillate and will howl until the volume control 73 is screwed down or until the undesired feedback has been re¬ duced. This problem and the consequences hereof can be re¬ moved or considerably reduced with the invention, in that the circuit 210 at different predetermined frequencies carries out an approximate calculation of the actual loop amplification, and multiplies this by the the setting of the volume control 73. If the result hereof is greater than a certain value, the amplification is reduced by means of the multiplication circuit 211 to a lower level in relation to that setting which the user has effected by means of the volume control 73. When the condition with the raised level in the undesired feedback path ceases or is reduced, the circuit 210 will take care that the hearing aid's ampli¬ fication is adjusted up again, and is adjusted back to that level selected by the user if this is possible, i.e. the circuit 210 receives current information concerning the filter coefficients in the correlation circuit 31. The set¬ ting back will naturallly take place in smaller steps, partly to avoid that the hearing aid starts to oscillate again, and partly in order to ensure that the regulation is noticeable by the user to the least possible degree. At the same time that the amplification is reduced, the al¬ gorithm control circuit 79 will be coupled so that it func¬ tions in accordance with the so-called statistically safe algorithm. If one expresses: the setting of the volume control: vol, the loop amplification: Gain (FIRCOEF), a constant: K, which can be frequency-dependent, then: vol ' Gain (FIRCOEF) > K => A < 1 where A indicates that factor by which the digital circuit 211 multiplies. The circuit's total open loop gain, i.e.: vol • Gain (FIRCOEF) * A < 1, is continuously calculated and for selected frequencies, so that the digital circuit 210 constantly carries out the re¬ gulation of A.

Claims: C L A I M S 1. Hearing aid in which acoustic feedback between the transducer (11) and the microphone (5) is compensated for electronically by means of an electrical feedback signal produced using an adjustable digital filter (27, 31), the coefficients of which are adusted in accordance with the actual acoustic feedback, and where the microphone signal is converted to digital signals (17) which pass an ampli- tude limiting circuit (15) which is arranged to prevent the transducer from entering its non-linear range, and where a digital noise signal (33, 21) and a digital compensation signal (27, 23) are added to the microphone signal, after which the composite signal is fed to a digital-analogue converter (19) from where the analogue signal is fed to the transducer (11) via an amplifier (9), c h a r a c t e r ¬ i z e d in that it further comprises a digital circuit (210) which is coupled to the digital filter (27, 31) for the purpose of scanning the current filter coefficients and thereby monitor the hearing aid's loop gain, and in that in the hearing aid's digital signal path between the circuit's A/D converter (17) and the circuit's D/A converter (19) there is placed at least one digital multiplication circuit (211), which is coupled to said additional digital circuit (210), and on the basis of which regulates the amplifica¬ tion of the hearing aid. 2. Hearing aid according to claim 1, c h a r a c t e r ¬ i z e d in that it further comprises a digital circuit (79) which in accordance with at least one certain function monitors and controls the updating of the digital filter (27), in that the digital circuit (210) is coupled hereto. 3. Hearing aid according to claim 1 or 2 and comprising a user-operated volume control (73) which, via an A/D con¬ verter (67), regulates the amplification in the hearing aid, c h a r a c t e r i z e d in that the additional di¬ gital circuit (210) is coupled (203) to the volume control circuit so that it receives digital signals which represent the setting of the volume control.

Inventors: HANSEN ROY SKOVGAARD, HANSEN, ROY, SKOVGAARD
Applicants: GN DANAVOX AS, HANSEN ROY SKOVGAARD, GN DANAVOX A/S
Application year: 1993
Main classification: H04R3
IPC codes: H04R3, H04R25