Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority, under 35 U.S.C. §119, of German application DE 10 2010 007 336.9, filed Feb. 9, 2010; the prior application is herewith incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method for compensating for a feedback signal in a hearing device with an input-transducer apparatus, a signal-processing apparatus and an output-transducer apparatus, by compensating for a feedback signal, which is fed back to the input-transducer apparatus from the output-transducer apparatus or the signal-processing apparatus. Moreover, the present invention relates to a corresponding hearing device. Here a hearing device is understood to mean any instrument that can be worn in or on the head and emits sound, more particularly a hearing aid, a headset, headphones or the like. 
     Hearing aids are portable hearing devices used to support the hard of hearing. In order to make concessions for the numerous individual requirements, different types of hearing aids are provided, e.g. behind-the-ear (BTE) hearing aids, hearing aids with an external receiver (receiver in the canal [RIC]) and in-the-ear (ITE) hearing aids, for example concha hearing aids or canal hearing aids (ITE, CIC) as well. The hearing aids listed in an exemplary fashion are worn on the concha or in the auditory canal. Furthermore, bone conduction hearing aids, implantable or vibrotactile hearing aids are also commercially available. In this case, the damaged sense of hearing is stimulated either mechanically or electrically. 
     In principle, the main components of hearing aids are an input transducer, an amplifier and an output transducer. In general, the input transducer is a sound receiver, e.g. a microphone, and/or an electromagnetic receiver, e.g. an induction coil. The output transducer is usually configured as an electroacoustic transducer, e.g. a miniaturized loudspeaker, or as an electromechanical transducer, e.g. a bone conduction receiver. The amplifier is usually integrated into a signal-processing unit. This basic configuration is illustrated in  FIG. 1  using the example of a behind-the-ear hearing aid. One or more microphones  2  for recording the sound from the surroundings are installed in a hearing aid housing  1  to be worn behind the ear. A signal-processing unit  3 , likewise integrated into the hearing aid housing  1 , processes the microphone signals and amplifies them. The output signal of the signal-processing unit  3  is transferred to a loudspeaker or receiver  4 , which emits an acoustic signal. If necessary, the sound is transferred to the eardrum of the equipment wearer using a sound tube, which is fixed in the auditory canal with an ear mold. A battery  5  likewise integrated into the hearing aid housing  1  supplies the hearing aid and in particular the signal-processing unit  3  with energy. 
     One of the greatest problems of hearing aids is the occurrence of feedback, which is often expressed as feedback whistling. Here, the sound leaving the loudspeaker of the hearing aid finds an acoustic feedback path to the microphones and is amplified again, leading to the typical whistling or resonance effects. Modern hearing systems are able to match the feedback path to the facial expression of the user and to compensate for the feedback signal in an appropriate fashion; the corresponding unit of the hearing system is called a feedback compensator. 
     As will be illustrated below, an adaptive filter (the feedback compensator) simulates the acoustic feedback path by minimizing the energy after the subtraction point. The problem here is that the desired signal or useful signal forms the unwanted signal from the point of view of the feedback compensator. Moreover, the useful signal is usually strongly correlated with the feedback signal as a result of the amplification caused by the hearing aid, and so it is almost impossible to distinguish between the feedback signal and the useful signal. 
     Hence, it is very important to set the adaptation speed for the feedback path correctly. If the adaptation is too slow, feedback whistling is audible for a certain period of time. If the adaptation is too fast, this results in so-called musical artifacts, i.e. the feedback compensator attempts to suppress the useful signal. Feedback detectors are often required for the correct adaptation speed. Moreover, the performance of the feedback detector is very important for the performance of the entire feedback compensator. 
     A typical configuration of a feedback compensator in a hearing aid with a feedback detector is illustrated in  FIG. 2 . A microphone  10  records a sound signal and transmits it to a signal-processing apparatus  11 . The output signal resulting from the signal-processing apparatus  11  is transmitted to an output transducer or loudspeaker  12 . The sound  13  leaving the loudspeaker partly advances to the eardrum or ear, and the other part is fed back as feedback signal  14  to the microphone  10  via the respectively current feedback path  15 . The fed-back sound is added to the useful signal  16 , and the sum provides the acoustic input signal for the microphone  10 . 
     The signal-processing apparatus  11  has a conventional signal processor  17  and a feedback compensator  18 . Provision is moreover made for a feedback detector  19 . The output signal from the signal processor  17  is fed to both the loudspeaker  12  and the feedback compensator  18 . The latter simulates the feedback path and supplies a corresponding compensation signal, which is subtracted from the signal from the microphone  10  by a subtractor  20 . The resulting signal is provided as an input signal to the signal processor  17 . The signal is moreover used for generating the feedback signal in the feedback compensator  18 . 
     The signal  30  from the microphone  10  and the difference signal  40  after the subtractor  20  are fed to the feedback detector  19 , which determines whether or not there is a feedback situation. The feedback compensator  18  and, if need be, the signal processor  17  as well are controlled as a function of this decision. The feedback compensator  18  often is an adaptive filter, which attempts to simulate the acoustic feedback path. Ideally, the feedback compensator  18  filters the output signal from the signal processor  17  like the acoustic feedback path  15 . This leads to a complete suppression of the feedback signal  14  at the subtractor  20 . However, the feedback compensator  18  is often mismatched or simply too slow for the rapid change in the feedback path. Hence, there often is the need for one or more feedback detectors  19  for adapting the adaptation speed of the feedback compensator  18 . These feedback detectors  19  usually analyze either the microphone signal  30  before the subtractor  20  or the compensated signal  40  after the subtractor  20 , which compensated signal should be without feedback. As already indicated above, the signal processor  17  can likewise be influenced such that feedback whistling is avoided, for example by reducing the amplification. 
     The now described detection methods are used in current feedback detectors. 
     1. Channel-Level-Based Detection. 
     Comparing the signal levels in different frequency channels allows feedback whistling to be detected by either searching for level peaks or classifying certain levels in particular frequency bands as feedback. 
     2. Detection Based on Sinusoidal Signal Components. 
     There are a number of methods for detecting sinusoidal signal components. If a sinusoidal signal component is detected in a feedback-critical frequency range, this indicates feedback. 
     3. Detection of a Phase Modulation. 
     The best method for detecting feedback is the detection of a phase modulation or frequency modulation to which the output signal from the hearing aid loudspeaker was subjected. In the process, the output-signal phase is modulated by a low, inaudible frequency. If precisely this frequency is detected at the input (microphone) as a phase modulation, it is a feedback signal in all probability. This method is the most robust feedback-detection method; in particular also in respect of false detections of the useful signal. 
     A problem in all these approaches is that there needs to be a high level of feedback whistling in order to be able to detect the feedback at all. The detection of the phase modulation also requires an input signal with a stable phase (a sinusoidal signal) in order to detect a modulation of this phase. This means that feedback whistling is necessary for suppressing the latter. None of the above methods are able to avoid the whistle in its entirety. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method for compensating for a feedback signal, and a hearing device which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, which recognizes a feedback situation as quickly as possible and, if need be, taking appropriate compensation steps. To this end, provision should be made for a corresponding method and a corresponding hearing device. 
     According to the invention, the object is achieved by a method for compensating for a feedback signal in a hearing device having an input-transducer apparatus, a signal-processing apparatus and an output-transducer apparatus. The method includes:
     a) compensating for a feedback signal, which is fed back to the input-transducer apparatus from the output-transducer apparatus or the signal-processing apparatus by establishing a probability of having a plurality of notches, equally spaced apart from one another, in the spectrum of an input signal, which originates directly from the input-transducer apparatus or which is a difference signal between the signal directly from the input-transducer apparatus and a compensation signal serving for compensation; and   b) modifying the compensation or an amplification of the signal-processing apparatus as a function of the established probability.   

     Moreover, according to the invention, provision is made for a hearing device. The hearing device includes an input-transducer apparatus, a signal-processing apparatus for processing the input signal emitted by the input-transducer apparatus to form an output signal, an output-transducer apparatus for converting the output signal into an acoustic output signal, and a compensation apparatus for compensating for a feedback signal, which is fed back to the input-transducer apparatus from the output-transducer apparatus or the signal-processing apparatus. A detection apparatus is provided for establishing a probability of the spectrum of the input signal having a plurality of notches, equally spaced apart from one another. The compensation apparatus can be controlled in dependence on an established probability. 
     In this case, “establishing a probability” is also understood to mean the “detection” (i.e. 100% probability) of notches (peaked minima). Thus, a feedback situation can advantageously be recognized simply by virtue of the fact that equally spaced-apart notches are detected in the transfer function and their distance to a transfer function is determined in the case of compensated feedback. Corresponding compensation can then be initiated as a function thereof, without feedback whistling having already occurred. 
     The probability is preferably established in a pause in the speech during the intended operation of the hearing device. This is because there generally is no useful signal, which could adversely affect the adaptation and the detection, during a pause in the speech. 
     The transfer function from the input signal to the output signal can correspond to a comb filter. If the feedback signal is taken into account, this then results in a constant transfer function for the useful signal. 
     Furthermore, the probability can be established in a noisy frequency range of the input signal. This generally provides a broadband input-signal, in which numerous notches are able to develop clearly. 
     The feedback signal can be verified by virtue of the fact that the output signal is frequency modulated or phase modulated and the notches are analyzed in respect of the frequency modulation or phase modulation. This can increase the reliability of the decision relating to the presence of a feedback situation. 
     The compensation is advantageously brought about by an adaptive filter and the adaptation speed is modified in dependence on the established probability. 
     More particularly, the compensation can be modified to the effect that the transfer function of a compensated signal, created by mixing the input signal with a compensation signal for compensating for the feedback signal, to the output signal is substantially without a gradient in the greatest part of a prescribed spectral range, which should be influenced by the compensation. If this is the case, an ideal compensation of the feedback signal has been achieved. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a method for compensating for a feedback signal, and a hearing device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a diagrammatic illustration of a hearing aid according to the prior art; 
         FIG. 2  is a block diagram of the hearing aid according to the prior art; 
         FIG. 3  is a graph showing a transfer function of a microphone signal at 100% compensation; 
         FIG. 4  is a graph showing a transfer function of a compensated signal at 100% compensation; 
         FIG. 5  is a graph showing a transfer function of the microphone signal at 80% compensation; 
         FIG. 6  is a graph showing a transfer function of a compensated signal at 80% compensation; 
         FIG. 7  is a graph showing a transfer function of a microphone signal at 50% compensation; 
         FIG. 8  is a graph showing a transfer function of a compensated signal at 50% compensation; 
         FIG. 9  is a graph showing a transfer function of a microphone signal at 30% compensation; 
         FIG. 10  is a graph showing a transfer function of the compensated signal with 30% compensation; and 
         FIG. 11  is a block diagram of a hearing aid according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The exemplary embodiments explained in more detail below constitute preferred embodiments of the invention. 
     The basic approach of the present invention consists of being able to detect a mismatch with respect to the feedback path without there being an audible feedback whistling. The invention utilizes the comb-filter effect, which is based on the superposition of a useful signal with a feedback signal. If two correlated signals are added with a small delay, this leads to destructive or constructive superposition, and notches or peaks can be identified in the frequency response (compare  FIG. 3 ). If the feedback compensator (FBC) is adapted in an ideal fashion (100% compensation), the transfer function TM of the microphone signal  30 , originating from the microphone  10  (compare  FIG. 2 ), is a finite impulse response from a comb filter with a typical distribution of approximately equally spaced-apart notches  21 . The transfer function TC of the compensated signal  40  to the compensated output signal is ideally completely flat, as illustrated in  FIG. 4 . It has no gradient and is constant over the entire observed frequency range (between 2000 and 4000 Hz in this case). 
     If, on the other hand, the feedback compensator  18  has been mismatched, the transfer function TM of the microphone signal  16  to the output signal  13  is an infinite impulse response from a comb filter with a typical distribution with significant frequency peaks. The feedback compensation is at 80% in  FIGS. 5 and 6 . Thus, there is a mismatch of 20%. Compared to the image in  FIG. 3 , the frequency peaks  22  in the transfer function TM of the microphone signal  30  to the output signal  13  are already slightly developed in  FIG. 5 . This mismatch leads to the transfer function TC of the compensated signal to the output signal  13  no longer being completely flat, as indicated in  FIG. 6 . 
     If there is a further increase in the mismatch, the transfer functions as per  FIGS. 7 and 8  result in the case of a compensation of 50%. The equally spaced-apart frequency peaks  23  can now be clearly identified in  FIG. 7 , i.e. there are clear constructive superpositions of the feedback signal  14  and the useful signal  16  in the frequency ranges of the frequency peaks  23 . 
     If the mismatch increases further, and the feedback compensation for example now is only 30%, this results in the transfer functions in  FIGS. 9 and 10 . Clearly developed frequency peaks  25  can now be identified in the transfer function TM of the microphone signal. The transfer function TC of the compensated signal  40  then likewise has significant peaks  26 , which are likewise equally spaced apart from one another. 
     Therefore, if there is complete feedback compensation (100%), there are notches in the transfer function TM of the microphone signal  30  and the transfer function TC of the compensated signal  40  is completely flat, i.e. the feedback has been perfectly compensated for. The smaller the degree of compensation becomes, the more peaks can be identified in the transfer functions, which peaks exceed the function mean. These peaks are an indication that feedback whistling will occur or has already occurred. Hence, the advantage of the comb-filter effect is that the reduction in the degree of compensation from 100% to 0% can easily be identified in the transfer functions. 
       FIGS. 3 to 10  show that the transfer function TM from the microphone signal  30  is primarily affected by notches  21  (minima with respect to the function mean) at 100% compensation, while the transfer function is mainly affected by frequency peaks  26  (maxima with respect to the function mean) at low compensation (30%). There is a smooth transition between the notch-affected transfer function and the peak-affected transfer function. The transition can be observed without audible artifacts having already occurred. The basic idea of the present invention is based on this. 
     The problem occurring when utilizing this effect is that it is only possible to observe the response level of the microphone signal  16  or the response level of the compensated output signal  13 , but not the transfer function TM of the microphone signal  30 . This means that all that is obtained is a convolution of the useful signal with the above-described transfer function TM. It follows that there is a need for robust detection methods, which are explained in more detail below. 
     The methods described below generally are independent of one another and can be used both individually and in combination. Most methods are based on the detection of notches or peaks in the frequency spectrum. There are a number of standard methods for this detection, in which methods either the spectrum itself can be observed with a high-resolution FFT or a plurality of adaptive notch/peak detectors or the like can be used. Use is not made of a specific method in this case; rather, the assumption is made that notch/peak detectors are available, which calculate a type of notch/peak probability. 
     1. Notch/Peak Spacing: 
     The aforementioned text alludes to the fact that there is a typical spacing between the notches or the peaks. The spacing results from the overall delay of the closed loop, which delay is usually a sum of the hearing-aid delay and the feedback-path delay. This delay is characteristic of a particular situation and hardly changes. Using this as a starting point, it is proposed to detect successive notches/peaks. If their spacing lies within a certain range, the assumption is made that the notches/peaks originate from the comb-filter effect and not from the useful signal. If the signal is more likely to have notches, the feedback compensator  18  has been adapted well. If it is more likely for peaks to occur, the compensator has been adapted badly. A threshold can be defined for this probability and it can be used to make a decision with respect to increasing the adaptation speed of the feedback compensator or reducing the amplification. 
     2. Detection in Pauses in the Speech: 
     It is advantageous to use the notch-peak detection in pauses in the speech only. In the process, the assumption may be made that the current useful signal corresponds to noise, and the detection of a notch directly allows deduction of the fact that the feedback compensation is operating well. 
     3. Detection in Noisy Frequency Ranges: 
     Furthermore, it is expedient to utilize the notch detection in noisy frequency ranges. These frequency ranges are not influenced by a useful signal, but only by background noise. It follows that notches in these frequency ranges allow deduction of the fact that the feedback compensation is operating well. 
     4. Comparison of Detection in the Microphone Input Signal and in the Compensated Output Signal: 
     It emerges from the aforementioned transfer functions that there usually is a clear difference between the microphone transfer function TM and the compensated transfer function TC. It is proposed that the notch/peak detection be applied to both signals  30 ,  40  (compare  FIG. 2 ). If a difference is determined during the intended operation of the hearing aid, it is very likely that the feedback compensation generates an appropriate performance. The difference can clearly be identified at 100% compensation in particular (compare  FIGS. 3 and 4 ). If the notches are detected in the spectrum of the microphone signal  30 , and there are no corresponding notches in the spectrum of the compensated signal  40 , then the feedback compensation is operating as desired. 
     5. Modulated Notches: 
     In order to verify that a notch is the result of the comb-filter effect, the output signal can also be subjected to an inaudible phase modulation (or frequency modulation). This phase modulation will lead to a modulation in the notch/peak frequencies. Use can then be made of a suitable notch/peak detector, by means of which the notch/peak frequency can be observed over time. If this frequency has the same modulation frequency as the phase modulation, the comb-filter effect is verified. This method is the most robust in respect of the useful signal. 
     The aforementioned methods can be used to assess the quality of the feedback adaptation. If the actual feedback path changes and the adapted, simulated feedback path no longer fits, the notches in the signal change to form small peaks. This allows the definition of a suitable threshold, by means of which the feedback path can be optimized before the hearing aid starts to whistle, or by means of which the amplification can be reduced before the aid starts to whistle. Therefore, the advantage of utilizing the comb-filter effect consists of being able to predict the occurrence of feedback whistling before the latter commences. Hence the feedback path can be adapted early enough for preventing the whistling. The invention therefore consists in examining the input signal in respect of contained comb-filter components in order to detect feedback-critical states at an early stage. In order to identify the comb filters unambiguously as the result of the input loudspeaker or receiver signal, a plurality of options have been described above. Probably the most reliable option is a combination made of the conventional so-called “phase shaker”, in which use is made of the modulation of the output signal. A modulation is impressed onto the output signal in a conventional fashion, which then leads to an oscillatory motion of the notches in the frequency response of the input signal. Hence a further feature is obtained for identifying feedback. 
       FIG. 11  shows an implementation of the above-described method for establishing a change in a feedback situation or for adaptation to a changed feedback situation in a hearing aid. The design of the hearing device including the feedback path  15  substantially corresponds to that of  FIG. 2 . Hence reference is made to the description of  FIG. 2  in respect of the components and reference signs that are the same in both figures. In place of the feedback detector  19 , the hearing aid in  FIG. 11  has a notch detector  24 , a threshold-decision unit  27 , a modulation detector  28  and an AND-element  29 . The notch detector  24  records the microphone signal  30  and establishes a probability w of a notch (i.e. peaked minimum) and the corresponding frequency f of the notch from this. The threshold-decision unit  27  decides whether there is a deviation from the ideal case by comparing the probability w to a threshold. An appropriate output signal is fed to the AND-element  29 . 
     The notch detector  24  feeds the notch frequency f to the modulation detector  28 . The latter examines whether the notch frequency f is undergoing an oscillatory motion. An appropriate output signal is guided to the AND-element  29 . If the respective conditions are satisfied in the two decision units  27  and  28 , the feedback compensator  18  is actuated appropriately by the output signal from the AND-element  29 , e.g. the adaptation speed is modified. 
     In order to verify the feedback situation, the hearing aid has a phase modulator  31  downstream of the signal processor  17 , which phase modulator modulates the phase of the output signal to the loudspeaker  12 . If there is a feedback situation, the feedback signal  14  likewise is phase-modulated and the modulation over the signal path through the microphone  10  and the notch detector  24  can be registered in the modulation detector  28 . If there is a modulation, and the probability of a notch falls below a certain threshold (see  FIGS. 5 ,  7  and  9 ), the adaptation speed of the feedback compensator is increased.

Technology Category: 5