Abstract:
The present invention combines audio compression and feedback cancellation in an audio system such as a hearing aid. The feedback cancellation element of the present invention uses one or more filters to model the feedback path of the system and thereby subtract the expected feedback from the audio input signal before hearing aid processing occurs. The hearing aid processing includes audio compression, for example multiband compression. The operation of the audio compression element may be responsive to information gleaned from the feedback cancellation element, the feedback cancellation may be responsive to information gleaned from the compression element, or both.

Description:
[0001]    Pending patent application Ser. No. 08/870,426, filed Jun. 6, 1997 and entitled “Spectral Sampling Multiband Audio Compressor,” pending patent application Ser. No. 08/972,265, filed Nov. 18, 1997 and entitled “Feedback Cancellation Apparatus and Methods,” and pending patent application Ser. No. 08/540,534, filed Oct. 10, 1995 and entitled “Digital Signal Processing Hearing Aid” are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to apparatus and methods for combining audio compression and feedback cancellation in audio systems such as hearing aids.  
           [0004]    2. Description of the Prior Art  
           [0005]    Mechanical and acoustic feedback limits the maximum gain that can be achieved in most hearing aids. System instability caused by feedback is sometimes audible as a continuous high-frequency tone or whistle emanating from the hearing aid. Mechanical vibrations from the receiver in a high-power hearing aid can be reduced by combining the outputs of two receivers mounted back-to-back so as to cancel the net mechanical moment; as much as 10 dB additional gain can be achieved before the onset of oscillation when this is done. But in most instruments, venting the BTE earmold or ITE shell establishes an acoustic feedback path that limits the maximum possible gain to less than 40 dB for a small vent and even less for large vents. The acoustic feedback path includes the effects of the hearing aid amplifier, receiver, and microphone as well as the vent acoustics.  
           [0006]    The traditional procedure for increasing the stability of a hearing aid is to reduce the gain at high frequencies. Controlling feedback by modifying the system frequency response, however, means that the desired high-frequency response of the instrument must be sacrificed in order to maintain stability. Phase shifters and notch filters have also been tried, but have not proven to be very effective.  
           [0007]    A more effective technique is feedback cancellation, in which the feedback signal is estimated and subtracted from the microphone signal. One particularly effective feedback cancellation scheme is disclosed in patent application Ser. No. 08/972,265, entitled “Feedback Cancellation Apparatus and Methods,” incorporated herein by reference.  
           [0008]    Another technique often used in hearings aids is audio compression of the input signal. Both single band and multiband dynamic range compression is well known in the art of audio processing. Roughly speaking, the purpose of dynamic range compression is to make soft sounds louder without making loud sounds louder (or equivalently, to make loud sounds softer without making soft sounds softer). Therefore, one well known use of dynamic range compression is in hearing aids, where it is desirable to boost low level sounds without making loud sounds even louder.  
           [0009]    The purpose of multiband dynamic range compression is to allow compression to be controlled separately in different frequency bands. Thus, high frequency sounds, such as speech consonants, can be made louder while loud environmental noises—rumbles, traffic noise, cocktail party babble—can be attenuated.  
           [0010]    Patent application Ser. No. 08/540,534, entitled “Digital Signal Processing Hearing Aid,” incorporated herein by reference, gives an extended summary of multiband dynamic range compression techniques with many references to the prior art.  
           [0011]    Patent application Ser. No. 08/870,426, entitled “Continuous Frequency Dynamic Range Audio Compressor,” incorporated herein by reference, teaches another effective multiband compression scheme.  
           [0012]    A need remains in the art for apparatus and methods to combine audio compression and feedback cancellation in audio systems such as hearing aids.  
         SUMMARY OF THE INVENTION  
         [0013]    The primary objective of the combined audio compression and feedback cancellation processing of the present invention is to eliminate “whistling” due to feedback in an unstable hearing aid amplification system, while make soft sounds louder without making loud sounds louder, in a selectable manner according to frequency.  
           [0014]    The feedback cancellation element of the present invention uses one or more filters to model the feedback path of the system and thereby subtract the expected feedback from the audio signal before hearing aid processing occurs. The hearing aid processing includes audio compression, for example multiband compression.  
           [0015]    As features of the present invention, the operation of the audio compression element may be responsive to information gleaned from the feedback cancellation element, the feedback cancellation may be responsive to information gleaned from the compression element, or both.  
           [0016]    A hearing aid according to a first embodiment of the present invention comprises a microphone for converting sound into an audio signal, feedback cancellation means including means for estimating a physical feedback signal of the hearing aid, and means for modelling a signal processing feedback signal to compensate for the estimated physical feedback signal, subtracting means, connected to the output of the microphone and the output of the feedback cancellation means, for subtracting the signal processing feedback signal from the audio signal to form a compensated audio signal, a hearing aid processor including audio compression means, connected to the output of the subtracting means, for processing the compensated audio signal, and a speaker, connected to the output of the hearing aid processor, for converting the processed compensated audio signal into a sound signal.  
           [0017]    In a second embodiment, the feedback cancellation means provides information to the compression means, and the compression means adjusts its operation in accordance with this information. For example, an increase in the magnitude of the zero coefficient vector can indicate the presence of an incoming sinusoid, which is likely due to feedback oscillations in the hearing aid. The maximum gain of the audio compression at low levels can be reduced if the feedback cancellation means detects an increase in the magnitude of the zero coefficient vector.  
           [0018]    In a third embodiment, the compression means provides information, for example input signal power levels at various frequencies, to the feedback cancellation means, and the feedback cancellation element adjusts its operation in accordance with this information. For example, the feedback cancellation adaptation constant can be adjusted based upon the power level of one or more of the frequency bands of the audio compressor. For example, the adaptation time constant of the feedback cancellation element could be adjusted based on the output of one of the compression bands or a weighted combination of two or more bands.  
           [0019]    In a fourth embodiment, the compression means provides information to the feedback cancellation means, and the feedback cancellation means provides information to the compression means, and each element adjusts its operation in accordance with the information obtained from the other. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 (prior art) is a flow diagram showing a hearing aid incorporating multiband audio compression.  
         [0021]    [0021]FIG. 2 (prior art) is a block diagram showing a hearing aid incorporating feedback cancellation.  
         [0022]    [0022]FIG. 3 is a block diagram showing a hearing aid according to the present invention, incorporating compression and feedback cancellation.  
         [0023]    [0023]FIG. 4 is a block diagram showing a hearing aid according to the present invention, incorporating compression and feedback cancellation, wherein the compression element modifies its operation according to information from the feedback cancellation.  
         [0024]    [0024]FIG. 5 is a block diagram showing a hearing aid according to the present invention, incorporating compression and feedback cancellation, wherein the feedback cancellation element modifies its operation according to information from the compression element.  
         [0025]    [0025]FIG. 6 is a flow diagram showing a hearing aid according to the present invention, incorporating compression and feedback cancellation, wherein the compression element modifies its operation according to information from the feedback cancellation, and the feedback cancellation element modifies its operation according to information from the compression element. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    [0026]FIG. 1 (prior art) is a flow diagram showing an example of a hearing aid  10  incorporating multiband audio compression  40 . This invention is described in detail in patent application Ser. No. 08/870,426, entitled “Spectral Sampling Multiband Audio Compressor.” An audio input signal  52  enters microphone  12 , which generates input signal  54 . Signal  54  is converted to a digital signal by analog to digital converter  15 , which outputs digital signal  56 . This invention could be implemented with analog elements as an alternative. Digital signal  56  is received by filter bank  16 , which is implemented as a Short Time Fourier Transform system, where the narrow bins of the Fourier Transform are grouped into overlapping sets to form the channels of the filter bank. However, a number of techniques for constructing filter banks in the frequency domain or in the time domain, including Wavelets, FIR filter banks, and IIR filter banks, could be used as the foundation for filter bank design.  
         [0027]    Filter bank  16  filters signal  56  into a large number of heavily overlapping bands  58 . Each band  58  is fed into a power estimation block  18 , which integrates the power of the band and generates a power signal  60 . Each power signal  60  is passed to a dynamic range compression gain calculation block, which calculates a gain  62  based upon the power signal  60  according to a predetermined function.  
         [0028]    Multipliers  22  multiply each band  58  by its respective gain  62  in order to generate scaled bands  64 . Scaled bands  64  are summed in adder  24  to generate output signal  68 . Output signal  68  may be provided to a receiver (not shown) in hearing aid  10  or may be further processed.  
         [0029]    [0029]FIG. 2 (prior art) is a block diagram showing a hearing aid incorporating feedback cancellation. This invention is described in detail in patent application Ser. No. 08/972,265, entitled “Feedback Cancellation Apparatus and Methods. Feedback path modelling  250  includes the running adaptation of the zero filter coefficients. The series combination of the frozen pole filter  206  and the zero filter  212  gives a model transfer function G(z) determined during start-up. The coefficients of the pole model filter  206  are kept at values established during start-up and no further adaptation of these values takes place during normal hearing aid operation. Once the hearing aid processing is turned, on zero model filter  212  is allowed to continuously adapt in response to changes in the feedback path as will occur, for example, when a telephone handset is brought up to the ear.  
         [0030]    During the running processing shown in FIG. 2, no separate probe signal is used, since it would be audible to the hearing aid wearer. The coefficients of zero filter  212  are updated adaptively while the hearing aid is in use. The output of hearing aid processing  240  is used as the probe. In order to minimize the computational requirements, the LMS adaptation algorithm is used by block  210 . The adaptation is driven by error signal e(n) which is the output of the summation  208 . The inputs to the summation  208  are the signal from the microphone  202 , and the feedback cancellation signal produced by the cascade of the delay  214  with the all-pole model filter  206  in series with the zero model filter  212 . The zero filter coefficients are updated using LMS adaptation in block  210 .  
         [0031]    [0031]FIG. 3 is a block diagram showing a hearing aid  300  according to the present invention, incorporating compression  340  and feedback cancellation  350 . Other types of hearing aid processing, for example direction sensitivity or noise suppression, could also be incorporated into block  340 . An example of a compression scheme which could be used is shown in block  40  of FIG. 1, but the invention is by no means limited to this particular compression scheme. Many kinds of compression could be used. Similarly, an example of feedback cancellation is shown in block  250  of FIG. 2, but many other types of feedback cancellation could be used instead, including algorithms operating in the frequency domain as well as in the time domain.  
         [0032]    Microphone  202  converts input sound  100  into an audio signal. Though this is not shown, the audio signal would generally be converted into a digital signal prior to processing. Feedback cancellation means  350  estimates a physical feedback signal of hearing aid  300 , and models a signal processing feedback signal to compensate for the estimated physical feedback signal. Subtracting means  208 , connected to the output of microphone  202  and the output of feedback cancellation means  350 , subtracts the signal processing feedback signal from the audio signal to form a compensated audio signal. Compression processor  340  is connected to the output of subtracting means  208 , for processing the compensated audio signal. Speaker  220 , connected to amplifier  218  at the output of hearing aid processor  340 , converts the processed compensated audio signal into a sound signal. If the processed compensated audio signal is a digital signal, it is converted back to analog (not shown).  
         [0033]    [0033]FIG. 4 is a block diagram showing a hearing aid  400  which is very similar to hearing aid  300  of FIG. 3, except that compression element  440  modifies its operation according to information from feedback cancellation  450 . Depending upon the type of feedback cancellation, the types of information available and useful to compression block  440  will vary. Taking as an example a feedback cancellation block  450  identical to  250  of FIG. 2, the coefficients of zero model  212  will change with time as feedback cancellation  350  attempts to compensation for feedback.  
         [0034]    Testing one or more of these coefficients to determine whether they are outside expected ranges in magnitude, or are changing faster than expected, gives a clue as to whether feedback cancellation  350  is having difficulty compensating for the feedback. For example, an increase in the magnitude of the zero coefficient vector might indicate the presence of an incoming sinusoid.  
         [0035]    If it appears that feedback compensation  450  is having trouble compensating for feedback, signal  406  would indicate to compression block  440  to lower gain at low levels, either for all frequencies or for selected frequencies. Thus, if compression block  440  is identical to compression block  100  of FIG. 1, signal  406  would be used to generate a control signal for one or more gain calculation blocks  20 . For example, the gain for frequencies between 1.5 KHz and 3 KHz might be lowered temporarily, as these are often the frequencies at which hearing aids are unstable. As another example, the kneepoint between the linear amplification function of compression  440  and the compression function at higher signal levels could be moved to a higher signal level. Once the zero model coefficients begin behaving normally, the gain applied by compression  440  can be partially or completely restored to normal. As a third example, the attack and/or release times of the compression  440  could be modified in response to changes in the zero model coefficients. The compressor release time, for example, can be increased when the magnitude of the zero filter coefficient vector increases and returned to its normal value when the magnitude of the zero coefficient vector decreases, thus ensuring that the compression stays at lower gains for a longer period of time when the magnitude of the zero coefficient vector is larger than normal.  
         [0036]    [0036]FIG. 5 is a block diagram showing a hearing aid  500  which is very similar to hearing aid  300  of FIG. 3, except that feedback cancellation element  550  modifies its operation according to information from compression element  540 . For example, the adaptation time constant of feedback cancellation  550  could be adjusted based on the output of one of the compression bands.  
         [0037]    The adaptive filter (zero model  212  in FIG. 2) used for feedback cancellation  550  adapts more rapidly and converges to a more accurate solution when the hearing aid input signal is broadband (e.g. White noise) than when it is narrowband (e.g. A tone). Better feedback cancellation system performance can be obtained by reducing the rate of adaptation when a narrowband input signal is detected. The rate of adaptation is directly proportional to the parameter (in the LMS update equation below. The spectral analysis performed by the multiband compression can be used to determine the approximate bandwidth of the incoming signal. The rate of adaptation for the adaptive feedback cancellation filter weight updates is then decreased ((made smaller) as the estimated input signal bandwidth decreases.  
         [0038]    As another example, the magnitude of the step size used in the LMS adaptation  210  (see FIG. 2) can be made inversely proportional to the power in one or more compression bands, for example as determined by power estimation blocks  18  (see FIG. 1). In this particular example, the adaptive update of the zero filter weights becomes:  
             b   k          (     n   +   1     )       =         b   k          (   n   )       +         2      μ         σ   x   2          (   n   )              e        (   n   )                 (     n   -   k     )             ,                         
 
         [0039]    b k (n+1) is the kth zero filter coefficient at time n+1,  
         [0040]    e(n) is the error signal provided by subtraction means  208 ,  
         [0041]    d(n−k) is the input to the adaptive filter at time n delayed by k samples, and  
         [0042]    σ x   2  (n) is the estimated power at time n from compression  540   
         [0043]    In particular, the filtered hearing aid input power can be obtained from one of the frequency bands of compression  540  (from one of power estimation blocks  18  shown in FIG. 1, for example). This adaptation approach offers the advantage of reduced computational requirements, since the power estimate is already available from compression  540 , while giving much faster adaptation at lower signal levels than is possible with a system which does not use power normalization  506 . Feedback compensation  550  will also adjust faster when normalized based on compression  540  input power rather than feedback compensation  550  input power, because the latter signal has been compressed, raising the level of less intense signals and thus reducing the adaptation step size after power normalization.  
         [0044]    Another example of adjusting feedback compensation  550  operation based upon information from compression  540  is the following. The cross correlation calculation used in LMS adapt block  210  (see FIG. 2) can overflow the accumulator if the input signal to hearing aid  500  is too high. By testing the power level of the input signal to compression  540 , it is possible to determine whether the input signal is high enough to make such an overflow likely, and freeze the filter coefficients until the high input signal level drops to normal.  
         [0045]    The test used is whether: 
           gpσ   x   2 ( n )&lt;θ,  
         [0046]    where  
         [0047]    σ x   2  (n) is the estimated power at time n of the hearing aid input signal,  
         [0048]    g is the gain in the filter band used to estimate power,  
         [0049]    q is the gain in pole filter  206 , and  
         [0050]    θ is the maximum safe power level to avoid overflow  
         [0051]    If this test is not satisfied, the adaptive filter update is not performed for that data block. Rather, the filter coefficients are frozen at their current level until the high input signal level drops to normal.  
         [0052]    As another example, the magnitude of the step size used in the LMS adaptation  210  (see FIG. 2) can be made dependent on the envelope fluctuations detected in one or more compression bands. A sinusoid will have very little fluctuation in its signal envelope, while noise will typically have large fluctuations. The envelope fluctuations can be estimated by detecting the peaks and valleys of the signal and taking the running difference between these two values. The adaptation step size can then be made smaller as the detected envelope fluctuations decrease.  
         [0053]    [0053]FIG. 6 is a flow diagram showing a hearing aid  600  which is very similar to hearing aid  300  of FIG. 3, except that feedback cancellation element  650  modifies its operation according to information from compression element  640 , and compression element  640  modifies its operation according to information from feedback cancellation  650 .  
         [0054]    An example of this is a combination of the processing described in conjunction with FIG. 4 with that described in conjunction with FIG. 5. The power estimated by the compressor or the detected envelope fluctuations in one or more bands is used to adjust the adaptive weight update, and the magnitude of the zero filter coefficient vector is used to adjust the compression gain or the compression attack and/or release times.  
         [0055]    While the exemplary preferred embodiments of the present invention are described herein with particularity, those skilled in the art will appreciate various changes, additions, and applications other than those specifically mentioned, which are within the spirit of this invention. In particular, the present invention has been described with reference to a hearing aid, but the invention would equally applicable to public address systems, telephones, speaker phones, or any other electroacoustical amplification system where feedback is a problem.