Patent Publication Number: US-8538053-B2

Title: Hearing device with frequency shifting and associated method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation application, under 35 U.S.C. §120, of copending U.S. provisional application No. 61/299,370, filed Jan. 29, 2010, this application also claims the priority, under 35 U.S.C. §119, of German patent application No. DE 10 2010 006 154.9, filed Jan. 29, 2010; the prior applications are herewith incorporated by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for operating a hearing device and to a hearing device with improved feedback suppression through the use of an optimized frequency filter. 
     A frequent problem with hearing devices is the feedback between the output of the hearing device and the input, which makes itself evident as an annoying whistling.  FIG. 1  shows the principle of acoustic feedback. A hearing device  1  has a microphone  2  which receives an acoustic useful signal  10 , converts it into an electrical microphone signal  11  and outputs it to a signal processing unit  3 . In the signal processing unit  3  the microphone signal  11  undergoes processing such as preparation, amplification and output to an earpiece  4  as an electrical earpiece signal  12 . In the earpiece  4  the electrical earpiece signal  12  is converted back into an acoustic output signal  13 , and output to an eardrum  7  of a hearing aid wearer. 
     The problem now is that part of the acoustic output signal  13  reaches the input of the hearing device  1  via an acoustic feedback path  14  where it superimposes itself on the useful signal  10  and is picked up by the microphone  2  as a sum signal. With an unfavorable phase position and amplitude of the fed back output signal the result is annoying feedback whistling. With an open hearing aid supply in particular the attenuation of the acoustic feedback is low, which exacerbates the problem. 
     Adaptive systems for feedback suppression have been available for some time as a solution. These involve simulating the acoustic feedback path  14  in the hearing device  1  digitally. The simulation is undertaken for example by an adaptive compensation filter  5  which is fed by the earpiece signal  12 . After filtering in the compensation filter  5 , a filtered compensation signal  15  is subtracted from the microphone signal  11 . In the ideal case the effect of the acoustic feedback path  14  is canceled out by this and a feedback-free input signal  16  is produced for the signal processing unit  3 . 
     For effective feedback suppression a regulation or adaptation of filter coefficients of the adaptive compensation filter  5  is required. To this end the microphone signal  11  is evaluated with the aid of a detection unit  6  and investigated for possible feedback. Artifacts can also be produced however by the regulation or adaptation of the filter coefficients, since with an adaptive compensation filter  5  which is not set optimally, additional signal components will be created or feedback whistling will occur. European patent EP 1 033 063 B1, corresponding to U.S. Pat. Nos. 6,104,822, 6,434,246, 6,831,986, 6,097,824, 6,072,884 and 6,498,858, discloses a hearing device with feedback suppression wherein, for improving the feedback suppression, two adaptive compensation filters operating in parallel are employed. 
     A high correlation between the useful signal  10  and the feedback signal  14  represents a major problem for an optimum feedback suppression, because input signal components will also be addressed by the correlation and incorrect adaptations of the compensation filter can occur. 
     A solution to this problem is disclosed in JASA Vol. 94, pt. 6, 1993-December, 3248 ff. A useful signal is decorrelated from a fed back noise signal by the frequency of the output signal of a hearing device and thereby the frequency of the fed back signal being shifted in relation to the frequency of the useful signal. 
     Unfortunately the frequency shifts or distortions also cause clearly perceptible artifacts. A distortion at low frequencies is not possible as a rule since in the low frequency range the human hearing reacts very sensitively to distortions. Therefore mostly only the high frequencies are shifted. Despite this the result can be an audible “detuning” of the useful signal. 
     Considerably more unpleasant are overlay artifacts in which a signal shifted in frequency and a non-shifted signal are perceived at the same time which leads with tonal signals to a marked modulation or fluctuation or to a roughness. Acoustic overlays are almost inevitable which arise through the inflow of direct sound, through the vent for example. 
     As a result of construction overlays can however also arise from non-ideal split-band filters. To enable only high-frequency frequency components to be shifted, these must be separated from the low-frequency components. This requires a frequency filter, also called a split-band filter. The filter cannot however carry out ideal separation, which means that disruptive overlays result in the area of the cut-off frequency of the filter. 
     Depending on the frequency shift, these overlays will be perceived as amplitude modulation or as signal roughness. In all cases described the overlays are disruptive, especially when an input signal involves music or more generally tonal signals. 
     Known filters in hearing devices are of the Butterworth type. They are not ideal and have a finite frequency overlap at their cut-off frequency GF.  FIG. 2  shows an example of the frequency curve of a 9th-order type Butterworth frequency filter of a hearing device with a cut-off frequency GF of 900 Hz. The curves K 1 , K 2  show the amplitude D in dB as a function of the frequency F in Hz in the range 0 to 1150 Hz. The curve K 1  shows a low-pass characteristic and the curve K 2  a high-pass characteristic. The sum curve K 3  of the curves K 1  and K 2  produces a flat, constant frequency response. The curve K 4  compared to the curve K 2  shows a high-path characteristic shifted by 25 Hz to higher frequencies. 
     With an addition of the signal components according to the curves K 1  and K 3  the result is, above all in the area of the cut-off frequency G 2 , overlays which cannot be ignored of signal components shifted in frequency and not shifted, which in an output signal of the hearing device is perceived as modulation or heavy roughness. Both effects are disruptive and are perceived by the hearing device wearer mostly markedly more strongly than frequency shifting. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a hearing device with frequency shifting and an associated method which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which reduces the perception of artifacts of a frequency shifting in hearing devices. 
     A hearing device has an adaptive feedback suppression unit and a signal processing unit. The hearing device also contains a low-pass filter characterized by a first cut-off frequency that couples a load frequency signal component out of an output signal of the signal processing unit, a high-pass filter characterized by a second cut-off frequency that couples a high-frequency signal component out of the output signal of the signal processing unit, and a frequency shifting unit which shifts the frequency of the high-frequency signal component to higher frequencies. Between the first and the second cut-off frequency there exists a predeterminable distance or a gap. Signal distortions caused by a frequency shift are effectively suppressed by the different limit frequencies. The reason is that fewer overlapping shifted and unshifted signal components arise in this way. This enables a feedback suppression to operate continuously at higher frequencies. The suppression is then undertaken quickly. 
     In a development the distance can be between 20 Hz and 50 Hz in size. Trials have shown that a distance between the limit frequencies of this size is sufficient. 
     In a further form of embodiment of the invention the frequency shift of the high-frequency signal component can amount to 10 Hz to 30 Hz. Acoustic feedback suppression is optimized by this. 
     Furthermore the hearing device contains an adder in which the low-frequency signal component and the high-frequency signal component shifted in frequency are summed, with an output signal of the hearing device being formed. 
     Preferably the low-pass filter and/or the high-pass filter can be embodied as Cauer filters (also referred to as elliptical filters). The great edge steepness of this filtered type more effectively prevents signal distortions. 
     The invention also relates to a method for frequency shifting in a hearing device. The method includes the steps of: 
     coupling out a low-frequency signal component from a signal-processed microphone signal (at the output of a signal processing unit) by a low-pass filter characterized by a first cut-off frequency; 
     coupling out a high-frequency signal component from a signal-processed microphone signal (at the output of a signal processing unit) by a high-pass filter characterized by a second cut-off frequency, with a predeterminable distance or a gap being present between the first and the second cut-off frequency; and 
     shifting the frequency of the high-frequency signal component to higher frequencies. 
     In a development of the method the distance between the limit frequencies can be selected between 20 Hz and 50 Hz. 
     In a further form of embodiment of the method the frequency of the high-frequency signal component can be shifted by 10 Hz to 30 Hz. 
     The method preferably also contains an addition of the low-frequency signal component and the high-frequency signal component shifted in the frequency, with an output signal of the hearing device being formed. 
     In addition the low-pass filter and/or the high-pass filter can be configured as a Cauer filter. 
     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 hearing device with frequency shifting and an associated method, 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 block diagram of a hearing device with acoustic feedback and feedback suppression according to the prior art; 
         FIG. 2  is a graph showing a frequency curve of a 9th-order Butterworth frequency filter according to the prior art; 
         FIG. 3  is a block diagram of a hearing device with feedback suppression and a frequency filter according to the invention; and 
         FIG. 4  is a graph showing frequency curves of two Cauer filters. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 3  thereof, there is shown a hearing device  1  with a microphone  2  picking up an acoustic input signal  101  and with an earpiece  4  outputting an acoustic output signal  13 . As described above, a part of the output signal  13  is fed back via a feedback path  14  to the microphone  2  of the hearing device  1 , wherein it is overlaid with a useful signal  10  to form an input signal  101 . The microphone  2  converts the acoustic input signal  101  into an electrical microphone signal  102 . 
     Any acoustic feedback that might arise is detected with the aid of a feedback suppression unit  17 , simulated from an earpiece input signal  108  and added as an inverted feedback suppression signal  109  to the microphone signal  102  in a second adder  22 . At the output of the second adder  22  a feedback-suppressed microphone signal  107  is thus produced which is fed to a signal processing unit  3 . An output signal  103  of the signal processing unit  3  is fed to the input of a frequency filter with a low-pass filter  18  and a high-pass filter  19 . 
     A low-pass output signal  105  is available at the output of the low-pass filter  18  and a high-pass output signal  104  is available at the output of the high-pass filter  19 . The high-pass output signal  104  is shifted with the aid of a frequency shift unit  25  by around 10 Hz to 30 Hz towards higher frequencies. The frequency-shifted high-pass output signal  106  is added in a first adder  21  to the low-pass output signal  105 . 
     An earpiece input signal  108  is available at the output of the first adder  21 , which is converted by the earpiece  4  into the acoustic output signal  13 . 
     In accordance with the invention the low-pass filter  18  and the high-pass filter  19  have different limit frequencies GF 1 , GF 2 , whereby practically no disruptive overlay effects can arise from original signal components and frequency-shifted signal components. Preferably the two filters  18 ,  19  are elliptical filters, also referred to as Cauer filters. They possess an especially steep edge, which can bring about an extreme reduction of an undesired signal overlay in the filter overlap area in addition to the different choice of the limit frequencies. 
     The invention is able to be used both for hearing devices with one microphone and for devices with a number of microphones. With a number of microphones there are also a number of feedback suppression units and a number of inventive frequency filters which are supplied by different signal-processed microphone signals. 
     Frequency curves K 5 , K 6 , K 7 , K 8 , K 9 , K 10  of corresponding Cauer filters employed in accordance with the invention are shown in  FIG. 4 . The two diagrams of  FIG. 4  show the amplitude D in dB as a function of the frequency F in kHz for a frequency range of 650 Hz to 1150 Hz. 
     The upper diagram of  FIG. 4  shows the frequency curves K 5 , K 6  of first Cauer filters with a narrow and deep notch of the sum frequency curve K 7  as a result of a corresponding embodiment of the first Cauer filters. The distance between the first cut-off frequency GF 1  of the low-pass (curve K 5 ) and the second cut-off frequency GF 2  of the high-pass (curve K 6 ) is selected relatively small. The first cut-off frequency GF 1  lies at around 890 Hz, the second cut-off frequency GF 2  at around 910 Hz. 
     The lower diagram of  FIG. 4  shows the frequency curves K 8 , K 9  of second Cauer filters with a broader and less deep notch of the sum frequency curve as a result of a corresponding embodiment of the second Cauer filters. A wider gap is selected between the first cut-off frequency GF 1  of the low-pass (curve K 8 ) and the second cut-off frequency GF 2  of the high-pass (curve K 9 ). The first cut-off frequency GF 1  lies at around 880 Hz and the second cut-off frequency GF 2  at around 920 Hz. 
     Trials have shown that the invention generates significantly lower signal disturbances in the hearing devices with frequency shifting, because no duplicated frequency components occur, which would cause a rough sound.