Patent Publication Number: US-9407998-B2

Title: Hearing device with analog filtering and associated method

Description:
RELATED APPLICATION DATA 
     This application is a continuation of U.S. patent application Ser. No. 13/720,652, filed on Dec. 19, 2012, pending, which claims priority to and the benefit of European Patent Application No. 12195007.5 filed on Nov. 30, 2012, pending, and Danish Patent Application No. PA 2012 70744 filed on Nov. 30, 2012, pending. The entire disclosures of all of the above-identified applications are expressly incorporated by reference herein. 
    
    
     FIELD 
     The present application relates to a hearing device or hearing aid with analog filtering and associated method. In particular, the present application relates to a hearing aid having improved wind noise reduction. 
     BACKGROUND 
     Wind noise is experienced as very unpleasant by a hearing device user and considerably reduces the user-friendliness of a hearing device. Known methods for reducing wind noise in hearing devices include filtering in the digital domain. Further, it is known to reduce the gain of a preamplifier to remove the uncomfortable sound levels. This however is at the expense of normal sound processing basically getting destroyed rendering the hearing device somewhat unusable. 
     SUMMARY 
     Despite the known solutions, there is still a need for improving the hearing device processing during windy conditions and to improve the signal to noise ratio (SNR) in the hearing device. 
     Accordingly, a hearing device is provided, the hearing device comprising: a microphone for converting audio into an audio signal; a preprocessing unit for analog processing of the audio signal and having an input and an output, wherein the input is connected to an output of the microphone; an A/D converter for converting the processed analog audio signal into a digital audio signal, the A/D converter having an input and an output, wherein the input is connected to the output of the preprocessing unit; and a processing unit for digital processing of A/D converter output, wherein the processing unit is connected to the preprocessing unit. The preprocessing unit is configured to apply a first transfer function to the audio signal in a first mode of operation and a second transfer function with a second cutoff frequency to the audio signal in a second mode of operation, depending on a control signal from the processing unit. 
     Also disclosed is a method for operating a hearing aid comprising a microphone, a preprocessing unit, an A/D converter, and a processing unit is provided, the method comprising: applying, in the preprocessing unit, a first transfer function to an audio signal from the microphone; converting the preprocessed audio signal to a digital audio signal; determining one or more noise parameters including a first noise parameter: and applying, in the preprocessing unit, a second transfer function, e.g. with a second cutoff frequency, to the audio signal from the microphone depending on a noise criterion, e.g. a first noise criterion, based on the one or more noise parameters. The method may comprise determining the one or more noise parameters while applying the second transfer function, and applying, in the preprocessing unit, the first transfer function or a third transfer function to the audio signal from the microphone depending on a noise criterion, e.g. a second noise criterion. 
     It is an advantage that saturation of A/D converter of the hearing device is reduced thereby providing improved SNR in the hearing device processing. 
     Further, the hearing device and method enable a more efficient use of signal processing resources by removing undesired noise at an early stage in the signal processing of the hearing device. Thereby, improved power management is provided. 
     A hearing device includes: a microphone for converting audio into an audio signal; a preprocessing unit for analog processing of the audio signal and having an input and an output, wherein the input is connected to an output of the microphone; an A/D converter for converting the processed analog audio signal into a digital audio signal, the A/D converter having an input and an output, wherein the input of the A/D converter is connected to the output of the preprocessing unit; and a processing unit for digital processing of an output from the A/D converter, wherein the processing unit is connected to the preprocessing unit; wherein the preprocessing unit is configured to apply a first transfer function to the audio signal in a first mode of operation, or a second transfer function with a cutoff frequency to the audio signal in a second mode of operation, depending on a control signal from the processing unit. 
     Optionally, the preprocessing unit may include a first capacitor circuit with a variable first capacitance value, wherein the first capacitance value depends on the control signal from the processing unit. 
     Optionally, the preprocessing unit may include a first resistor circuit with a variable first resistance value, wherein the first resistance value depends on the control signal from the processing unit. 
     Optionally, the preprocessing unit may include a second resistor circuit with a variable second resistance value, wherein the second resistance value depends on the control signal from the processing unit. 
     Optionally, the preprocessing unit may include an amplifier. 
     Optionally, the processing unit may include a detector unit configured to determine one or more noise parameters including a first noise parameter, and wherein the processing unit is configured to send the control signal indicative of one of the first and second modes of operation to the preprocessing unit depending on a noise criterion based on the one or more noise parameter(s). 
     Optionally, the preprocessing unit may be in the first mode of operation, and the processing unit may be configured to send a control signal indicative of a second mode of operation to the preprocessing unit when a first noise criterion is fulfilled in the first mode of operation. 
     Optionally, the preprocessing unit may be in the second mode of operation, and the processing unit may be configured to send a control signal indicative of a first mode of operation to the preprocessing unit when a second noise criterion is fulfilled in the second mode of operation. 
     Optionally, the preprocessing unit may be configured to apply a third transfer function with a cutoff frequency to the audio signal in a third mode of operation as an alternative to the first mode and the second mode of operation depending on the control signal from the processing unit. 
     Optionally, the first transfer function may be a high pass filter function and having a cutoff frequency, wherein the cutoff frequency of the first transfer function is anywhere in a range from 100 Hz to 1 kHz. 
     Optionally, the cutoff frequency of the second transfer function may be larger than the cutoff frequency of the first transfer function. 
     Optionally, the second transfer function may be a high pass filter function, and wherein the cutoff frequency may be anywhere in a range from 100 Hz to 2 kHz. 
     Optionally, the audio signal may include a first part that corresponds with a first time point, and a second part that corresponds with a second time point; and the preprocessing unit may be configured to apply one of the first transfer function and the second transfer function to the first part of the audio signal based on a first value of the control signal from the processing unit, and another one of the first transfer function and the second transfer function to the second part of the audio signal based on a second value of the control signal from the processing unit. 
     A method for operating a hearing aid comprising a microphone, a preprocessing unit, an A/D converter, and a processing unit, includes: applying, in the preprocessing unit, a first transfer function to a first part of an audio signal from the microphone to obtain a preprocessed audio signal; converting the preprocessed audio signal to a digital audio signal; determining one or more noise parameters including a first noise parameter; and applying, in the preprocessing unit, a second transfer function with a cutoff frequency to a second part of the audio signal from the microphone depending on a noise criterion based on the one or more noise parameters; wherein the first part and the second part of the audio signal correspond to different respective time points. 
     Optionally, the one or more noise parameters may be determined while the second transfer function is being applied. 
     Optionally, the method may also include applying, in the preprocessing unit, the first transfer function or a third transfer function to a third part of the audio signal from the microphone depending on another noise criterion. 
     Other and further aspects and features will be evident from reading the following detailed description of the embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate the design and utility of embodiments, in which similar elements are referred to by common reference numerals. These drawings are not necessarily drawn to scale. In order to better appreciate how the above-recited and other advantages and objects are obtained, a more particular description of the embodiments will be rendered, which are illustrated in the accompanying drawings. These drawings depict only typical embodiments and are not therefore to be considered limiting of the scope of the claims. 
         FIG. 1  schematically illustrates an exemplary hearing device, 
         FIG. 2  schematically illustrates an exemplary preprocessing circuit of the hearing device in  FIG. 1 , 
         FIG. 3  schematically illustrates an exemplary preprocessing circuit of the hearing device in  FIG. 1 , 
         FIG. 4  shows an exemplary resistance circuit with a variable resistance value, 
         FIG. 5  shows an exemplary capacitance circuit with a variable capacitance value, 
         FIG. 6  shows exemplary first and second transfer functions, 
         FIG. 7  is a flow diagram of an exemplary method, 
         FIG. 8  is a flow diagram of an exemplary method, and 
         FIG. 9  shows an exemplary processing unit. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments are described hereinafter with reference to the figures. It should be noted that the figures are not drawn to scale and that the elements of similar structures or functions are represented by like reference numerals throughout the figures. It should be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the invention or as a limitation on the scope of the claimed invention. In addition, an illustrated embodiment need not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated. Also, reference throughout this specification to “some embodiments” or “other embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiments is included in at least one embodiment. Thus, the appearances of the phrase “in some embodiments” or “in other embodiments” in various places throughout this specification are not necessarily referring to the same embodiment or embodiments. 
     The preprocessing unit processes the audio signal from the microphone and has an input and an output. The input comprises a first input terminal connected to an output, e.g. a first output terminal, of the microphone. The preprocessing unit may comprise one or more control terminals for receiving and/or sending control signal(s) from/to the processing unit. 
     The preprocessing unit may comprise a first capacitor circuit with a first capacitance value. The first capacitance value may be variable, e.g. the first capacitance value may depend on the control signal from the processing unit. The preprocessing unit may comprise a second capacitor or second capacitor circuit having a second capacitance value. The second capacitance value may be variable, e.g. the second capacitance value may depend on the control signal from the processing unit. A variable first and/or second capacitance circuit may reduce the microphone requirements with regard to DC biasing or drifting. A capacitor or capacitor circuit has a first and a second terminal having a capacitance value therebetween. 
     The preprocessing unit may comprise a first resistor circuit with a first resistance value. The first resistance value may be variable, e.g. the first resistance value may depend on the control signal from the processing unit. The preprocessing unit may comprise a second resistor circuit with a second resistance value. The second resistance value may be variable, e.g. the second resistance value may depend on the control signal from the processing unit. A resistor or resistor circuit has a first and a second terminal having a resistance value therebetween. 
     The preprocessing unit may be a passive unit or an active unit. Accordingly, the preprocessing unit may comprise an amplifier. The amplifier has a first and optionally a second input terminal and an output terminal. The preprocessing unit may comprise a first part or unit and a second part or unit, where the first part is a passive filter part with adjustable filter parameters via the control signal and the second part is an active part with an amplifier. Accordingly, the preprocessing unit may comprise a passive adjustable filter unit followed by an active amplifier unit with adjustable gain. 
     The processing unit comprises an input with one or more input terminals including a first input terminal. The processing unit comprises one or more control terminals connected to corresponding control terminals of the preprocessing unit for sending and/or receiving control signal(s) indicative of mode of operation to/from the preprocessing unit. 
     The processing unit is configured to send a control signal to the preprocessing unit indicative of a mode of operation. The control signal may comprise one or more bits, e.g. depending on the number of modes/transfer functions. The control signal may have a first value indicative of the first transfer function (e.g. Bit1=0) and a second value indicative of a second transfer function (e.g. Bit1=1). The processing unit may comprise a detector unit configured to determine one or more noise parameters including a first noise parameter and/or a second noise parameter. The noise parameter(s) may be indicative of the amount of wind noise in the audio signal. The processing unit may be configured to send a control signal indicative of mode of operation to the preprocessing unit depending on a noise criterion based on one or more of the noise parameters. The detector unit may be configured to determine a saturation parameter indicative of whether the A/D converter is close to or in saturation. The processing unit may be configured to send a control signal indicative of mode of operation to the preprocessing unit based on the saturation parameter. 
     A noise criterion may be applied for determining the mode of operation. For example, a first mode (first transfer function) may be selected when a noise criterion, e.g. a first noise criterion, based on noise parameter(s) is not fulfilled, e.g. if a first noise parameter is less than, equal to or larger than a noise threshold (e.g. a first noise threshold). Further, a second mode (second transfer function) may be selected when a noise criterion, e.g. the first noise criterion or a second noise criterion, based on the noise parameter(s) is fulfilled, e.g. if the first noise parameter is less than, equal to or larger than a noise threshold (e.g. the first noise threshold or a second noise threshold). Different criteria may be applied depending on the mode of operation. 
     In an exemplary hearing device, a noise parameter may be based on the sound pressure level (SPL) of the digital audio signal and/or the sound pressure level (SPL) in one or more frequency bands of the digital audio signal, e.g. a first frequency band F 1  and/or a second frequency band F 2 . It is contemplated that a noise criterion and/or the noise parameter may be based on SPL or other suitable input signal properties determined by the detector unit or other sound classification units in the hearing device. 
     In an exemplary hearing device, a noise criterion in the first mode of operation is given by
 
SPL total   &gt;T   1  
 
where the first noise parameter SPL total  is the sound pressure level of the digital audio signal and T 1  is a first noise threshold. A first transfer function is applied (first mode of operation) if the noise criterion is not fulfilled (false, indicative of no or little wind noise) and a second transfer function is applied (second mode of operation) if the noise criterion is fulfilled (true, indicative of wind noise present).
 
     In an exemplary hearing device, the noise criterion in the first mode of operation is given by
 
SPL F1   &gt;T   1  
 
where the first noise parameter SPL F1  is the sound pressure level of the digital audio signal in a first frequency band F 1  and T 1  is a first noise threshold. A first transfer function is applied (first mode of operation) if the noise criterion is not fulfilled (false, indicative of no or little wind noise) and the preprocessing unit switches to applying a a second transfer function (second mode of operation) if the noise criterion is fulfilled (true, indicative of wind noise present).
 
     The noise criterion may be based on a plurality of noise parameters. In an exemplary hearing device, the noise criterion in the first mode of operation is given by
 
(SPL total   &gt;T   1 ) AND (SPL F1   &gt;T   2 )
 
where the first noise parameter SPL total  is the sound pressure level of the digital audio signal, T 1  is a first noise threshold, the second noise parameter SPL F1  is the sound pressure level of the digital audio signal in a first frequency band F 1 , and T 2  is a second noise threshold.
 
     A first noise criterion may be applied in a first mode of operation and a second noise criterion different from the first noise criterion may be applied in a second mode of operation. The hearing device may be configured to operate in the second mode for a predetermined time period and then switch back to the first mode. For example, the second noise criterion may consist of or include whether the hearing device has operated in the second mode for a certain period of time. A noise criterion may comprise one or more logical expressions. 
     The preprocessing unit may be configured to apply a third transfer function with a third cutoff frequency to the audio signal in a third mode of operation depending on the control signal from the processing unit. 
     The preprocessing unit is configured to apply a plurality of different transfer functions including a first transfer function and a second transfer function to the output signal from the microphone in different modes of operation, e.g. depending on control signal(s) from the processing unit. 
     The first transfer function may be a high pass filter function having a first cutoff frequency f 1 . The first cutoff frequency may be selected in the range from 5 Hz to 1 kHz. Exemplary first cutoff frequencies are 20 Hz, 50 Hz, 80 Hz, 100 Hz, 200 Hz, 400 Hz, 600 Hz, 1 kHz or any ranges therebetween. 
     The second transfer function may be a high pass filter function. The second cutoff frequency f 2  may be selected in the range from 50 Hz to 2 kHz. Exemplary second cutoff frequencies are 50 Hz, 80 Hz, 100 Hz, 200 Hz, 400 Hz, 600 Hz, 800 Hz, 1 kHz, 2 kHz or any ranges therebetween. The first cutoff frequency may be less than the second cutoff frequency, e.g. f 1 =80 Hz and f 2 =400 Hz. 
     The transfer functions applied in different modes may be band-pass filter functions with lower cutoff corresponding to the first and second cutoff frequencies, respectively. 
     The processing unit may be configured to send a control signal indicative of a second mode of operation to the preprocessing unit when a first noise criterion is fulfilled in a first mode of operation where a first transfer function is applied. Additionally or as an alternative, the processing unit may be configured to send a control signal indicative of a first mode of operation to the preprocessing unit when a second noise criterion is fulfilled in a second mode of operation where a second transfer function is applied. 
     The method according some embodiments comprises applying, in the preprocessing unit, a second transfer function with a second cutoff frequency to the audio signal from the microphone if the noise parameter fulfills a criterion. The method may comprise determining the noise parameter(s) while applying the second transfer function, and applying, in the preprocessing unit, the first transfer function or a third transfer function to the audio signal from the microphone if the noise parameter(s) fulfills a criterion. A first criterion may be applied when the first transfer function is applied in the preprocessing unit and a second criterion different from the first criterion may be applied when the second transfer function is applied in the preprocessing unit. 
     The hearing device according to some embodiments provides improved degree of freedom in the design of the hearing device by reducing the requirements to microphone performance. 
     Further, one or more embodiments described herein allow for utilizing low frequency audio signals for secondary applications not necessarily related to hearing loss compensation. 
       FIG. 1  schematically illustrates an exemplary hearing device. The hearing device  2  comprises a microphone  4  for converting audio into an audio signal  5 , and a preprocessing unit  6  for analog processing of the audio signal  5 . The preprocessing unit has an input  8  for audio signal  5  and an output  10 , wherein the input  8  is connected to an output  12  of the microphone  4 . Further, the hearing device  2  comprises an A/D converter  14  for converting the processed analog audio signal  16  into a digital audio signal  18 , the A/D converter  14  having an input  20  and an output  22 , wherein the input  20  is connected to the output  10  of the preprocessing unit  6 . The hearing device comprises a processing unit  24  for digital processing of the digital audio signal  18 . The processed digital audio signal  26  is fed to a receiver device (not shown) for conversion of the processed digital audio signal  26  into a compensated audio signal. The processing unit  24  is connected to the preprocessing unit  6  on connection  28  for feeding and/or receiving control signal(s) to/from the preprocessing unit. 
     The preprocessing unit  6  is configured to apply a first transfer function H 1  to the audio signal in a first mode of operation and a second transfer function H 2  with a second cutoff frequency to the audio signal in a second mode of operation, depending on a control signal  30  on connection  28  from the processing unit  24 . 
       FIG. 2  shows an exemplary preprocessing unit  6 ′ of the hearing aid  2 . The preprocessing unit  6 ′ comprises a first capacitance circuit  40  with a variable first capacitance value C1 depending on a control signal on control terminal(s)  42 . Further, the preprocessing unit  6 ′ comprises a second capacitance circuit  44  with a second capacitance value C2. The preprocessing unit  6 ′ comprises a first resistance circuit  46  with a variable first resistance value R1 depending on a control signal on control terminal(s)  42 ′. The preprocessing unit  6 ′ is an active unit comprising an amplifier or amplifier circuit  48  having a first input terminal  49 A, a second input terminal  49 B, and output terminal  50  coupled to the capacitance and resistance circuits  40 ,  44 ,  46 . The first capacitance circuit  40  is coupled between a first input terminal  47  of the preprocessing unit input  8  and the first input terminal  49 A. The second capacitance circuit  44  and the first resistance circuit  46  are coupled in parallel between the first input terminal  49 A and the output terminal  50 . The second input terminal  49 B is connected to virtual ground. 
       FIG. 3  shows an exemplary preprocessing unit  6 ″ of the hearing aid  2 . The preprocessing unit  6 ″ comprises a first resistance circuit  46  with a variable first resistance value R1 depending on a control signal on control terminal(s)  42 ′. Further, the preprocessing unit  6 ″ comprises a second resistance circuit  52  with a variable second resistance value R2 depending on a control signal on control terminal(s)  42 ″. The preprocessing unit  6 ″ is an active unit comprising an amplifier or amplifier circuit  48  having a first input terminal  49 A, a second input terminal  49 B, and output terminal  50 . The first resistance circuit  46  is coupled between the second input terminal  49 B and the output terminal  50 . The second resistance circuit  52  is coupled between the second input terminal  49 B and the second input terminal  47 ″ of the preprocessing unit. A capacitance circuit (not shown) is coupled external to the preprocessing unit between the second input terminal  47 ′ and ground. As an alternative or in combination, a first capacitance circuit (C1) may be incorporated in the preprocessing unit  6 ″ in series with the second resistance circuit  52  between ground and the second input terminal  49 B. 
       FIG. 4  shows an exemplary resistance circuit with a variable resistance value. Different resistance values may be selected by controlling the switches  56  (open or closed) with a control signal in order to couple or decouple resistors  54 . 
       FIG. 5  shows an exemplary capacitance circuit with a variable capacitance value. Different capacitance values may be selected by controlling the switches  56  (open or closed) with a control signal in order to couple or decouple capacitors  58 . 
       FIG. 6  shows exemplary first and second transfer functions applied to the audio signal in first and second modes of operation, respectively. The first transfer function  90  is a high pass filter with a first cutoff frequency f 1  of about 100 Hz. The second transfer function  92  is a high pass filter with a second cutoff frequency f 2  of about 400 Hz. 
       FIG. 7  illustrates an exemplary method for operating a hearing aid comprising a microphone, a preprocessing unit, an A/D converter, and a processing unit according to some embodiments. The method  100  comprises applying  102 , in the preprocessing unit, a first transfer function H 1  to an audio signal from the microphone and converting  104  the preprocessed audio signal to a digital audio signal with the A/D converter. Further, the method comprises determining  106  one or more noise parameters including a first noise parameter and/or a second noise parameter of the digital audio signal, and applying  110 , in the preprocessing unit, a second transfer function H 2  with a second cutoff frequency f 2  to the audio signal from the microphone depending on a noise criterion  108  based on the one or more noise parameters. In the method  100 , the noise criterion applied in  108  depends on the mode of operation. If the preprocessing unit applies a first transfer function corresponding to the first mode of operation, a first criterion based on sound pressure levels is applied. If the first criterion is fulfilled, e.g. one or more sound pressure levels (total and/or selected bandwidths) are larger than respective thresholds, a control signal indicative of second mode is fed to the preprocessing unit and the preprocessing unit applies a second transfer function  110 . If the first criterion is not fulfilled (indicative of no wind noise), the preprocessing unit continues applying the first transfer function corresponding to the hearing device operating in the first mode of operation. If the preprocessing unit applies a second transfer function corresponding to the second mode of operation, a second criterion is applied in  108 . The second criterion may be fulfilled if the hearing device has operated in the second mode for a certain time period, i.e. if t 2 &gt;T 1 , where t 2  is the present time of operation in the second mode and T 1  is a threshold time, e.g. in the range from 1 second to 60 seconds. If the second criterion is fulfilled, a control signal indicative of first mode is fed to the preprocessing unit and the preprocessing unit applies a first transfer function  102 . If the second criterion is not fulfilled (indicative of wind noise present), the preprocessing unit continues applying the second transfer function and the hearing device continues operating in the second mode of operation. 
       FIG. 8  illustrates an exemplary method for operating a hearing aid comprising a microphone, a preprocessing unit, an A/D converter, and a processing unit according to some embodiments. The method  100 ′ comprises applying  102 , in the preprocessing unit, a first transfer function H 1  to an audio signal from the microphone and converting  104  the preprocessed audio signal to a digital audio signal with the A/D converter. Further, the method comprises determining  106  one or more noise parameters including a first noise parameter and/or a second noise parameter of the digital audio signal. Further, an operation mode of the hearing device is determined based on the one or more noise parameters and the current mode of operation in  106 . The operation mode may be selected from two or more operation modes with corresponding transfer functions. In  110 , the transfer function corresponding to the mode of operation determined in  106  is applied to the audio signal and the method returns to determining the mode of operation and accordingly the transfer function to be applied in the preprocessing unit. 
       FIG. 9  shows an exemplary processing unit  24 . The processing unit  24  receives the digital audio signal  18  at input  93 . The digital audio signal  18  is fed to detector unit  94  and hearing compensation processing unit  96 . The detector unit  94  is configured to determine at least a first noise parameter of the digital audio signal  18 . The noise parameter(s) are fed to mode selector  98  configured to determine the mode of operation of the hearing aid, e.g. based on the noise parameter(s) and/or current mode of operation. The mode selector  98  may be embedded in the hearing compensation processing unit  96 . The mode selector  98  is configured to send a control signal  30  indicative of mode of operation to the preprocessing unit via processing unit control output  25 . 
     Although particular embodiments have been shown and described, it will be understood that they are not intended to limit the claimed invention, and it will be made obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the claimed invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. The claimed invention is intended to cover alternatives, modifications, and equivalents. 
     LIST OF REFERENCES 
     
         
           2  hearing device 
           4  microphone 
           5  audio signal 
           6  preprocessing unit 
           8  preprocessing unit input 
           10  preprocessing unit output 
           12  microphone output 
           14  analog-digital (A/D) converter 
           16  processed analog audio signal 
           18  digital audio signal 
           20  A/D converter input 
           22  A/D converter output 
           24  processing unit 
           25  processing unit control output 
           26  processed digital audio signal 
           28  connection 
           30  control signal 
           40  first capacitance circuit 
           42 ,  42 ′,  42 ″ control terminal(s) 
           44  second capacitance circuit 
           46  first resistance circuit 
           47  first input terminal 
           48  amplifier 
           49 A first input terminal of amplifier 
           49 B second input terminal of amplifier 
           50  output terminal of amplifier 
           52  second resistance circuit 
           54  resistor 
           56  switch 
           58  capacitor 
           90  first transfer function 
           92  second transfer function 
           93  digital audio input 
           94  detector unit 
           96  hearing compensation processing unit 
           98  mode selector