Abstract:
Feedback reduction in hearing aids is simplified and improved in an amplification device that has two separate compensation paths, an acoustic input and an inductive input. In the first compensation path, a first filter device acts to compensate acoustic feedback, and in the second compensation path, a second filter device acts to compensate inductive feedback. The two very different feedbacks can be individually compensated without increased outlay.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention concerns an amplifier circuit for hearing aids of the type having an amplification device with an input at which acoustically-generated signals and inductively-generated signals are present, and a filter device that is connected to the amplification device for compensation of an acoustic feedback by adaptation to the acoustic feedback. In addition, the present invention concerns a hearing aid with such an amplifier circuit. Furthermore, the present invention concerns a corresponding method for the amplification of input signals of a hearing aid.  
           [0003]    2. Description of the Prior Art  
           [0004]    In a hearing aid, acoustic input signals are acquired by a microphone, amplified in an amplification device, and the amplified signals are supplied to a speaker (earphone). The sound emitted from the speaker is partially fed back to the microphone, resulting in noticeable interference.  
           [0005]    Besides the microphone and the speaker, a hearing aid often also has a telecoil for the inductive infeed of auditory signals, for example from a telephone. Since the speaker operates electromagnetically, it normally causes a magnetic feedback at the telecoil from the magnet of the speaker.  
           [0006]    In mixed mode, the microphone and the telecoil are simultaneously operated, and a number of input signals exist. Correspondingly, a number of feedback paths are also created in the mixed mode.  
           [0007]    In this context, a device for electromagnetically reducing feedback in communications devices is known from the European Application EP 1 119 218 A1. A digital filter is used in order to simulate the combined pulse response of the components of the D/A converter, speaker, electromagnetic feedback path, pick-up coil, and A/D converter. The coefficients of the digital filter are adjusted or set according to a selected feedback estimation algorithm.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to improve and simplify feedback reduction in hearing aids.  
           [0009]    This object is achieved in accordance with the invention in an amplifier circuit for hearing aids having an amplification device with an input at which acoustically-generated signals and inductively-generated signals are present, and a first filter device that is connected to the amplification device for compensation of acoustic feedback, as well as a second filter device that is connected to the amplification device and separate from the first filter device, for compensation of inductive feedback.  
           [0010]    The object is also achieved in a method for the amplification of input signals for a hearing aid, by providing acoustic and inductive input signals and compensating acoustic feedback by means of a first filter device, as well as compensating an inductive feedback by means of a second filter device.  
           [0011]    The basis of the invention is to compensate a number of feedback paths with a number of compensation paths. Conventional feedback reduction methods in hearing aids operate only with one internal compensation path. These methods are based on only one feedback path. Approaches or methods have not been pursued or presented, that are concerned with the simultaneous compensation(feedback reduction) of a number of simultaneously active feedback paths by means of a number of compensation paths, wherein one of the active feedback paths may not be acoustic.  
           [0012]    With the present invention, it is possible to adequately react to a number of feedback paths that may possibly carry different input signals. Inventively, separate feedback compensation or reduction is provided for each of the various paths. Specifically, the problem of operating the telephone can well solved in a satisfactory manner by its own compensation filter. In particular, inductive feedback can be compensated by a static filter, and acoustic feedback can be compensated by an adaptive filter.  
           [0013]    The relatively expensive and elaborate earphone encapsulations can be dispensed with, the assembly of the telecoil is subject to fewer limitations, as a result of compensation of the magnetic feedback. Moreover, the invention makes it possible to simulate the telecoil path with an existing adaptive compensation filter, to subsequently read out the coefficients thereof, and to use these as a preset for a second static- though slow-adapting compensation filter. Consequently, the actual adaptive compensator remains almost completely free for another, preferably acoustic, path. Thus, a number of mostly independent feedback paths are analyzed, especially in the context of non-acoustic paths, in the inventive device and in the inventive method, and used for compensation. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0014]    The single FIGURE is a schematic circuit diagram of the inventive amplifier circuit. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    An amplification device  1  is circuited between a microphone  2  and a telecoil or telephone coil  3  on one side and speaker  4  on the other side. Thus, the hearing aid has an inductive input and an acoustic input. Acoustic and magnetic output signals are generated by the speaker  4 .  
         [0016]    The telecoil  3  can be disconnected from the amplification device  1  by a first switch  5 . In the same manner, the microphone  2  can be separated from the amplification device  1  by a second switch  6 . The signals of the telecoil  3  and the microphone  2  are merged in an adder  7  and supplied to the amplification device  1 . Alternatively, the signals can also be amplified separately and subsequently added.  
         [0017]    The output signal of the amplification device  1  is fed back via an adaptive filter  8  to its input. In an adder  9 , the feedback signal of the adaptive filter  8  is subtracted from the summed signal of the adder  7 . The adaptive filter  8  is adapted continuously to the conditions of the acoustic environment on the basis of the summed signal E of the adder  9 .  
         [0018]    Furthermore, the output signal of the amplification device  1  is fed back via a static filter  10  to its input. This feedback signal is also subtracted from the summed signal of the adder  7  in the adder  9 . A third switch  11  enables the deactivation of the feedback path of the static filter  10 .  
         [0019]    There is a data connection  12  to the adaptive filter  8  for setting or adjusting the filter coefficients in the second static filter  10 .  
         [0020]    Sound is emitted by the speaker  4  and is fed back into the microphone  2  via an acoustic feedback path  13  that has a transfer function H acoustic . In the same manner, a magnetic field emitted by speaker  4  is fed back into the telecoil  3  via a magnetic feedback path  14 , which has a transfer function H magnetic . As a result of the adaptation, the adaptive filter  8  has a transfer function Ĥ acoustic , and the static filter  10  has a transfer function Ĥ magnetic , so that both feedback signals can be appropriately compensated by the acoustic path  13  and the magnetic path  14 .  
         [0021]    The reason for the use of an adaptive filter  8  and a simple static filter  10  is that the second static filter  10  can be fashioned less elaborately, since the magnetic feedback path is less dynamic than the acoustic feedback path. The magnetic feedback is normally constant, whereas the acoustic feedback can be varied merely by the hearing aid user putting his/her hand to his/her ear. It is therefore beneficial to differentiate between the magnetic feedback path and the acoustic feedback path, and use filters that differ in complexity for each.  
         [0022]    Moreover, there is a transit time delay between the acoustic feedback and the magnetic feedback that must be taken into account for in the filter compensation. Furthermore, the acoustic feedback is centered in a higher frequency range than the magnetic feedback. It is also for this reason that different compensation paths are used for each feedback. A single filter for both compensations would clearly be much more elaborate than each of the individually described filters  8  and  10 .  
         [0023]    Although it is possible to use an adaptive filter for each of the filters  8  and  10 , a static filter suffices for compensation of the magnetic feedback, due to the minimal dynamics in the magnetic feedback path.  
         [0024]    The adjustment or setting of the filter coefficients now will be described in more detail. The filter system has, as stated above, at least one microphone  2 , a telecoil  3 , an amplification device  1 , and an earphone or speaker  4 . Furthermore, it has at least one adaptive feedback compensation filter  8  and an adaptive or static feedback compensation filter  10 . During production of the device, or during customization, telephone coil operation is switched on and all other signal inputs are disconnected. For this, the second switch  6  and the third switch  11  are opened and the first switch  5  is closed. Consequently, the adaptive filter  8  is confronted only with the magnetic feedback path  14 . For adjustment, only the gain of the amplification device  1  is increased, until the device feeds back, or is set just short of feeding back. The adaptive feedback compensation filter  8  is thereupon activated, until it has suppressed the feedback. The adaptation is thus stopped and the coefficients are either stored in the device, or sent via a transmission path to or from a customization computer as pre-settings for the second feedback compensation filter  10  located in the hearing aid. This can—as already mentioned—be adaptive as well. When the telephone coil is switched on during normal operation, then the second, now optimally preset compensation filter  10  is activated. This effectively compensates the magnetic feedback path  14 , and the actual adaptive system can be used unrestricted for the acoustic feedback path.  
         [0025]    The adaptation of the adaptive filter  8  to the acoustic feedback path  13  continuously ensues during microphone operation. Consequently, the switches  5  and  11  are opened while the switch  6  is closed.  
         [0026]    In mixed mode, i.e., during operation of the microphone  2  and the telecoil  3  for pick-up of input signals, all switches  5 ,  6 , and  11  are closed. In this condition, the adaptive filter  8  compensates the acoustic feedback path  13 , and the static filter  10  compensates the magnetic feedback path  14 .  
         [0027]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.