1.  Field of the Invention
The present invention is directed to a hearing aid of the type having two microphones with automatic balancing of the output signals of the respective microphones, as well as to a method for operating such a hearing aid. 
2.  Description of the Prior Art
Hearing aids are known wherein a subtraction of the signals of two omni-directional microphones ensues for producing a directional microphone characteristic.  The desired directional microphone characteristic of the hearing aid arises due to the phase difference which occurs as a result of the different transit times of the microphone signals. 
In order to achieve a directional microphone characteristic, the two microphones of the hearing aid must exhibit identical frequency responses and sensitivities.  It is therefore necessary to employ microphone pairs and microphone groups that are specifically selected and adapted to one another.  Nevertheless, an undesired shift of the directional microphone characteristic due to different post-assembly modification of the characteristics of the individual microphones, for example due to aging, temperature or radiation influences, cannot be avoided. 
An object of the present invention is to provide a hearing aid as well as a method for the operation of a hearing aid wherein an undesired change of the directional microphone characteristic of the hearing aid is avoided. 
In the inventive hearing aid and method, a first difference element is used to subtract the respective signals from two microphones, with the output of this first difference element being supplied to the earphone of the hearing aid, and a subtraction of average values of the output signals of the microphones ensues with a second difference element.  Proceeding from the identified deviation of the average values of the output signals of the microphones, the amplification of the output signal of at least one of the microphones is regulated by an analysis/control unit following the second difference element until no deviation of the average values of the output signals of the microphones subtracted from one another can be found following the second difference element. 
This makes it possible to combine more economical microphones that are not precisely adapted to one another into microphone pairs or microphone groups in a hearing aid, and to recognize and eliminate the differences of the characteristics of the microphones, for example in the frequency response or in the sensitivity, that exist from the very start or occur during the useful life. 
The manufacturing costs are lowered by expanding the combination possibilities of different microphones.  Further, modifications in the characteristics occurring during the useful life of the microphones can be recognized and corrected, so that a uniformly high microphone quality and accurate directional microphone characteristic is achieved over the entire service life of the hearing aid. 
In one embodiment, an adjustable amplifier element for controlling the amplification of the corresponding output signal is allocated to at least one of the microphones. 
Average values of the output signals of the microphones are subtracted from one another in the second difference element of the inventive hearing aid.  The output signals of the microphones are preferably rectified before being supplied as input signals to the second difference element. 
In another embodiment, the analysis/control unit is an I-regulator, so that a constant repetitive error is not obtained in the control event.  For further stabilization of the control event, the analysis/control unit can be a PI regulator. 
The level of the output signals of the microphones and/or of the output signal of the second difference element can be acquired via a level acquisition element such as a threshold element, in order, for example, to cut-in the automatic microphone balancing only after upward transgression of a minimum level. 
In another embodiment, the output signals of the microphones pass through a filter element (for example, a low-pass filter or a bandpass filter) with which the control event of the inventive hearing aid is activated in an identified frequency range. 
In the inventive method, average values are initially formed proceeding from the output signals of the microphones.  Subsequently, the deviation of the average values from one another is identified and the amplification of at least one of the output signals of a microphone is potentially controlled as needed in order to reduce the deviation of the average values and, ultimately, to eliminate it. 
For example, the RMS values (root mean square) or peak values can be employed as average values of the output signals.  A balancing of the RMS values is more complicated but is also more precise, whereas a balancing of the peak values can ensue within a shorter time. 
In an embodiment of the method, the amplification, in particular, of the less sensitive microphone is boosted when a deviation of the average values from one another is found.  The less sensitive microphone given a hearing aid with two microphones can be identified on the basis of the operational sign of the deviation of the average values. 
The signal amplification of a number of microphones can be adjusted in the same or opposite directions for automatic microphone balancing. 
In order to avoid the intrinsic low level noise of the microphones from being amplified, the inventive method for the automatic microphone balancing can be activated only when an adjustable minimum level is upwardly transgressed. 
When this level is downwardly transgressed, an optimum amplification (gain value) for the output signals of the individual microphones that has already been identified or stored can be set.  The optimum individual amplification of the output signals of the microphones is again individually identified only when the limit level value is exceeded. 
In order to avoid distortions, it can also be advantageous not to implement any automatic microphone balancing outside of an identifiable frequency range.  Thus, for example, the inventive method can be implemented only in a specific frequency range on the basis of a bandpass filter.