Abstract:
An apparatus for and method of employing an electronic hearing aid device to assist a hearing impaired patient. The resonance curve of the outer auditory canal of the patient is determined. A device in accordance with the present invention is tuned to a frequency response curve which matches the measured resonance curve. The device is tuned by adjusting the overshoot of a low pass filter stage which is interposed between the microphone input and a class D output stage.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a circuit for and method of processing an audio frequency signal and more particularly relates to hearing aid signal processing. 
   2. Description of the Prior Art 
   It is well known in the art to utilize electronic devices to assist the hearing impaired. The earliest such instruments consisted of a microphone coupled to an electronic amplifier which was in turn coupled to an earphone. Quite apart from the technical difficulties experienced, these early hearing aids were sufficiently large and intrusive that the hearing impaired could be readily identified providing a degree of self-consciousness. 
   The coming of electronic miniaturization and sub-miniaturization permitted the manufacture of hearing aid systems which are totally inserted in the outer auditory canal during use. The resulting systems provide substantially greater hearing assistance along with a much more pleasing (and almost unnoticeable) aesthetic appearance. A modern, totally in-the-ear device has a microphone acoustically coupled to the ambient with all of the electronics packaged in a form factor which is accommodated by the outer ear of the patient. A transducer is electronically coupled to the output stage of the hearing aid circuit and acoustically coupled to the distal portion of the outer auditory canal. 
   U.S. Pat. No. 4,689,818, issued to Ammitzboll on Aug. 25, 1987, purports to describe the circuitry and operation of the Siemens Custom In-The-Ear Hearing Aid 007, sold by Siemens Hearing Instruments, Inc. This is a typical example of a totally in-the-ear device. 
   A key problem in the miniaturization process is reducing the size of the battery. Whereas substantial progress has been made in battery development, much credit is also appropriately given to designers of low power consumption electronic circuitry. Current state of the art instruments utilize class D output stages which are particularly helpful in reducing overall power consumption. However, as is known to those of skill in the art, the class D output stage tends to have a frequency response curve whose peak gain frequency is not easily modified to accommodate differences in patient pathologies. Yet, abnormalities in middle ear functioning are known to shift the peak in the unaided ear canal resonance to a lower frequency. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the disadvantages of the prior art by providing a technique for utilizing the power saving characteristics of a class D output stage within a system which has sufficient adjustability in frequency response peak gain frequency to accommodate various differences in patient-to-patient middle ear pathology. Specifically, the present invention employs an active low-pass filter which has adjustable overshoot. This filter is coupled through a buffering stage to the class D output amplifier. By adjusting the degree of overshoot, the level of the peak in the frequency response of the entire system is readily adjustable within a given therapeutic range even though the class D output amplifier is inherently difficult to tune. 
   When practicing the present invention, the resonance curve of the outer auditory canal of the patient is determined utilizing existing techniques. This curve is relatively consistent for patients having normal ear physiology. However, various middle ear pathologies often lower the frequency of the basic resonance producing a unique frequency response curve for a given patient. 
   In accordance with the present invention, the overshoot of the low pass filter stage is adjusted such that the frequency response curve of the hearing aid system most nearly matches the resonance curve of the patient&#39;s outer auditory canal. Thus, when the hearing aid is properly inserted, the resulting interface between the hearing assistance device and the patient&#39;s middle ear are very closely correlated. 
   As a result of this frequency response match, the patient is provided with a smooth insertion frequency response without extra amplification at the frequency of the ear canal resonance. The advantages of lower power consumption, lessened probability of acoustic feedback, and improved auditory acuity are the direct products of practicing the present invention. 
   In a preferred mode of practicing the present invention and not to be deemed to be limiting of the scope of the invention, the output of the preamp or signal processing stage is applied to a standard R-C circuit. The resulting signal is coupled through a variable resistor to an amplifying stage, wherein the resistance variability adjusts the overshoot. The active low pass filter output is capacitively coupled to a buffering stage employing a normal operational amplifier. The output of the buffering stage is applied directly to the class D output amplifier. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein: 
       FIG. 1  is the 2 cc coupler frequency response of a typical ITE hearing aid with a class D output stage in the hearing aid receiver; 
       FIG. 2  are real ear IG frequency response curves in: a) the unoccluded outer auditory canal of a patient with normal middle ear function (REUR—bottom) and b) with the hearing aid of  FIG. 1  (REAR—top); 
       FIG. 3  is the response curve of  FIG. 1  superimposed over the response curve shifted with the active low pass filter for a patient with abnormal middle ear pathology; and 
       FIG. 4  is a detailed electronic schematic diagram of the signal processing circuit of the preferred mode of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is described in accordance with several preferred embodiments which are to be viewed as illustrative without being limiting. In the preferred mode, the present invention is employed as a totally within the ear hearing aid system having a class D output stage. 
     FIG. 1  is diagram  10  showing the 2 cc coupler frequency response of a typical ITE hearing aid with a class D output stage in the hearing aid receiver. Abscissa  14  is a logarithmic plot of frequency in kilohertz. Ordinate  12  shows the gain at each frequency plotted in decibels. 
   In a patient having normal middle ear physiology, the ear canal can be thought of as an open organ pipe having a primary resonance at about 2.8 kilohertz and a relatively flat response from about 300 hertz to about 3 kilohertz. As shown in diagram  10 , gain curve  16  for the hearing aid is deliberately designed to match this response to replace the peak in gain lost when the ear canal is occluded by an ear mold. Gain peak  18  occurs at about 2.8 kilohertz. 
     FIG. 2  is diagram  11  showing the real ear IG frequency response curves in: a) the unoccluded outer auditory canal of a patient with normal middle ear function (bottom) and b) with the hearing aid of  FIG. 1  (top). The bottom curve is a typical resonance curve of the unoccluded outer auditory canal (REUR) of a patient having normal middle ear physiology. Abscissa  17  is a logarithmic plot of frequency in kilohertz. Ordinate  19  shows the resonance at each frequency plotted in decibels. The top curve is the typical real ear output of the hearing aid of  FIG. 1  in the ear canal whose unaided ear canal response is shown by the REUR curve. 
   As explained above, the ear canal can be thought of as an open organ pipe having a primary resonance at about 2.8 kilohertz and a relatively flat response from about 300 hertz to about 3 kilohertz. As shown in diagram  11 , REUR curve  15  shows the resonance curve for the typical patient. Resonance peak occurs at about 2.8 kilohertz. 
   For a hearing impaired patient having a totally in-the-ear hearing aid device, the outer auditory canal is totally or partially blocked thus removing the natural resonance at resonance peak  18 . However, it is typical that the class D amplifiers employed in current devices deliberately have a corresponding response peak at about 2.8 kilohertz (see also  FIG. 1 ). Thus, the totally in-the-ear hearing aid device having the class D amplifier can easily provide hearing assistance with a response similar to the non-hearing impaired ear as shown by REAR curve  13 . 
     FIG. 3  is a diagram  20  showing a 2 cc coupler response curve  16  of  FIG. 1  superimposed upon shifted response curve in a 2 cc coupler  22  for a patient having a typical middle ear pathology which lowers the primary resonance of resonance curve  22  to resonance peak  24 . For this example, peak  24  occurs at about 1.2 kilohertz.-8—8- 
   A number of various problems can cause this lowering of the resonance of the outer auditory canal including punctured ear drum, abnormal middle ear bone physiology, etc. If a standard totally in-the-ear hearing aid device, having a class D output amplifier, is utilized in the patient of resonance curve  22 , there will be a substantial mismatch in the frequency response curve of the hearing aid device and that of the open ear of the patient. 
   This mismatch renders most hearing aids incapable of providing enough amplification at the abnormally low resonant peak of frequency of the patient. The result is under-amplification at this frequency and a jagged insertion gain frequency response. 
     FIG. 4  is a detailed electronic schematic diagram  26  showing the critical circuitry of the preferred mode of the present invention. To properly practice the invention, the unaided ear canal resonance curve (REUR) of a given patient is obtained by measurement in accordance with standard procedures and the circuit of the present invention is tuned to match this measured resonance curve, before the device is released to the patient for use. 
   Microphone  28  is a standard hearing aid microphone acoustically coupled to the ambient. The signal produced by microphone  28  is coupled through standard preamplifier  29  and standard signal processing stage  31  to the low pass filter consisting of resistor  30  and capacitor  32 . Variable resistor  38  couples the filtered signal to operational amplifier  42  and forms another pole of the low pass filter with capacitor  40 . In this way, variable resistor  38  controls the amplification gain of the overshoot and the peak frequency of the low pass filter. Thus, variable resistor  38  controls frequency of peak gain in the frequency response curve of the entire hearing aid system. 
   The processed audio frequency signal is capacitively coupled via capacitor  44  to operational amplifier  50  via resistor  46 . Resistor  48  provides feedback for operational amplifier  50  which functions as a buffering stage between the active low pass filter stage and the class D output amplifier. 
   The output of operational amplifier  50  is capacitively coupled via capacitor  52  to standard class D output amplifier  54 . 
   Having thus described the preferred embodiments of the present invention, those of skill in the art will be readily able to adapt the teachings found herein to yet other embodiments within the scope of the claims hereto attached. 
   It will be understood that this disclosure, in many respects, is only illustrative. Changes may be made in details, particularly in matters of shape, size, material, and arrangement of parts without exceeding the scope of the invention. Accordingly, the scope of the invention is as defined in the language of the appended claims.