Abstract:
A filter for a hearing aid is disclosed. The hearing aid comprises a microphone, an amplifier and a receiver. The filter mitigates the interfering effect of an RF burst on the hearing aid.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/076,571, filed Mar. 2, 1998. 
    
    
     TECHNICAL FIELD 
     The present invention relates to hearings aids and, more particularly, to a system for mitigating the effect of RF interference on a hearing aid. 
     BACKGROUND OF THE INVENTION 
     Technical improvements in digital, cellular telephones using radio transmission have produced a problem for users of such cellular telephones who wear hearing aids. Early cellular telephone system implementations used a system called analog transmission and were not a serious problem. Each user had an exclusive radio channel. The subsequent need for more channels resulted in a technical change in the method of transmission. To obtain better utilization of the radio spectrum, systems were devised to gather signals during short periods of time and transmit the signals in much shorter bursts of radio energy. At the receiving end, this burst was expanded to its original length of time, resulting in a signal which sounded continuous. During the time between bursts, other callers could be using the same channel. To accomplish this, the radio transmitter had to be turned off between bursts. This intermittent radio transmission interferes with many electronic devices. While the analog cellular telephones also interacted with these devices, because it transmitted continuously, it only made a momentary disruption at the beginning and end of the call. If the information stream did not interact with the intended use of the device, it produced no real harm. 
     On the other hand, the improved transmission system turned the transmitter on and off at a rate that produced disruptions at a frequency that was quite audible to users of hearing aids. The rate of these disruptions for practical reasons was in the lower audible range. Such systems need to operate at pulse rates between tens of pulses per second to a few hundred pulses per second. There are at this time, systems pulsing at 50 times per second and at 217 times per second. While in use, the radio transmitter in the telephone handset must necessarily be in close proximity to the hearing aid, therefore, there is an intense radio frequency signal intercepting the hearing aid. This produces disturbances in the electronics of conventional hearing aids, which make their use impossible. 
     The problem in the hearing aid was the result of the radio frequency signals interacting with the semiconductor components in the hearing aid that are necessary for its operation. There are many avenues that this disruptive RF signal can enter the hearing aid circuitry. The conventional prior art methods ameliorating this problem are to prevent the entry of the radio signal to the sensitive portions of the hearing aid circuitry. Common approaches include shielding, reducing the sensitivity to radio frequency pick up by arranging the wiring and attenuating the propagation of the radio frequency as it approaches demodulating components in the hearing aid. These helped the hearing aid&#39;s performance, but frequently left an annoying residual buzz. 
     While the previously mentioned precautions are necessary, if a way could be devised to reduce the residual interference, the quality of the transmission could be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a frequency domain representation of the signal output of a typical digital cellular telephone; 
     FIG. 2 is a frequency domain representation of a comb filter; 
     FIG. 3 is a block diagram of an apparatus for mitigating RF interference in a hearing aid in accordance with the present invention; 
     FIG. 4 is a simplified schematic diagram of another embodiment of an apparatus for mitigating RF interference; 
     FIG. 5 illustrates the frequency response of the various outputs of the apparatus shown in FIG. 4; 
     FIG. 6 is a simplified schematic diagram of yet another embodiment of a device for mitigating RF interference in a hearing aid; 
     FIG. 7 depicts the frequency response of the device of FIG. 6; and, 
     FIG. 8 is a phase plot of the device of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail a preferred embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiment illustrated. 
     The realization of a method and apparatus by which these residual RF interferences could be attenuated is the subject of this invention. A hearing aid typically consists of a series of functions that describe its workings. First, a microphone receives the acoustic signal. This produces a weak electrical signal transmitted to an amplifier usually containing signal-crossing functions, which increase the magnitude of the signal. This is followed by a third function where this signal is routed to a receiver, which converts the signal into an amplified acoustic signal in the user&#39;s ear. It is at this point, if any remaining disruptive signal could be removed the quality of the signal could be improved. 
     It is necessary to understand the nature of this unwanted component of the signal. The unwanted component results from the handset transmitter being turned on and off repeatedly for very short times separated by longer spacing intervals. In electrical terms, this resolves into a signal having frequency components that are harmonics (multiples) of this repetition rate. A spectrum representing these frequencies is illustrated in FIG.  1 . In accordance with the invention, a filter is provided to suppress these harmonic frequencies, and the effective quality of the desired signal transmission is improved. These interfering signals are rigid in their arrangement, and the signal processing used for the therapeutic aims of the hearing aid does not change their frequencies. The filter that will suppress such an array is known as a “comb” filter, because the plot of transmission characteristics produces a series of low transmission frequencies at regular intervals, creating the visual impression of a comb. See FIG.  2 . The object is to make these attenuated transmission frequencies coincide with the frequencies of the disruptive signals, while letting the desired signal pass relatively unimpeded. 
     FIG. 3 is a block diagram of one arrangement that produces the desired result. Sound enters the microphone  12 , and is converted to an electrical signal. The electrical signal is passed on to the amplifier and signal processor  14  to perform the normal functions of the hearing aid. If there is an offending digital cellular telephone handset in operation near this hearing aid, or some other similarly offending device, the RF bursts from the telephone will be rectified in the semiconductors, if the defensive measures in the hearing aid mentioned earlier are not completely effective. The RF burst will then create reoccurring pulses in the signal string being fed to a first pulse detector  16 , a signal delay apparatus  18  and a signal differencing amplifier  20 . After the pulse traverses the signal delay apparatus  18 , it will be again be recognized by a second pulse detector  22 . If the pulse from the next burst arrives from the first pulse detector  16  at the same time as the first pulse arrives at the pulse timing detector, the time delay will be as desired. If they are not coincident, a pulse timing comparator  24  applies a correction to the signal delay apparatus  18  to make the delay for the next pair of pulses coincident. 
     When the two consecutive pulses arrive simultaneously at the signal differencing amplifier  20 , being equal or very nearly equal, they cancel the first pulse as a component of the signal to the receiver. The signal, unless it is repeating at the same rate as the pulses, will be different and is not canceled. Some information is lost, but enough is retained to leave an improved signal at the output  26  of the hearing aid. 
     It is contemplated that the filter would be located within the hearing aid, potentially within the receiver itself. Although the filter is shown in block diagram fort in FIG. 3, those skilled in the art will readily realize that the present invention can be implemented as a program for controlling the operation of a digital signal processor within a hearing aid. 
     Turning to FIG. 4, a simplified schematic diagram is depicted of another embodiment of an apparatus for mitigating RF interference. In particular, the device  110  includes two delay lines to increase bandwidth and provide a mid-point tap. The input of the first delay line  112  is operably coupled to a microphone  114  and the inverting inputs of a first amplifier  116  and a second amplifier  118  having preferred gains of about 1 and 0.8125, respectively. The output of the first delay line  112  is operably coupled to the input of the second delay line  120  and the non-inverting input of the first amplifier  116 . The output of the second delay line  120  is operably coupled to the non-inverting input of the second amplifier  118  and a pull-down resistor  122  tied to ground with a resistance of about 100 ohms. Coupled between the outputs of the amplifiers  116 , 118  are a pair of serially connected resistors  124 , 126  having individual resistance values of about 1000 ohms each. As shown in FIG. 5, the frequency response of the various outputs of the device  110  are depicted wherein the RF interference is substantially mitigated at the output  128  between the serially connected resistors  124 , 126 . 
     Turning to FIG. 6, a simplified schematic diagram is depicted of another embodiment of a filter device  210  for mitigating RF interference that uses only one delay line  220 . In this embodiment, a pair of serially connected resistors  224 , 226  are coupled between a microphone  214  and the output of a second amplifier  218 . The resistors  224  and  226  preferably have resistance values of about 1000 and 1500 ohms, respectively. 
     Coupled to the junction  217  between the resistors  224 , 226  is the non-inverting input of a first amplifier  216  with its inverting input attached to ground and having a preferred gain of about 1. The output of the first amplifier  216  is attached to the input of the delay line  220  and the inverting input of the second amplifier  218 . The output of the delay line  220  is coupled to the non-inverting input of the second amplifier  218  and a pull down resistor  222  tied to ground. Preferably, the second amplifier  218  has a gain of about 10 and the resistor  222  has a resistance value of about 100 ohms. 
     In FIG. 6, all signals from the microphone  214  pass through the filter device  210  to provide continuous filtering. Thus, comb filtering is always achieved. 
     In particular, the first amplifier  216  drives the filter wherein the output  228  of the filter is summed with the input signal from the microphone  214  by an amount determined by the resistance values of resistors  224  and  226 . Accordingly, the signal arriving from the output  228  and through resistor  226  is similar to a negative feedback amplifier for producing flat responses. Thus, device  210  flattens the response by increasing the transmission near the comb frequencies, but the notch frequencies cannot reach the output terminal  228 . 
     The result of the filtering by device  210  is depicted in FIGS. 7 and 8 wherein, for convenience, the device is set to 200 Hz rather than the desired frequency of 217 Hz. In particular, FIG. 7 depicts the frequency response of the device  210  at output  228 . Further, FIG. 8 provides a phase plot of the device  210 . 
     While the specific embodiment has been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention and the scope of protection is only limited by the scope of the accompanying Claims.