WO2000015001A2 - Hearing aid device incorporating signal processing techniques - Google Patents

Hearing aid device incorporating signal processing techniques Download PDF

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Publication number
WO2000015001A2
WO2000015001A2 PCT/US1999/020841 US9920841W WO0015001A2 WO 2000015001 A2 WO2000015001 A2 WO 2000015001A2 US 9920841 W US9920841 W US 9920841W WO 0015001 A2 WO0015001 A2 WO 0015001A2
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output
input
element
hearing
log
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PCT/US1999/020841
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French (fr)
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WO2000015001A3 (en
Inventor
Douglas M. Chabries
William C. Borough
Richard W. Christiansen
Thomas G. Stockham, Jr.
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Sonic Innovations, Inc.
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Priority to US09/169,547 priority
Application filed by Sonic Innovations, Inc. filed Critical Sonic Innovations, Inc.
Publication of WO2000015001A2 publication Critical patent/WO2000015001A2/en
Publication of WO2000015001A3 publication Critical patent/WO2000015001A3/en

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    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03GCONTROL OF AMPLIFICATION
    • H03G9/00Combinations of two or more types of control, e.g. gain control and tone control
    • H03G9/005Combinations of two or more types of control, e.g. gain control and tone control of digital or coded signals
    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03GCONTROL OF AMPLIFICATION
    • H03G9/00Combinations of two or more types of control, e.g. gain control and tone control
    • H03G9/02Combinations of two or more types of control, e.g. gain control and tone control in untuned amplifiers
    • H03G9/025Combinations of two or more types of control, e.g. gain control and tone control in untuned amplifiers frequency-dependent volume compression or expansion, e.g. multiple-band systems
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04RLOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R25/00Deaf-aid sets, i.e. electro-acoustic or electro-mechanical hearing aids; Electric tinnitus maskers providing an auditory perception
    • H04R25/45Prevention of acoustic reaction, i.e. acoustic oscillatory feedback
    • H04R25/453Prevention of acoustic reaction, i.e. acoustic oscillatory feedback electronically

Abstract

A hearing compensation system for the hearing impaired comprises a plurality of bandpass filters having an input connected to an input transducer and each bandpass filter having an output connected to the input of one of a plurality of multiplicative AGC circuits whose outputs are summed together and connected to the input of an output transducer. The multiplicative AGC circuits attenuate acoustic signals having a constant background level without the loss of speech intelligibility. The identification of the background noise portion of the acoustic signal is made by the constancy of the envelope of the input signal in each of the several frequency bands. The background noise that will be suppressed includes multi-talker speech babble, fan noise, feedback whistle, fluorescent light hum, and white noise.

Description

S P ECI F I C A T I O N

HEARING AID DEVICE INCORPORAΗNG SIGNAL PROCESSING TECHNIQUES

RELATED APPLICATIONS

This application is a continuation-in-part of United States patent application, Serial No.

08/697,412, filed August 22, 1996, which is a continuation-in-part of United States patent application, Serial No. 08/585,481, filed January 12, 1996, which is a continuation of United States patent application, Serial No. 08/272,927, filed July 8, 1994, now United States Patent No. 5,500,902.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

The present invention relates to electronic hearing devices and electronic systems for sound reproduction. More particularly the present invention relates to noise suppression to preserve the fidelity of signals in electronic hearing aid devices and electronic sound systems. According to the present invention, the noise suppression devices and methods utilize both analog and digital signal processing techniques.

2. The Prior Art.

One of the most common complaints made by hearing aid users is the inability to hear in the presence of noise. Accordingly, the elimination of noise has long been the focus of researchers, and many approaches to solving the noise elimination problem have been proposed. In one approach, an independent measure of the noise is made and then subtracted from the signal being processed. This technique is typically applied to signals that are expressed as follows: s(t) = d(t) + n(t) Wherein s(t) is the signal being processed, d(t) is the desired portion of the signal s(t), and n(t) the noise in the signal s(t). For example, one or more sensors may be employed along with adaptive techniques to form an independent measure of the estimate of the noise, ne(t) from interference. By subtracting the noise estimate, ne(t), from the signal, s(t), an improved version of the desired signal, d(t), is obtained. To emphasize the subtraction of the noise estimate, ne(t), this technique is commonly referred to as "noise canceling." This noise canceling technique has been applied to both sonar systems, medical fetal electrocardiograms, and has further been found to be effective to process acoustic signals containing both speech and interference. See for example, Douglas M. Chabries, et al., "Application of Adaptive Digital Signal Processing to Speech Enhancement for the Hearing Impaired," Journal of Rehabilitation Research and Development, Vol. 24, No. 4, pp. 65-74, and Robert H. Brey, et al., "Improvement in Speech Intelligibility in Noise Employing an Adaptive Filter with Normal and Hearing- Impaired Subjects," Journal of Rehabilitation Research and Development, Vol, 24, No. 4, pp. 75-86.

When no independent sample or estimate of the noise is available, other techniques to provide noise elimination have been employed. In several instances, researchers have exploited the differences in the temporal properties of speech and noise to enhance the intelligibility of sound. These techniques are typically referred to as noise suppression or speech enhancement. See for example, United States Patent 4,025,721 to Graupe, United States Patent 4,185,168 to Graupe, and S. Boll, "Suppression of Acoustic Noise in Speech Using Spectral Subtraction," IEEE Trans, on ASSP, Vol. ASSP-27, pp. 113-120, April

1979, H. Sheikhzadeh, et, al., "Comparative Performance of Spectral Subtraction and HMM- Based Speech Enhancement Strategies with Application to Hearing Aid Design," Proc. IEEE ICASSP, pp. 1-13 to 1-17, 1994, and P.M Crozier, BMG Cheethan, C. Holt, and E. Munday, "Speech enhancement employing spectral subtraction and linear predictive analysis," Electronic Letters, 29(12):1094-1095, 1993.

These approaches have been shown to enhance particular signals in comparison to other signals that have been defined as noise. One researcher, Mead Killion, has noted that none of these approaches has enhanced speech intelligibility. See Mead Killion, Etymotic Update, Number 15, Spring 1997. However in low noise environments compression techniques have been shown to relieve hearing deficits. See Mead Killion, "The SIN report: Circuits haven't solved the hearing-in-noise problem," The Hearing Journal, Vol. 50, No. 20, October 1997, pp. 28-34. With these techniques, researchers have generally noted a decrease in speech intelligibility testing when noise contaminated speech is processed, despite the fact that measures of quality or preference increase. Typically, the specification of the noise characteristics or the definition of the speech parameters distinguish the various techniques in the second category of noise elimination from one another. It has been demonstrated that acoustic signals can be successfully processed according to these techniques to enhance voiced or vowel sounds in the presence of white or impulsive noise, however, these techniques are less successful in preserving unvoiced sounds such as fricatives or plosives.

Other noise suppression techniques have been developed wherein speech is detected and various proposed methods are employed to either turn off the amplifier in a hearing aid when speech is not present or to clip speech and then turn off the output amplifier in the absence of detectable speech. See for example, Harry Teder, "Hearing Instruments in Noise and the Syllabic Speech-to- Noise Ration," Hearing Instruments, Vol. 42, No. 2, 1991. Further examples of the approach to noise elimination by suppressing noise to enhance the intelligibility of sound are found in United States Patents 4,025,721 to Graupe, 4,405,831 to Michaelson, 4,185,168 to Graupe et al., 4,188,667 to Graupe et al, 4,025,721 to Graupe et al., 4,135,590 to Gaulder, and 4,759,071 to Heide et al.

Other approaches have focussed upon feedback suppression and equalization (United States Patents 4,602,337 to Cox, and 5,016,280 to Engebretson, and see also Leland C. Best, "Digital Suppression of Acoustic Feedback in Hearing Aids, " Thesis, University of

Wyoming, May 1995 and Rupert L. Goodings, Gideon A. Senensieb, Phillip H. Wilson, Roy S. Hansen, "Hearing Aid Having Compensation for Acoustic Feedback," United States Patent 5,259,033 issued Nov. 2, 1993.), dual microphone configurations (United States Patents 4,622,440 to Slavin and 3,927,279 to Nakamura et al.), or upon coupling to the ear in unusual ways (e.g., RF links, electrical stimulation, etc.) to improve intelligibility. Examples of these approaches are found in United States Patents 4,545,082 to Engebretson, 4,052,572 to Shafer, 4,852,177 to Ambrose, and 4,731,850 to Levitt.

Still other approaches have opted for digital programming control implementations which will accommodate a multitude of compression and filtering schemes. Examples of such approaches are found in United States Patents 4,471,171 to Kopke et al. and 5,027,410 to Williamson. Some approaches, such as that disclosed in United States Patent 5,083,312 to Newton, utilize hearing aid structures which allow flexibility by accepting control signals received remotely by the aid. United States Patent 4,187,413 to Moser discloses an approach for a digital hearing aid which uses an analog-to-digital converter, a digital-to-analog converter, and implements a fixed transfer function H(z). However, a review of neuro-psychological models in the literature and numerous measurements resulting in Steven's and Fechner's laws (see S. S. Stevens, Psychophysics, Wiley 1975; G. T. Fechner, Elemente der Psychophysik, Breitkopf u. Hartel, Leipzig, 1960) conclusively reveal that the response of the ear to input sound is nonlinear. Hence, no fixed linear transfer function H(z) exists which will fully compensate for hearing.

United States Patent 4,425,481 to Mangold, et. al. discloses a programmable digital signal processing (DSP) device with features similar or identical to those commercially available, but with added digital control in the implementation of a three-band (lowpass, bandpass, and highpass) hearing aid. The outputs of the three frequency bands are each subjected to a digitally-controlled variable attenuator, a limiter, and a final stage of digitally- controlled attenuation before being summed to provide an output. Control of attenuation is apparently accomplished by switching in response to different acoustic environments.

United States Patents 4,366,349 and 4,419,544 to Adelman describe and trace the processing of the human auditory system, but do not reflect an understanding of the role of the outer hair cells within the ear as a muscle to amplify the incoming sound and provide increased basilar membrane displacement. These references assume that hearing deterioration makes it desirable to shift the frequencies and amplitude of the input stimulus, thereby transferring the location of the auditory response from a degraded portion of the ear to another area within the ear (on the basilar membrane) which has adequate response.

Mead C. Killion, The k-amp hearing aid: an attempt to present high fidelity for persons with impaired hearing, American Journal of Audiology, 2(2): pp. 52-74, July 1993, states that based upon the results of subjective listening tests for acoustic data processed with both linear gain and compression, either approach performs equally well. It is argued that the important factor in restoring hearing for individuals with losses is to provide the appropriate gain. Lacking a mathematically modeled analysis of that gain, several compression techniques have been proposed, e.g., United States Patent 4,887,299 to Cummins; United States Patent 3,920,931 to Yanick, Jr. ; United States Patent 4, 118,604 to Yanick, Jr. ; United States Patent 4,052,571 to Gregory; United States Patent 4,099,035 to Yanick, Jr. and United States Patent 5,278,912 to Waldhauer. Some involve a linear fixed high gain at soft input sound levels and switch to a lower gain at moderate or loud sound levels. Others propose a linear gain at the soft sound intensities, a changing gain or compression at moderate intensities and a reduced, fixed linear gain at high or loud intensities. Still others propose table look-up systems with no details specified concerning formation of look-up tables, and others allow programmable gain without specification as to the operating parameters.

Switching between the gain mechanisms in each of these sound intensity regions has introduced significant distracting artifacts and distortion in the sound. Further, these gain- switched schemes have been applied typically in hearing aids to sound that is processed in two or three frequency bands, or in a single frequency band with pre-emphasis filtering.

Insight into the difficulty with prior art gain-switched schemes may be obtained by examining the human auditory system. For each frequency band where hearing has deviated from the normal threshold, a different sound compression is required to provide for normal hearing sensation to result. The application of gain schemes which attempt to combine more than a critical band (i.e., resolution band in hearing as defined in Jack Katz (Ed.) Handbook of Clinical Audiology, Williams & Wilkins, Baltimore, third ed., 1985) in frequency range cannot produce the appropriate hearing sensation in the listener. If, for example, it is desired to combine two frequency bands then some conditions must be met in order for the combination to correctly compensate for the hearing loss. These conditions for the frequency bands to be combined are that they have the same normal hearing threshold and dynamic range and require the same corrective hearing gain. In general, this does not occur even if a hearing loss is constant in amplitude across several critical bands of hearing. Failure to properly account for the adaptive full-range compression will result in degraded hearing or equivalently, loss of fidelity and intelligibility by the hearing impaired listener. Therefore, prior art which does not provide sufficient numbers of frequency bands to compensate for hearing losses will produce degraded hearing.

Several schemes have been proposed which use multiple bandpass filters followed by compression devices (see United States Patents 4,396,806 to Anderson, 3,784,750 to Stearns et al., and 3,989,904 to Rohrer).

One example of prior art in United States Patent No. 5,029,217 to Chabries focussed on an FFT frequency domain version of a human auditory model. The FFT implements an efficiently-calculated frequency domain filter which uses fixed filter bands in place of the critical band equivalents which naturally occur in the ear due to its unique geometry, thereby requiring that the frequency resolution of the FFT be equivalent to the smallest critical band to be compensated. The efficiency of the FFT is in large part negated by the fact that many additional filter bands are required in the FFT approach to cover the same frequency spectrum as a different implementation with critical bandwidth filters. This FFT implementation is complex and likely not suitable for low-power battery applications.

The prior-art FFT implementation introduces a block delay into the processing system inherent in the FFT itself. Blocks of samples are gathered for insertion into the FFT. This block delay introduces a time delay into the sound stream which is annoying and can induce stuttering when one tries to speak or can introduce a delay which sounds like an echo when low levels of compensation are required for the hearing impaired individual.

For acoustic input levels below hearing threshold(i.e. soft background sounds which are ever present), the FFT implementation described above provides excessive gain. This results in artifacts which add noise to the output signal. At hearing compensation levels greater than 60 dB, the processed background noise level can become comparable to the desired signal level in intensity thereby introducing distortion and reducing sound intelligibility.

As noted above, the hearing aid literature has proposed numerous solutions to the problem of hearing compensation for the hearing impaired. While the component parts that are required to assemble a high fidelity, full-range, adaptive compression system have been known since 1968, no one has to date proposed the application of the multiplicative AGC to the several bands of hearing to compensate for hearing losses.

As will be appreciated by those of ordinary skill in the art, there are three aspects to the realization of a high effectiveness aid for the hearing impaired. The first is the conversion of sound energy into electrical signals. The second is the processing of the electrical signals so as to compensate for the impairment of the particular individual which includes the elimination of noise from the acoustic signal being input to a hearing aid user while preserving the intelligibility of the acoustic signal.. Finally, the processed electrical signals must be converted into sound energy in the ear canal.

Modern electret technology has allowed the construction of extremely small microphones with extremely high fidelity, thus providing a ready solution to the first aspect of the problem. The conversion of sound energy into electrical signals can be implemented with commercially available products. A unique solution to the problem of processing of the electrical signals to compensate for the impairment of the particular individual is set forth herein and in parent application serial No. 08/272,927 filed July 8, 1994, now United States Patent No. 5,500,902. The third aspect has, however, proved to be problematic, and is addressed by the present invention.

An in-the-ear hearing aid must operate on very low power and occupy only the space available in the ear canal. Because the hearing-impaired individual has lower sensitivity to sound energy than a normal individual, the hearing aid must deliver sound energy to the ear canal having an amphtude large enough to be heard and understood. The combination of these requirements dictates that the output transducer of the hearing aid must have high efficiency.

To meet this requirement transducer manufacturers such as Knowles have designed special iron-armature transducers that convert electrical energy into sound energy with high efficiency. To date this high efficiency has been achieved at the expense of extremely poor frequency response.

The frequency response of prior art transducers not only falls off well before the upper frequency limit of hearing, but also shows resonances starting at about 1 to 2 kHz, in a frequency range where they confound the information most useful in understanding human speech. These resonances are also primarily responsible for the feedback oscillation so commonly associated with hearing aids, and subject signals in the vicinity of the resonant frequencies to severe intermodulation distortion by mixing them with lower frequency signals. These resonances are a direct result of the mass of the iron armature, which is required to achieve good efficiency at low frequencies. In fact it is well known by those of ordinary skill in the art of transducer design that any transducer that is highly efficient at low frequencies will exhibit resonances in the mid-frequency range.

A counterpart to this problem occurs in high-fidelity loudspeaker design, and is solved in a universal manner by introducing two transducers, one that provides high efficiency trans- duction at low frequencies (a woofer), and one that provides high-quality transduction of the high frequencies (a tweeter). The audio signal is fed into a crossover network which directs the high frequency energy to the tweeter and the low frequency energy to the woofer. As will be appreciated by those of ordinary skill in the art, such a crossover network can be inserted either before or after power amplification. From the above recitation, it should be appreciated that many approaches have been taken in the hearing compensation art to improve the intelligibility of the acoustic signal being input to the user of a hearing compensation device. These techniques include both compensating for the hearing deficits of the hearing impaired individual by various methods, and also for removing or suppressing those aspects of the acoustic signal, such as noise, that produce an undesirable effect on the intelligibility of the acoustic signal. Despite the multitude of approaches, as set forth above, that have been taken to provide improved hearing compensation for hearing impaired individuals, there remains ample room for improvement.

BRIEF DESCRIPTION OF THE INVENTION According to the present invention, a hearing compensation system for the hearing impaired comprises a plurality of bandpass filters having an input connected to an input transducer and each bandpass filter having an output connected to the input of one of a plurality of multiplicative AGC circuits whose outputs are summed together and connected to the input of an output transducer.

The multiplicative AGC circuits attenuate acoustic signals having a constant background level without the loss of speech intelligibility. The identification of the background noise portion of the acoustic signal is made by the constancy of the envelope of the input signal in each of the several frequency bands. It is presently contemplated mat examples of background noise that will be suppressed according to the present invention include multi-talker speech babble, fan noise, feedback whistle, florescent light hum, and white noise.

BRIEF DESCRIPTION OF THE DRAWING FIGURES FIG. 1 illustrates a block diagram of a hearing compensation system according to the present invention.

FIG. 2A illustrates a block diagram of a first embodiment of a multiplicative AGC circuit suitable for use according to the present invention.

FIG. 2B illustrates a block diagram of an alternative embodiment of the multiplicative AGC circuit shown in FIG. 2A suitable for use according to the present invention.

FIG. 2C illustrates a block diagram of a first embodiment of a multiplicative AGC circuit with noise elimination according to the present invention. FIG. 3 is a plot of the response characteristics of the filter employed in the multiplicative AGC circuit of FIG. 2A.

FIGS. 4A-4C illustrate plots of the response characteristics of the filters employed in the multiplicative AGC circuit of FIG. 2C according to the present invention.

FIG. 5 A illustrates a block diagram of a second embodiment of a multiplicative AGC circuit suitable for use according to the present invention.

FIG. 5B illustrates a block diagram of an alternative embodiment of the multiplicative AGC circuit shown in FIG. 5A suitable for use according to the present invention.

FIG. 5C illustrates a block diagram of a second embodiment of a multiplicative AGC circuit with noise elimination according to the present invention.

FIG. 5D illustrates a block diagram of a third embodiment of a multiplicative AGC circuit with noise elimination according to the present invention.

FIG. 6 illustrates the implementation of a high pass filter suitable for use according to the present invention.

FIGS. 7 A and 7B illustrate plots of the response characteristics of the filters employed in the multiplicative AGC circuit of FIGS. 5C and 5D according to the present invention.

FIG. 8 illustrates a noise estimator suitable for replacing the filters depicted in FIGS 5C and 5D according to the present invention.

FIG. 9A illustrates a block diagram of a third embodiment of a multiplicative AGC circuit suitable for use according to the present invention.

FIG. 9B illustrates a block diagram of an alternative embodiment of the multiplicative AGC circuit shown in FIG. 9A suitable for use according to the present invention.

FIG. 10 A illustrates a block diagram of a presently preferred embodiment of a multiplicative AGC circuit according to the present invention. FIG. 10B illustrates a block diagram of a fourth embodiment of a multiplicative AGC circuit having noise elimination according to the present invention.

FIG. 11 illustrates a plot of the three slope gain regions of the multiplicative AGC circuits of FIGS. 10A and 10B according to the present invention.

FIG. 12 is a block diagram of an in-the-ear hearing compensation system according to the present invention employing two electrical signal-to-acoustical energy transducers.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Those of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons.

It has been discovered that the appropriate approach to high fidelity hearing compensation is to separate the input acoustic stimulus into frequency bands with a resolution at least equal to the critical bandwidth, which for a large range of the sound frequency spectrum is less than 1/3 octave, and apply a multiplicative AGC with either a fixed or variable exponential gain coefficient for each band.

According to the present invention, the multiplicative AGC attenuates acoustic signals having a constant background level without the loss of speech intelligibility. The portion of the input signal which comprises the background noise portion of the acoustic signal is attenuated in amphtude without distortion to preserve the intelligibility of the acoustic input signal. The identification of the background noise portion of the acoustic signal is made by the constancy of the envelope of the input signal in each of the several frequency bands, as will be described below.

During highly dynamic variations in sound level, the output signal of the hearing compensation circuit due to noise suppression will be nearly the same as the output of the hearing compensation system without noise suppression, and that during the quiescent periods between words that the output signal will have a significantly quieter background level due to the noise suppression of the present invention. It is presently contemplated that examples of background noise that will be suppressed according to the present invention include multi- talker speech babble, fan noise, feedback whistle, florescent light hum, and white noise. Those of ordinary skill in the art will recognize that the principles of the present invention may be applied to audio applications other than hearing compensation for the hearing impaired. Non-exhaustive examples of other applications of the present invention include music playback for environments with high noise levels, such as automotive environments, voice systems in factory environments, and graphic sound equalizers such as those used in stereophonic sound systems.

As will be appreciated by persons of ordinary skill in the art, the circuit elements of the hearing compensation apparatus of the present invention may be implemented as either an analog circuit or as a digital circuit, preferably a microprocessor or other computing engine performing digital signal processing (DSP) functions to emulate the analog circuit functions of the various components such as filters, amplifiers, etc. It is presently contemplated that the DSP version of the circuit is the preferred embodiment of the invention, but persons of ordinary skill in the art will recognize that an analog implementation, such as might be integrated on a single semiconductor substrate, will also fall within the scope of the invention. Such skilled persons will also realize that in a DSP implementation, the incoming audio signal will have to be time sampled and digitized using conventional analog to digital conversion techniques.

Referring first to FIG. 1, a block diagram of a presently preferred hearing compensation system 8 according to the present invention is presented. The hearing compensation system 8 according to a presently preferred embodiment of the invention includes an input transducer 10 for converting acoustical energy (shown schematically at reference numeral 12) into an electrical signal corresponding to that acoustical energy. Various known hearing-aid microphone transducers, such as a model EK 3024, available from Knowles Electronics of Ithaca, Illinois, are available for use as input transducer 10, or other microphone devices may be employed.

In FIG. 1, three audio bandpass filters are shown at reference numerals 14-1, 14-2 . . . 14-n to avoid over complicating the drawing. According to a presently preferred embodiment of the invention, n will be an integer from 9 to 15, although persons of ordinary skill in the art will understand that the present invention will function if n is a different integer.

There are preferably nine audio bandpass filters 14-1 to 14-n having a bandpass resolution of approximately 1/2 octave. The bandpass filters 14-1 through 14-n are preferably realized as fifth-order Chebychev band-split filters which provide smooth frequency response in the passband and about 65 dB attenuation in the stopband. The design of 1/2 octave bandpass filters is well within the level of skill of the ordinary worker in the art. Therefore the details of the circuit design of any particular bandpass filter, whether implemented as an analog filter or as a DSP representation of an analog filter, will be simply a matter of design choice for such skilled persons.

In an alternative embodiment, audio bandpass filters 14-1 to 14-n preferably have a bandpass resolution of 1/3 octave or less, but in no case less than about 125 Hz, and have their center frequencies logarithmically spaced over a total audio spectrum of from about 200 Hz to about 10,000 Hz. The audio bandpass filters may have bandwidths broader than 1/3 octave, i.e., up to an octave or so, but with degrading performance. In this alternative embodiment, the bandpass filters 14-1 through 14-n are realized as eighth-order Elliptic filters with about 0.5 dB ripple in the passband and about 70 dB rejection in the stopband.

Those of ordinary skill in the art will recognize that several bandpass filter designs including, but not limited to, other Elliptic, Butterworth, Chebyshev, or Bessel filters, may be employed. Further, filter banks designed using wavelets, as disclosed, for example, in R. A. Gopinath, "Wavelets and Filter Banks- New Results and Applications", PhD Dissertation, Rice University, Houston, Texas, May 1993, may offer some advantage. Any of these bandpass filter designs may be employed without deviating from the concepts of the invention disclosed herein.

Each individual bandpass filter 14-1 to 14-n is cascaded with a corresponding multiplicative automatic gain control (AGC) circuit. Three such devices 16-1, 16-2, and 16-n are shown in FIG. 1. Multiplicative AGC circuits are known in the art and an exemplary configuration will be disclosed further herein.

The outputs of the multiplicative AGC circuits are summed together and are fed to an output transducer 18, which converts the electrical signals into acoustical energy. As will be appreciated by those of ordinary skill in the art, output transducer 18 may be one of a variety of known available hearing-aid earphone transducers, such as a model ED 1932, available from Knowles Electronics of Ithaca, Illinois, in conjunction with a calibrating amplifier to ensure the transduction of a specified electrical signal level into the correspondingly specified acoustical signal level. Alternately, output transducer 18 may be another earphone-like device or an audio power amplifier and speaker system. Referring now to FIG. 2A, a more detailed conceptual block diagram of a typical multiplicative AGC circuit 16-n suitable for use according to the present invention is shown. As previously noted, multipUcative AGC circuits are known in the art. An illustrative multiplicative AGC circuit which will function in the present invention is disclosed in the article T. Stockham, Jr., The Application of Generalized Linearity to Automatic Gain Control, IEEE Transactions on Audio and Electroacoustics, AU-16(2): pp 267-270, June 1968. A similar example of such a multipUcative AGC circuit may be found in United States Patent No. 3,518,578 to Oppenheim et al.

Conceptually, the multiplicative AGC circuit 16-n which may be used in the present invention accepts an input signal at amplifier 20 from the output of one of the audio bandpass filters 14-n. Amplifier 20 is set to have a gain of l/emax, where emax is the maximum allowable value of the audio envelope for which AGC gain is applied (i.e., for input levels above emax , AGC attenuation results). Within each band segment in the apparatus of the present invention, the quantity emax is the maximum acoustic intensity for which gain is to be appUed. This gain level for emax (determined by audiological examination of a patient) often corresponds to the upper comfort level of sound. In an analog implementation of the present invention, amphfier 20 may be a known operational amphfrer circuit, and in a DSP implementation, amplifier 20 may be a multiplier function having the input signal as one input term and the constant 1/e^^ as the other input term.

The output of amplifier 20 is processed in the "LOG" block 22 to derive the logarithm of the signal. The LOG block 22 derives a complex logarithm of the input signal, with one output representing the sign of the input signal and the other output representing the logarithm of the absolute value of the input. Those of ordinary skill in the art wiU recognize that by setting the gain of the amplifier 20 to llemax, the output of amphfier 20 (when the input is less than emαχ,) will never be greater than one and the logarithm term out of LOG block 22 will always be 0 or less.

In a DSP implementation, LOG block 22 is reahzed preferably by employing a circuit that converts binary numbers to a floating point format in a manner consistent with the method described in "ADSP-2100 Family Applications Handbook," Volume 1, published by Analog Devices, pp. 46-48. In this implementation, several different bases for the logarithm may be employed. The LOG block 22 may be alternatively implemented as a software subroutine running on a microprocessor or similar computing engine as is well known in the art, or from other equivalent means such as a look-up table. Examples of such implementations are found in Knuth, Donald E., The Art of Computer Programming, Vol. 1, Fundamental Algorithms, Addison-Wesley Publishing 1968, pp. 21-26 and Abramowitz, M. and Stegun, A., Handbook of Mathematical Functions, US Department of Commerce, National Bureau of Standards, Appl. Math Series 55, 1968.

In an analog implementation of the present invention, LOG block 22 may be, for example, an amplifier having a logarithmic transfer curve, or a circuit such as the one shown in FIGS. 8 and 9 of United States Patent No. 3,518,578.

The first output of LOG block 22 containing the sign information of its input signal is presented to a Delay block 24, and a second output of LOG block 22 representing the logarithm of the absolute value of the input signal is presented to a filter 26 having a characteristic preferably like that shown in FIG. 3. ConceptuaUy, filter 26 may comprise both high-pass filter 28 and low-pass filter 30 followed by amphfier 32 having a gain equal to K. As wUl be appreciated by those of ordinary skiU in the art, high-pass filter 28 may be synthesized by subtracting the output of the low-pass filter 30 from its input.

Both high-pass filter 28 and low-pass filter 30 have a cutoff frequency that is determined by the specific appUcation. In a hearing compensation system application according to the embodiments depicted in FIGS. 2A-2C where the LOG operation is performed prior to the low-pass operation, it is preferred that a nominal cutoff frequency of about 16 Hz is employed. However, it should be appreciated that other cutoff frequencies may be chosen for low-pass filter 30 up to about 1/8 of the critical bandwidth associated with the frequency band being processed without deviating from the concepts of this invention. Those of ordinary skUl in the art wiU recognize that filters having response curves other than that shown in FIG. 3 may be used in the present invention. For example, other non-voice appUcations of the present invention may require a cutoff frequency higher or lower than 16 Hz.

The sign output of the LOG block 22 which feeds delay 24 has a value of either 1 or 0 and is used to keep track of the sign of the input signal to LOG block 22. The delay 24 is such that the sign of the input signal is fed to the EXP block 34 at the same time as the data representing the absolute value of the magnitude of the input signal, resulting in the proper sign at the output. In the present invention, the delay is made equal to the delay of the high- pass filter 28. Those of ordinary skiU in the art wiU recognize that many designs exist for ampUfiers and for both passive and active analog filters as well as for DSP filter implementations, and that the design for the filters described herein may be elected from among these available designs. For example, in an analog implementation of the present invention, high-pass filter 28 and low-pass filter 30 may be conventional high-pass and low-pass fUters of known designs, such as examples found in Van Valkenburg, M.E., Analog Filter Design, Holt, Rinehart and Winston, 1982, pp 58-59. Amplifier 32 may be a conventional operational amplifier. In a digital implementation of the present invention, amphfier 32 may be a multipUer function having the input signal as one input term and the constant K as the other input term. DSP filter techniques are well understood by those of ordinary skill in the art.

The outputs of high-pass filter 28 and amplifier 32 are combined and presented to the input of EXP block 34 along with the unmodified output of LOG block 22. EXP block 34 processes the signal to provide an exponential function. In a DSP implementation, EXP block 34 is preferably realized as described in "ADSP-2100 FamUy Applications Handbook," Volume 1, 1995, published by Analog devices, pp52-67. EXP block 34 preferably has a base that corresponds to the base employed by LOG block 22. The EXP block 34 may alternatively be implemented as a software subroutine as is well known in the art, or from other equivalent means such as a look-up table. Examples of known implementations of this function are found in the Knuth and Abramowitz et al. references, and United States Patent No. 3,518,578, previously cited.

In an analog implementation of the present invention, EXP block 34 may be an amplifier with an exponential transfer curve. Examples of such circuits are found in FIGS. 8 and 9 of United States Patent No. 3,518,578.

Sound may be conceptuaUzed as the product of two components. The first is the always positive slowly varying envelope and may be written as eft), and the second is the rapidly varying carrier which may be written as v(t). The total sound may be expressed as: s(t) = e(t) v(t)

Since an audio waveform is not always positive (i.e., v(t) is negative about half of the time), its logarithm at the output of LOG block 22 wUl have a real part and an imaginary part. If LOG block 22 is configured to process the absolute value of s(t) , its output wUl be the sum of log (e(t)/emax) and log \v(t)\. Since log \v(t)\ contains high frequencies, it wUl pass through high-pass filter 28 essentially unaffected. The component log (eiήle,^ contains low frequency components and wUl be passed by low-pass filter 30 and emerge from amplifier 32 as K log (e(t)/emax). The output of EXP block 34 will therefore be:

(e(t)/emm ' v(t)

The output of EXP block 34 is fed into amplifier 36 with a gain of emax in order to rescale the signal to properly correspond to the input levels which were previously scaled by l/e,^ in ampUfier 20. Amplifiers 20 and 36 are similarly configured except that their gains differ as just explained.

When -RT<1, it may be seen that the processing in the multipUcative AGC circuit 16-n of FIG. 2A performs a compression function. Persons of ordinary skiU in the art wiU recognize that embodiments of the present invention using these values of K are useful for appUcations other than hearing compensation.

According to this embodiment of the invention employed as a hearing compensation system, K may be about between zero and 1. The number K will be different for each frequency band for each hearing impaired person and may be defined as foUows: K = [1 - (HL / (UCL - NHT)] where HL is the hearing loss at threshold (in dB), UCL is the upper comfort level (in dB), and NHT is the normal hearing threshold (in dB). Thus, the apparatus of the present invention may be customized to suit the individual hearing impairment of the wearer as determined by examination. The multipUcative AGC circuit 18-n in the present invention provides no gain for signal intensities at the upper sound comfort level and a gain equivalent to the hearing loss for signal intensities associated with the normal hearing threshold.

When K>\, the AGC circuit 16-n becomes an expander. Useful applications of such a circuit include noise reduction by expanding a desired signal.

Those of ordinary skiU in the art wiU recognize that when K is negative (in a typical useful range of about zero to -1), soft sounds will become loud and loud sounds wiU become soft. Useful applications of the present invention in this mode include systems for improving the inteUigibiUty of a low volume audio signal on the same signal line with a louder signal.

Despite the fact that multipUcative AGC has been avaUable in the literature since 1968, and has been mentioned as a candidate for hearing aid circuits, it has been largely ignored by the hearing aid literature. Researchers have agreed, however, that some type of frequency dependent gain is necessary. Yet even this agreement is clouded by perceptions that a bank of filters with AGC will destroy speech inteUigibiUty if more than a few bands are used, see, e.g., R. Plomp, The Negative Effect of Amplitude Compression in Hearing Aids in the Light of the Modulation-Transfer Function, Journal of the Acoustical Society of America, 83, 6, June 1983, pp. 2322-2327. The understanding that a separately configured multiplicative AGC for a plurality of sub-bands across the audio spectrum may be used according to the present invention is a substantial advance in the art.

FIG. 2B is a block diagram of a circuit which is a variation of the circuit shown in FIG. 2A. Persons of ordinary skill in the art wiU recognize that amphfier 20 may be eUminated and its gain (l/emax) may be equivalently implemented by subtracting the value log e max fr°m the output of low pass filter 30 in subtractor circuit 38. Similarly, in FIG. 2B, amplifier 36 has been eUminated and its gain (e,^) has been equivalently implemented by adding the value log emax to the output from amphfier 32 in adder circuit 40 without departing from the concept of the present invention. In a digital embodiment of FIG. 2B, the subtraction or addition my be achieved by simply subtracting/adding the amount log emax ; while in an analog implementation, a summing amplifier such as shown in examples in "Microelectronic Circuits", by A.S. Sedra and K.C. Smith, Holt Rinehart and Winston, 1990, pp 62-65, may be used.

When noise is present, the input signal to the multiplicative system may be characterized as foUows: s(t) = (ed(t) + en (t))v (t) Where ed(t) is the dynamic part of the envelope, and en(t) is the near stationary part of the envelope.

According to a preferred embodiment of the multiplicative AGC circuit 16 of the present invention, FIG. 2C iUustrates noise ehmination that is performed on the near stationary parts of the envelope, en(t). In FIG. 2C, the second output of LOG block 22 is connected to high pass filter 28, bandpass filter 42, and low-pass filter 44. The high pass filter 28 is preferably set to 16 Hz as described above to separate loglv(t)l and log(ed(t)+en(t)).

In the prefened embodiment, the band pass filter 42 is implemented with a single order pole at 16 Hz, a single order pole that is consistent with the desired operation of separating the ed (t) and en(t) signals of the envelope, and a zero (i.e. a zero response) at D.C. According to the present invention, sounds that remain nearly constant in envelope amplitude for more than 6 seconds are characterized as stationary. Accordingly, the specification of the lower cutoff frequency to be 1/6 Hz for the band-pass filter 42 corcesponds to signals with a 6 second duration. It wiU be appreciated by those of ordinary skill in the art that other cut off frequencies and filter orders may be selected to implement the desired specifications for separating the ed (t) and en(t) signals portions of the envelope according to the present invention.

FIGS. 4A-4C Ulustrate the transfer functions of the high pass 28, the band pass filter 42 and the low pass filter 44, respectively. In FIG. 4A, the output of the high pass filter 28 is the log lv(t)l. In FIG. 4B, the output of the band pass filter 42, is the log of the dynamic or rapidly varying time envelope, log(ed(t)) . In FIG. 4C, the output of the low pass filter 44 is the log of the near stationary or slowly varying time envelope, log(en(t)). The near stationary envelope is most often associated with noise, such as for example, a multi-talker speech background that provides a constant din, a fan with a constant level of output hum, or white or colored noise with a constant power level.

According to the present invention, the noise, en(t), may be reduced by a linear attenuation factor, atten, wherein the amplitude is changed to equal the original amphtude times the atten factor. A reduction in the level of the constant component of sound is obtained by adding the log of the attenuation to en(t). In FIG. 2C, log(αtten), the value of which is negative for atten values less than one, is added to the output of the low-pass filter 44 along with - log(emax) at node 38-2. It should be appreciated that the inclusion of - log(emax) is made in place of the amphfier 20 as taught with respect to node 38 Ulustrated in FIG. 2B, and as such, - log(emax) is also added to the output of the band pass filter 42 at node 38-1.

The outputs of the amplifiers 32-1 and 32-2 are connected to exponent blocks 46-1 and 46-2, respectively. The exponent blocks 46-1 and 46-2 are implemented in a manner similar to the exponent block 34 described above. The outputs of exponent block 46-1 and 46-2 are summed at summing junction 48 to form the output, e^.(ed(t)+ attend (t)). The output of summing junction 48 is connected to LOG block 50.

Log block 50 is implemented in the same manner as log block 22 described above. The output of log block 50 is summed with the output of the high pass filter 28 at summing junction 52. The output of summing junction 52 is connected to a second input of exponent block 34. The first input of exponent block 34 contains the sign information of v(t), and when combined with the input at the second input of exponent block 34 forms an output of exponent block 34 as follows: ed(t) + (αtten) x en(t) K v(t) ema)c

Accordingly, the multipUcative AGC circuit 16 set forth in FIG. 2C wiU attenuate an acoustic signal having a relatively constant amphtude for more than approximately six seconds. Preferably, the value of atten, the log of which is added to the output of the low pass filter 44 may be under the control of the user of the hearing aid. In this manner, the user of the hearing aid may set the background noise attenuation in a way that is analogous to the selection of volume by a volume control. It wiU be appreciated by those in ordinary skill in the art that any variety of known volume control devices typicaUy employed in hearing aids or stereo sound systems may be employed to adjust the background noise attenuation level in either a digital or an analog system.

By introducing the attenuation factor, atten, at the output of the low pass filter 44, and by appropriately selecting the value of K in amplifier 32- 1 and 32-2 in each of the channels of the hearing compensation depicted in FIG. 1, the attenuation in each of the channels can be perceived as being the same. It should be appreciated, however, that if desired, the attenuation perceived in each of the channels may be made different.

Referring now to FIG. 5A, a block diagram is presented of an alternate embodiment of the multiplicative AGC circuit 16-n of the present invention wherein the log function foUows the low-pass filter function. Those of ordinary skiU in the art wiU appreciate that the individual blocks of the circuit of FIG. 5A which have the same functions as corresponding blocks of the circuit of FIG. 2A may be configured from the same elements as the corresponding ones of the blocks of FIG. 2A.

Like the multipUcative AGC circuit 16-n of FIG. 2A, the multipUcative AGC circuit

16-n of FIG. 5 A accepts an input signal at amplifier 20 from the output of one of the audio bandpass filters 16-n. Amplifier 20 is set to have a gain of lemax, where emax is the maximum allowable value of the audio envelope for which AGC gain is to be applied.

The output of amplifier 20 is passed to absolute value circuit 60. In an analog implementation, there are numerous known ways to implement absolute value circuit

60, such as given, for example, in A. S. Sedra and K. C. Smith, Microelectronic Circuits, Holt, Rinehart and Winston Publishing Co., 2nd ed. 1987. In a digital implementation, this is accompUshed by taking the magnitude of the digital number.

The output of absolute value circuit 60 is passed to low-pass filter 30. Low-pass filter 30 may be configured in the same manner as disclosed with reference to FIG. 2A. Those of ordinary skill in the art wiU recognize that the combination of the absolute value circuit 60 and the low-pass filter 30 provide an estimate of the envelope e(t) and hence is known as an envelope detector. Several implementations of envelope detectors are weU known in the art and may be used without departing from the teachings of the invention. Since, in the embodiment of FIG. 5A, the low-pass filter 30 precedes the LOG block 22, it is preferred that the cutoff frequency be up to 1/8 of the critical bandwidth of the cutoff frequency. It should be appreciated, however, that a nominal cutoff frequency of 16 Hz may also be employed.

In a presently preferred embodiment, the output of low-pass filter 30 is processed in the LOG block 22 to derive the logarithm of the signal. The input to the LOG block 22 is always positive due to the action of absolute value block 60, hence no phase or sign term from the LOG block 22 is used. Again, because the gain of the amplifier 20 is set to Uemax, the output of amplifier 20 for inputs less than emax, wUl never be greater than one and the logarithm term out of LOG block 22 will always be 0 or less.

In FIG. 5 A, an alternative implementation of LOG block 22 from the description provided with respect to FIG. 2A may be made, because less accuracy is required in the LOG block 22 implementation in FIG. 5 A. It should be understood that this alternative implementation is not considered suitable for use in the implementation of LOG block 22 of FIG. 2A because an unacceptably high level of noise is created by the inaccuracies. In this alternative embodiment of LOG block 22. The exponent and the fractional part of the mantissa of the floating point representation of the input to LOG block 22 are added together to form the output of the LOG block 22. For example, the floating point representation of the number 12 pursuant to IEEE standard 754-1985 format is 1.5 x 23. In accordance with the alternative implementation of LOG block 22, the value of log212 is treated as 3.5, since the sum of the exponent of 23 and the fractional part of 1.5 is calculated as 3 + .5 = 3.5. The true value of log212 is 3.58496. The error of approximately 2% is considered acceptable.

The logarithmic output signal of LOG block 22 is presented to an amplifier 62 having a gain equal to K - 1. Other than its gain being different from amplifier 32 of FIG. 2A, amplifiers 32 and 62 may be simUarly configured. The output of amplifier 62 is presented to the input of EXP block 34 which processes the signal to provide an exponential (anti-log) function.

The output of EXP block 34 is combined with the input to amphfier 20 in multipUer 64. There are a number of known ways to implement multipUer 64. In a digital implementation, this is simply a multiplication. In an analog implementation, an analog multiplier such as shown in A. S. Sedra and K. C. Smith, Microelectronic Circuits, Holt, Rinehart and Winston Publishing Co., 3rd ed. 1991 (see especially page 900) is required. As in the embodiment depicted in FIG. 2 A, the input to amplifier 20 of the embodiment of FIG. 5A is delayed prior to presentation to the input of multipUer 64. Delay block 66 has a delay equal to the group delay of low pass filter 30.

FIG. 5B is a block diagram of a circuit which is a variation of the circuit shown in FIG. 5 A. Those of ordinary skill in the art wiU recognize that amplifier 20 may be eUminated and its gain, lemax, may be equivalently implemented by subtracting the value log emax from the output of log block 22 in sub tractor circuit 68, as shown in FIG. 5B, without deviating from the concepts herein.

FIG. 5C Ulustrates a preferred embodiment of a multiplicative AGC circuit 16 including noise suppression according to the present invention. The multipUcative AGC circuit 16 is similar to the multipUcative AGC circuit 16 depicted in FIGS. 5A and 5B, except that the noise suppression components according to the present invention have been included. Accordingly, only the additional circuit elements Ulustrated in FIG. 5C wiU be described herein.

According to the present invention, the log(e(t)) is connected to the high pass filter 70 and the low pass filter 72. The implementation of the low pass filter 72 may be made with a simple order low pass filter characteristic having a corner at 1/6 Hz, embodiments of which are weU known to those of ordinary skiU in the art. The high pass filter 70 may be implemented with the understanding that the first order high pass filter transfer function is the low pass filter function subtracted from 1. A high pass filter 70 implemented in this manner is depicted in FIG. 6, and is well known to those of ordinary skill in the art. The transfer functions for the high pass filter 70 and the low pass filter 72 are iUustrated in FIGS. 7A and 7B, respectively. It wiU be appreciated that filter orders and cut off frequencies other than those recited herein may be selected as a matter of design choice according to the present invention. Alternatively, the high pass filter 70 and the low pass filter 72 may be replaced with a noise estimator in a manner iUustrated in FIGS. 8 A and 8B, respectively. Various implementations of noise estimators are well known to those of ordinary skiU in the art. A suitable implementation of a noise estimator is suggested in the article by Harry Teder, "Hearing Instruments in Noise and the Syllabic Speech-to-Noise Ration," Hearing

Instruments, Vol. 42, No. 2, 1991 recited above. In this embodiment, switching artifacts result as the noise estimator switches between an estimate of the noise when speech is present and an estimate when the speech is absent.

Turning again to FIG. 5C, the output of the high pass filter 70 is log(ed(t)), representing the dynamic portion of the acoustic signal envelope. The output of the low pass filter 72 is log(en(t)), representing the near stationary portion of the signal envelope. At summing junctions 68-1 and 68-2, the value -log(emax) is added to both the output of the high pass filter 70 and the low pass filter 72 in a manner analogous to the value added to the summing junction 68 in FIG. 5B. According to the present invention, the value log(αtten) is also added to the output of the low pass filter 72 at the summing junction 68-2 to form the output log(en(t)) - log(emax) + log(atten)).

The outputs of the summing junctions 68-1 and 68-2 are input to the amplifiers 42-1 and 42-2, respectively. The outputs of the amplifier 42-1 and 42-2 have an exponentiation performed on them by exponent blocks 34-1 and 34-2 in the manner described above. The outputs of exponent blocks 34-1 and 34-2 are summed at summing junction 74 to form the / atten • en (t) A' 1 output max

The output of the summing junction 74 is multiplied by the value of the input signal through the delay block 66 by multiplier 64. The selection of K as described above, along with the selection of the attenuation value, atten, may be made in two or more of the multipUcative AGC circuits 16 to provide a similar attenuation of the background noise across several of the channels. The attenuation value, atten, may be controUed by a volume control circuit in a manner as described above.

FIG. 5D illustrates an alternative embodiment of noise eUmination according to the present invention. In FIG. 5D the attenuation of noise represented in the output of low-pass filter 72 is made by a variable gain amphfier 80. In this embodiment, the value for K used in amphfier 42-1 must be readjusted each time the gain of the amphfier 80 is changed. While the multiplicative AGC circuits 16-n shown in FIGS. 2A-2C and FIGS. 5A-5C are implemented differently, it has been determined that the output resulting from either the log-lowpass implementation of FIGS. 2A-2C and the output resulting from the lowpass-log implementation of FIGS. 5A-5C are substantiaUy equivalent, and the output of one cannot be said to be more desirable than the other. In fact, it is thought that the outputs are sufficiently similar to consider the output of either a good representation for both. Listening results of tests performed for speech data to determine if the equivalency of the log-lowpass and the lowpass-log was appropriate for the human auditory multiplicative AGC configurations indicate the intelUgibiUty and fidelity in both configurations was nearly indistinguishable.

Although intelUgibUity and fideUty are equivalent in both configurations, analysis of the output levels during calibration of the system for specific sinusoidal tones revealed that the lowpass-log maintained calibration while the log-lowpass system deviated slightly from caUbration. While either configuration would appear to give equivalent listening results, calibration issues favor the low-pass log implementation of FIGS. 5A-5C.

The multi-band multiplicative AGC adaptive compression approach of the present invention has no exphcit feedback or feedforward. With the addition of a modified soft-limiter to the multiplicative AGC circuit 16-n, stable transient response and a low noise floor is ensured. Such an embodiment of a multiplicative AGC circuit for use in the present invention is shown in FIG. 9A.

The embodiment of FIG. 9A is similar to the embodiment shown in FIG. 5 A, except that, instead of feeding the absolute value circuit 60, amphfier 20 follows the low-pass filter 30. In addition, a modified soft Umiter 86 is interposed between EXP block 34 and multipUer 64. In an analog implementation, soft Umiter 86 may be designed, for example, as in A. S. Sedra and K. C. Smith, Microelectronic Circuits, Holt, Rinehart and Winston Publishing Co., 2nd ed. 1987 (see especiaUy pp. 230-239) with the slope in the saturation regions asymptotic to zero. The output of the EXP block 34 is the gain of the system. The insertion of the soft Umiter block 86 in the circuit of FIG. 9A limits the gain to the maximum value which is set to be the gain required to compensate for the hearing loss at threshold.

In a digital implementation, soft Umiter 86 may be reaUzed as a subroutine which provides an output to multiplier 44 equal to the input to soft Umiter 86 for all values of input less than the value of the gain to be reaUzed by multiplier 64 required to compensate for the hearing loss at threshold and provides an output to multipUer 64 equal to the value of the gain required to compensate for the hearing loss at threshold for all inputs greater than this value. Those of ordinary skUl in the art will recognize that multipUer 64 functions as a variable gain amplifier whose gain is timited by the output of soft Umiter 86. It is further convenient, but not necessary to modify the soft Umiter to Umit the gain for soft sounds below threshold to be equal to or less than that required for hearing compensation at threshold. If the soft limiter 86 is so modified, then care must be taken to ensure that the gain below the threshold of hearing is not discontinuous with respect to a small change in input level.

Use of the modified soft Umiter 86 provides another beneficial effect by eliminating transient overshoot in the system response to an acoustic stimulus which rapidly makes the transition from sUence to an uncomfortably loud intensity. The stabilization effect of the soft limiter 86 may also be achieved by introducing appropriate delay into the system, but this can have damaging side effects. Excessive delayed speech transmission to the ear of one's own voice causes a feedback delay which can induce stuttering. Use of the modified soft Umiter 86 eUminates the acoustic delay used by other techniques and simultaneously provides stability and an enhanced signal-to-noise ratio.

FIG. 9B is a block diagram of a variation of the circuit shown in FIG. 9A. Those of ordinary skill in the art wiU recognize that amplifier 20 may be eliminated and its gain function may be reaUzed equivalently by subtracting the value log emax from the output of log block 22 in subtracter circuit 88 as shown in FIG. 9B without deviating from the concepts herein.

Turning now to FIG. 10 A, a preferred embodiment of multipUcative AGC circuit 16 implementing a three slope gain curve according to the present invention is iUustrated. In FIG. 10A, the output of the LOG block 22 is connected to first and second compare circuits 90-1 and 90-2. The LOG block 22 may alteratively be implemented as in FIG. 2A or 5B. The outputs of first and second compare circuits are connected to the first and second select inputs of gain multiplexer 92 and normalization multiplexer 94. The first, second and third inputs, KQ', K , and K2'to gain multiplexer 92 provide the value of K-l in the amplifier 42 according to the expression:

K'=K-1

The first, second and third inputs, Ay', A, ', and A2'to normalization multiplexer 94 provides the normalization implemented by the amplifier 20 in FIGS. 2 A, 5 A, and 9 A by adding the value -KTog emax to the output of amphfier 42 by summing node 96. Since the normaUzation occurs after the operation of amplifier 42, it should be appreciated that the value of K is included in each of the three inputs to the normalization multiplexer 94. Further, the value of K included in each of the three inputs corresponds to the value of K that is employed by amplifier 42 in response to the output from gain multiplexer 92.

According to this embodiment of the present invention, compare circuits 90-1 and 90-2 divide the amphtude of the output from LOG block 22 into expansion, compression and saturation regions. An exemplary graph of the gain provided to the input in these region is iUustrated in FIG. 11. The upper limit of the expansion region is set by the threshold hearing loss determined during a fitting of the hearing aid user. When the amplitude of the output from LOG block 22 is below the threshold hearing loss, the inputs Ky' and AQ' wUl be selected, and the gain of the amphfier 42 wiU preferably provide expansive gain to the input. For input signal energy at low levels constituting unwanted noise, expansion is useful by reducing the gain to those low level signals.

The lower limit of the compression region is set by the threshold hearing loss, and the upper limit is set by compression provided to the signal in the compression region and the compression provided in the saturation region. When the amphtude of the output from LOG block 22 is above the threshold hearing loss, and below the upper hmit of the compression region, the inputs K, ' and A2' will be selected, and the gain of the amplifier 42 will preferably provide compressive gain to the input. The compression provided in each channel wUl be determined during the fitting of the hearing aid.

When the amplitude of the output from LOG block 22 is above the upper limit of the compression region, the inputs K2' and A2' wUl be selected, and the gain of the amplifier 42 wUl preferably provide compressive gain to the input. The compression in the saturation region wiU typically be greater than the compression in the compression region. In the saturation region, the output is limited to a level below the maximum output capabiUty of the output transducer. This is preferred to other types of output limiting, such as peak chpping.

In FIG. 10B, an embodiment of the three gain region multiplicative AGC circuit 16 including noise eUmination as described above is illustrated. In this embodiment, the high- pass filter 70, the low-pass filter 72, and the summing junction 68 and their operations as described above to provide noise elimination are further included. The low-pass filter 72 is preferably implemented as described with respect to FIG. 5 A. The gain of the amplifiers 42-1 and 42-2, and the value added by the summing junctions 96- 1 and 96-2 are determined in the manner set forth with regard to FIG. 10 A.

An alternate method for achieving stabiUty is to add a low level (i.e., an intensity below the hearing threshold level) of noise to the inputs to the audio bandpass filters 14-1 through 14-n. This noise should be weighted such that its spectral shape follows the threshold-of-hearing curve for a normal hearing individual as a function of frequency. This is shown schematically by the noise generator 100 in FIG. 1. Noise generator 100 is shown injecting a low level of noise into each of audio bandpass filters 14-1 through 14-n. Numerous circuits and methods for noise generation are well known in the art.

In the embodiments of FIGS. 5A-5D, FIGS. 9A and 9B, and FIGS. 10A and 10B the subcircuit comprising absolute value circuit 60 foUowed by low-pass filter 30 functions as an envelope detector. The absolute value circuit 60 may function as a half- wave rectifier, a full- wave rectifier, or a circuit whose output is the RMS value of the input with an appropriate scaling adjustment. Because the output of this envelope detector subcircuit has a relatively low bandwidth, the envelope updates in digital reaUzations of this circuit need only be performed at the Nyquist rate for the envelope bandwidth, a rate less than 500 Hz. Those of ordinary skiU in the art wiU appreciate that this wiU enable low power digital implementations.

The multiplicative AGC full range adaptive compression for hearing compensation differs from the eartier FFT work in several significant ways. The multi-band multipUcative AGC adaptive compression technique of the present invention does not employ frequency domain processing but instead uses time domain filters with similar or equivalent Q based upon the required critical bandwidth. In addition, in contrast to the FFT approach, the system of the present invention employing multiplicative AGC adaptive compression may be implemented with a minimum of delay and no exphcit feedforward or feedback.

In the prior art FFT implementation, the parameter to be measured using this prior art technique was identified in the phon space. The presently preferred system of the present invention incorporating multi-band multiplicative AGC adaptive compression inherently includes recruitment phenomenalogicaUy, and requires only the measure of threshold hearing loss and upper comfort level as a function of frequency in the embodiments iUustrated in FIGS. 2A-2C, FIG. 5A-5D, and FIGS. 9A and 9B. FinaUy, the multi-band multipUcative AGC adaptive compression technique of the present invention utitizes a modified soft limiter 86 or alternatively a low level noise generator 100 which eliminates the additive noise artifact introduced by prior-art processing and maintains sound fidelity. However, more importantly, the prior-art FFT approach wUl become unstable during the transition from silence to loud sounds if an appropriate time delay is not used. The presently preferred multiplicative AGC embodiment of the present invention is stable with a minimum of delay.

The multi-band, multipUcative AGC adaptive compression approach of the present invention has several advantages. For the embodiments described with respect to FIGS. 2A- 2C, FIG. 5A-5D, and FIGS. 9A and 9B, only the threshold and upper comfort levels for the person being fitted need to be measured. The same lowpass filter design is used to extract the envelope, e(t), of the sound stimulus s(t), or equivalently the log(e(t)), for each of the frequency bands being processed. Further, by using this same filter design and simply changing the cutoff frequencies of the low-pass filters as previously explained, other appUcations may be accommodated including those where rapid transition from silence to loud sounds is anticipated.

The multi-band, multiplicative AGC adaptive compression approach of the present invention has a minimum time delay. This eliminates the auditory confusion which results when an individual speaks and hears their own voice as a direct path response to the brain and receives a processed delayed echo through the hearing aid system.

Normalization with the factor emax, makes it mathematicaUy impossible for the hearing aid to provide a gain which raises the output level above a predetermined upper comfort level, thereby protecting the ear against damage. For sound input levels greater than emax the device attenuates sound rather than amplifying it. Those of ordinary skUl in the art wUl recognize that further ear protection may be obtained by Umiting the output to a maximum safe level without departing from the concepts herein.

A separate exponential constant K is used for each frequency band which provides precisely the correct gain for all input intensity levels, hence, no switching between linear and compression ranges occurs. Switching artifacts are eUminated.

The multi-band, multipUcative AGC adaptive compression approach of the present invention has no exphcit feedback or feedforward. With the addition of a modified soft limiter, stable transient response and a low noise floor are ensured. A significant additional benefit over the prior art which accrues to the present invention as a result of the minimum delay and lack of explicit feedforward or feedback in the multiplicative AGC is the ameUoration of annoying audio feedback or regeneration typical of hearing aids which have both the hearing aid microphone and speaker within close proximity to the ear.

The multipUcative AGC may be implemented with either digital or analog circuitry due to its simplicity. Low power implementation is possible. As previously noted, in digital realizations, the envelope updates (i.e., the operations indicated by amplifier 20, LOG block 22, amphfier 42) need only be performed at the Nyquist rate for the envelope bandwidth, a rate less than 500 Hz, thereby significantly reducing power requirements.

The multi-band, multiplicative AGC adaptive compression system of the present invention is also applicable to other audio problems. For example, sound equalizers typically used in stereo systems and automobUe audio suites can take advantage of the multi-band multipUcative AGC approach since the only user adjustment is the desired threshold gain in each frequency band. This is equivalent in adjustment procedure to current graphic equalizers, but the AGC provides a desired frequency boost without incurring abnormal loudness growth as occurs with current systems.

According to another aspect of the present invention, an in-the-ear hearing compensation system employs two electrical signal-to-acoustical energy transducers. Two recent developments have made a dual-receiver hearing aid possible. The first is the development of miniaturized moving-coil transducers and the second is the critical-band compression technology disclosed herein and also disclosed and claimed in parent application serial No. 08/272,927 filed July 8, 1994, now United States Patent No. 5,500,902.

Referring now to FIG. 12, a block diagram of an in-the-ear hearing compensation system 110 employing two electrical-signal to acoustical-energy transducers is presented. A first electrical-signal to acoustical-energy transducer 112, such as a conventional iron-armature hearing-aid receiver is employed for low frequencies (e.g., below 1 kHz) and a second electrical-signal to acoustical-energy transducer 114 is employed for high frequencies (e.g., above 1 kHz).

Demand for high-fidelity headphones for portable electronic devices has spurred development of moving-coU transducers less than 1/2 inch diameter that provide flat response over the entire audio range (20-20,000 Hz). To fit in the ear canal, a transducer must be less than 1/4 inch in diameter, and therefore the commerciaUy avaUable transducers are not appUcable. A scaling of the commercial moving-coU headphone to 3/16 in diameter yields a transducer that has exceUent efficiency from 1 kHz to weU beyond the upper frequency hmit of human hearing. The system of the present invention uses such a scaled moving-coU transducer 114 as the tweeter, and a standard Knowles (or similar) iron-armature hearing-aid transducer 112 as the woofer. Both of these devices together can easily be fit into the ear canal.

The hearing compensation system shown in FIG. 12 is ConceptuaUy identical to the parent invention except that the processing channels, each containing a bandpass filter and multiplicative AGC gain control, are divided into two groups. The first group, comprising bandpass filters 14-10, 14-11, and 14-12 and multiplicative AGC circuits 16-10, 16-11, and 16-12, processes signals with frequencies below the resonance of the iron- armature transducer 112. The second group, comprising bandpass filters 14-20, 14-21, and 14-22 and multiplicative AGC circuits 16-20, 16-21, and 16-22 processes signals above the resonance of the iron-armature transducer 114. The outputs of the first group of processing channels are summed in summing element 116-1, and fed to power amplifier 118-1, which drives iron- armature transducer 112. The outputs of the second group of processing channels are summed in summing element 116-2, and fed to power amplifier 118-2, which drives high- frequency moving-coil transducer 114. The inputs to both processing channels are supplied by electret microphone 120 and preamplifier 122.

Using the arrangement shown in FIG. 12 where the frequency separation into high and low components is accomplished using the bandpass filters, no crossover network is needed, thereby simplifying the entire system. Persons of ordinary skUl in the art wiU appreciate that processing and amplifying elements in the first group may be specialized for the frequency band over which they operate, as can those of the second group. This specialization can save considerable power dissipation in practice. Examples of such specialization include using power ampUfiers whose designs are optimized for the particular transducer, using samphng rates appropriate for the bandwidth of each group, and other weU-known design optimizations.

An alternative to a miniature moving-coil transducer for high-frequency transducer 114 has also been successfuUy demonstrated by the authors. Modern electrets have a high enough static polarization to make their electro-mechanical transduction efficiency high enough to be useful as high-frequency output transducers. Such transducers have long been used in ultrasonic appUcations, but have not been applied in hearing compensation applications. When these electret devices are used as the high-frequency transducer 114, persons of ordinary skiU in the art wUl appreciate that the design specializations noted above should be followed, with particular emphasis on the power amplifier, which must be speciaUzed to supply considerably higher voltage than that required by a moving-coU transducer.

WhUe embodiments and appUcations of this invention have been shown and described, it would be apparent to those skiUed in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.

Claims

What is claimed is:
1. A multipUcative automatic gain control (AGC) circuit with noise suppression comprising: a logarithmic element having an input connected to said output of said first amplifier element, said logarithmic element having a first output carrying a signal indicating the sign of a signal at said input of said logarithmic element and a second output carrying a signal proportional to the logarithm of the absolute value of said signal at said input of said logarithmic el-ment; a filter element having a throughput delay including: a high-pass filter having an input connected to said second output of said logarithmic element and an output connected to a first summing means; a band-pass filter having an input connected to said second output of said logarithmic element and an output; a first adder element having a first input, a second input and an output, said first input of said first adder connected to said output of said band-pass filter, said a second input of said first adder element coupled to a value -log emax to add -log emax to said output of said band-pass filter; a first amphfier with gain of less than unity having an input connected to said output of said first adder element and an output; a first exponential element having an input connected to said output of said first amphfier and an output; a low-pass filter having an input connected to said second output of said logarithmic element and an output; a second adder element having a first input, a second input and an output, said first input of said first adder connected to said output of said low-pass filter, said a second input of said first adder element coupled to a value -log emax and a value log(atten) to add -log emax and log (atten) to said output of said low-pass filter; a second amphfier with gain of less than unity having an input connected to said output of said second adder element and an output; a second exponential element having an input connected to said output of said second amplifier and an output; a third adder element having a first input, a second input and an output, said first input of said first adder connected to said output of first exponential element and said second input of said third adder connected to said output of second exponential element; a second logarithmic element having an input connected to said output of said third adder element and an output; and a fourth adder element having a first input, a second input and an output, said first input of said first adder connected to said output of second logarithmic element, said second input of said fourth adder connected to said output of said high-pass filter, and said output of said fourth adder element forming said output of said filter element; a delay element having an input connected to said first output of said logarithmic element and an output, said delay element having a delay equal to said throughput delay; an exponential element having a first input connected to said output of said delay element, a second input connected to said output of said filter element, and an output; and a third amplifier element having an input and an output, said input connected to said output of said exponential element, said second amphfier element having a gain of emax.
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JP2003516003A (en) * 1999-11-22 2003-05-07 ブリガム ヤング ユニヴァーシティ Hearing aids incorporating signal processing technology
EP2375787A1 (en) * 2010-04-12 2011-10-12 Starkey Laboratories, Inc. Methods and apparatus for improved noise reduction for hearing assistance devices
US8737654B2 (en) 2010-04-12 2014-05-27 Starkey Laboratories, Inc. Methods and apparatus for improved noise reduction for hearing assistance devices
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US9331649B2 (en) 2012-01-27 2016-05-03 Cochlear Limited Feature-based level control
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