Hearing Aid Signal Processing Research Articles

A portable programmable digital sound processor for cochlear implant research

A programmable sound processor which utilizes digital signal processing has been developed for hearing prosthesis research. It incorporates a Motorola DSP56001 integrated circuit, 32 K words of memory, a 12 b analog-to-digital converter, and a data formatter and transmitter which conveys control codes to the receiver-stimulator of a cochlear implant. The processor is pocket-sized and battery powered. It has been programmed to emulate the Spectral Maxima Sound Processor for the University of Melbourne/Nucleus 22 electrode implant, and is currently being used by several implantees. In continuing research, speech processing programs are being improved, and other applications, including signal processing for binaural implants and advanced hearing aids, are being developed. >

Maximum length sequence based testing of hearing aids

An automated dual-channel maximum-length sequence (MLS) test system for the electroacoustic characterization of hearing aids has been developed. This test system applies a speech-shaped MLS acoustically to a hearing aid and measures the electro-acoustic frequency response. This method provides results that compare favorably to those obtained using the method employed by Kates [J. M. Kates, J. Rehab. Res. Develop. 27, 255–278 (1990)] and the ANSI standard method [ANSI S3.42 (1992)]. Test results show that MLS-based testing is significantly faster than noise-based testing. Two signal-biased MLS-based testing methods have also been developed. These methods apply a bias signal to force the hearing aid into a mode of operation where the frequency response, with adaptive or subtractive bias signal cancellation, is measured using a low-level, speech-shaped MLS. This method has proved valuable for the characterization of automatic signal processing hearing aids. [Work supported by ORTC and OMH.]

The perception of complex harmonic patterns by profoundly hearing-impaired listeners.

In providing profoundly hearing-impaired persons with processed speech through a signal-processing hearing aid, it is important that the new speech code matches their auditory capacities. This processing capacity for auditory information was investigated in this study. In part 1, the subjects' ability to judge similarities among 8 different but related harmonic complexes was studied. The patterns contained different numbers of harmonics to a 125-Hz fundamental frequency; the harmonics had been spread over the spectrum in various ways. The perceptual judgments appeared to be based on a temporal cue, beat strength, and a spectral cue, related to the balance of high and low frequency components. In part 2, three sets of synthetic vowels were presented to the subjects. Each vowel was realized by summing harmonically related in-phase sinusoids at two formant frequencies. The sets differed in the number of sinusoids per formant: 1, 2 or 3. It was found that the subjects used spectral cues and vowel length for differentiating among the vowels. The overall results show the limited but perhaps usable ability of the profoundly impaired ear to handle spectral information. Implications of these results for the development of signal-processing hearing aids for the profoundly hearing impaired are discussed.

Feedback suppression in digital signal processing hearing aids

Acoustic feedback in digital signal processing hearing aids is suppressed by using signal processing techniques in the digital processor. A first processing technique causes the data to the main signal processing path in the digital signal processor to be delayed by varying amounts over time, preferably in a periodic manner, to disrupt the buildup of feedback resonances. In a second technique, a digital filter receives the input data and has its coefficients adjusted so that the output of the filter is substantially an optimal estimate of the current input sample based on past input samples. The output of the filter is then subtracted from the input signal data to provide difference signal data which substantially cancels out the resonant frequencies. In a third technique, the acoustic feedback path from the output to the input of the hearing aid is modeled in the digital signal processor as a delay and a linear filter. The output of the main signal processing path in the digital signal processor is delayed and the delayed data passed through the linear filter, with the output of the filter then being substracted from the input signal data to provide difference signal data which is provided to the main signal processing path. The coefficients of the digital filter in the feedback path are adjusted so that the signal passed through the feedback filter substantially corresponds to of the acoustic feedback signal to thereby cancel the same.

Study of adaptive feedback stabilization of hearing aid.

Instability is a problem with high-power and ITE hearing aids. Gain margins can be improved by 15 dB using an adaptive LMS filter to estimate the feedback characteristic and actively cancel it [Engebretson etal., ‘‘Adaptive feedback stabilization of hearing aids,’’ Second International Workshop on Hearing Impairment and Signal-Processing Hearing Aids, London, England, June 1991]. This algorithm was evaluated with a wearable digital hearing aid (WDHA). Nine hearing-impaired subjects were tested at 55 dB SPL, in quiet and in multitalker babble (6-dB SNR), with their own aid and with the WDHA programmed, with and without feedback stabilization (FBS), to match the REAR of their own aid. Subjects adjusted overall gain to their liking for each condition. Real ear measurements were taken to verify REAR. Correlation of word identification scores (PHFWL) with the subject’s own aid and the WDHA (FBS disabled) was high. With FBS enabled, subjects chose additional gain that ranged from 2 to 10 dB and scores improved by 7 percentage points on the average across subjects. [Work supported by DVA.]

Adaptive, programmable signal processing and filtering for hearing aids

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Future Directions in Signal Processing Hearing Aids

Digital hearing aids offer many advantages over conventional analog hearing aids, such as programmability, memory, extremely precise, flexible control of electroacoustic characteristics, and advanced signal processing capabilities for noise reduction and speech enhancement. At the present stage of development, digital hearing aids are subject to severe practical constraints with respect to size and power consumption. Hybrid analog/digital hearing aids have been developed which combine some of the advantages of digital technology with the practicality of small, cosmetically acceptable instruments. Recent studies with all-digital and hybrid analog/digital hearing aids have identified trends which are likely to influence future hearing aid design.

Modeling Normal and Impaired Hearing

A cochlear simulation has been developed to model normal and impaired hearing. The simulation includes the middle ear, the mechanical motion of the cochlear partition, and the mechanical to neural transduction of the inner hair cells. The outer hair cells are postulated to provide an active feedback mechanism that adjusts the gain and shape of the auditory filters. Auditory impairment is simulated by reducing the efficacy of the outer hair cells and by modifying the inner hair cell transduction process. The effects of simulated impairment are illustrated for two speech sounds, /ba/ and /ka/, with the neural firing patterns from the impaired ear compared with those of a normal ear. The differences in the neural firing patterns are interpreted in the context of hearing aid signal processing.

信号処理型補聴器の将来 (&lt小特集&gt聴覚障害補償へのディジタル技術の利用)

信号処理型補聴器の将来 (&lt小特集&gt聴覚障害補償へのディジタル技術の利用)

An evaluation of two signal-processing hearing aids

Various forms of signal processing are used in modern ASP hearing aids. The present study investigated the relationship between sentence recognition ability and two types of signal processing used in commercially available hearing aids. Results indicated a significant improvement in sentence recognition ability employing an instrument with adaptive compression (variable release time) versus an instrument with an adaptive high-pass filter with short attack and release times. Data were obtained for a single-talker competing message at several message-to-competition ratios. The adaptive compression system may prove to be beneficial to hearing-impaired listeners in certain background noise environments.

Real-time portable multi-layer perceptron voice fundamental-period extractor for hearing aids and cochlear implants

Previous work has shown that it is possible to train a multi-layer perceptron to estimate the voice fundamental period ( Tx) for multiple speakers in the presence of high levels of background noise. The algorithm has been implemented in real-time on a TMS320C25 based development system. A prototype pocket-sized portable device has been constructed and the real-time software transferred to it. This will provide the basis for a new generation of signal processing hearing aids for the profoundly and totally deaf. Power supply current is sufficiently low for battery operation for periods of 12 hours between charges. The basic algorithm has been adapted to provide the higher time resolution which will make it applicable to a wide range of other applications.

Evaluation of two voice-separation algorithms using normal-hearing and hearing-impaired listeners.

Two signal-processing algorithms, designed to separate the voiced speech of two talkers speaking simultaneously at similar intensities in a single channel, were compared and evaluated. Both algorithms exploit the harmonic structure of voiced speech and require a difference in fundamental frequency (F0) between the voices to operate successfully. One attenuates the interfering voice by filtering the cepstrum of the combined signal. The other uses the method of harmonic selection [T. W. Parsons, J. Acoust. Soc. Am. 60, 911-918 (1976)] to resynthesize the target voice from fragmentary spectral information. Two perceptual evaluations were carried out. One involved the separation of pairs of vowels synthesized on static F0's; the other involved the recovery of consonant-vowel (CV) words masked by a synthesized vowel. Normal-hearing listeners and four listeners with moderate-to-severe, bilateral, symmetrical, sensorineural hearing impairments were tested. All listeners showed increased accuracy of identification when the target voice was enhanced by processing. The vowel-identification data show that intelligibility enhancement is possible over a range of F0 separations between the target and interfering voice. The recovery of CV words demonstrates that the processing is valid not only for spectrally static vowels but also for less intense time-varying voiced consonants. The results for the impaired listeners suggest that the algorithms may be applicable as components of a noise-reduction system in future digital signal-processing hearing aids. The vowel-separation test, and subjective listening, suggest that harmonic selection, which is the more computationally expensive method, produces the more effective voice separation.

EEC hearing impairment programme: collaboration in Europe, 1974-1988.

The Commission of the European Communities funds a 'concerted action' programme in the area of hearing impairment. The programme does not fund research directly, but is intended to provide funds for coordination of research in different countries. Previous projects funded by the programme include a comprehensive study of the prevalence of childhood deafness in the European Community, and meetings on cochlear implants, human temporal bone histopathology, diagnostic techniques and hearing impairment in children. The programme is currently concerned with new technologies for communication in the hearing impaired and activities in 1988 will include workshops on signal processing hearing aids and early diagnosis in children.

Signal-Processing Hearing Aids for the Totally and Profoundly Deaf

We have developed a family of single-channel signal-processing aids for the profoundly and totally deaf. Common to them all are the analysis of speech into the components most important to the deaf lipreader; the synthesis of stimuli which make the best use of the patient's sensory abilities; and facilities to ensure accurate matching of the aid to the patient. The totally deaf are electrically stimulated by electrodes on the promontory or on the round window of the cochlea using charge-balanced controlled current square waves automatically adjusted to be at a comfortable level. Many potential candidates for electrocochlear stimulation have significant low frequency residual hearing, but do not find conventional hearing aids to be useful. We have found that they can often make very effective use of the voice fundamental frequency presented as an acoustic sinusoid. Our approach to these patients avoids the need for implant surgery but preserves that option should total loss of hearing occur in the future. Both electrocochlear and acoustic methods of signal presentation are implemented with similar hardware. The speech signal from a microphone or other source is analyzed by a voice fundamental frequency extractor and a voiceless sound detector. Their outputs are processed by a single chip microcomputer that synthesizes the output waveform. In both devices the aid is tailored to the patient using a desktop computer that stores amplitude-frequency characteristics and frequency mapping tables into a read-only memory.

The effect of enhanced spectral contrast on the internal representation of vowel-shaped noise

Two steady-state noise-excited vowel sounds of 200-ms duration were synthesized: one similar to a naturally produced whispered vowel, the other with energy removed outside a nominal 50-Hz band centered at each formant peak. The threshold for a brief sinusoidal probe tone was measured at frequencies corresponding to the peaks and troughs in the vowel spectra, in both forward and simultaneous masking. The masking functions for a flat-spectrum masker at each frequency were also measured, so that the vowel masking patterns could be expressed in terms of the equivalent broadband masker level. In the first experiment, masking patterns for the two maskers were measured with three normal-hearing subjects. Increasing spectral contrast was effective in increasing peak–valley differences in the masking patterns up to 2.5 kHz. Contrast was greater in forward than in simultaneous masking, suggesting that the internal representation of a vowel may be enhanced by suppression. In a second experiment, masking patterns and flat-spectrum masking functions were measured with three listeners with moderate sensorineural hearing impairments. The enhancement produced by increasing spectral contrast was less than that obtained with normal-hearing listeners, but still present up to 2.5 kHz. A comparison of forward- and simultaneous-masking patterns again revealed an increase in peak–valley differences, suggesting that suppression mechanisms were still effective in these listeners. The ability to enhance the internal representation of speechlike sounds in hearing-impaired listeners may have implications for the design of signal-processing hearing aids.

Hearing aid signal processing for noise and nonsense syllables

Transfer function magnitude and phase, coherence, cross power spectrum, and cross correlation measurements for vented hearing aid fittings worn in situ near onset of acoustic feedback oscillation were obtained with linear system identification techniques utilizing a FFT spectrum analyzer. These measurements were repeated for various methods of suppressing the oscillation tendency. Time segments containing formant transitions of selected CV and VC nonsense syllables found to be difficult to perceive by hearing‐impaired persons were passed through the vented hearing aid fittings worn in situ. The ability of the hearing aids to process difficult‐to‐perceive speech stimuli is discussed in light of interpreting the cross correlation and cross spectrum measurements.

Generalized signal processing hearing aid

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