Electrical audio signal processing systems and devices – Sound effects – Reverberators
Patent
1985-06-17
1989-02-07
Ng, Jin F.
Electrical audio signal processing systems and devices
Sound effects
Reverberators
381 29, 381 684, 381106, H04R 2500, H03G 500, H03G 700, H03G 920
Patent
active
048037320
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention concerns hearing aids. More particularly, it concerns a multi-band amplification system for use in hearing aids.
BACKGROUND ART
Conventional hearing aids amplify all received sounds to increase their loudness. In most instances, however, the hearing loss of a deaf or partially deaf person is different at different frequencies. Furthermore, the range of intensities which the partially deaf person can comfortably hear is often different at different frequencies. More recent hearing aids, therefore, have been constructed so that their amplification is frequency and level dependent. In this way the amplified signal can be matched to the impaired hearing of the user of the aid.
One example of such a "matched" hearing aid is one that was developed in 1973. This well-known hearing aid uses a multi-band compression system, and is set up so that the signal from each frequency channel (band) varies over a reduced range of levels. This hearing aid can be used to fit the residual hearing of its user fairly accurately, with the result that a user certainly hears the incident sound in each frequency band equally.
Such a hearing aid, however, does not permit the user of the aid to discriminate easily between different sounds. It has now been found that this inability to discriminate between sounds is due to the reduction in the range of spectral shapes that are associated with different speech sounds.
DISCLOSURE OF THE PRESENT INVENTION
It is an objective of the present invention to provide an amplification circuit for use in a hearing aid in which not only is the frequency-dependent amplification for a user having impaired hearing maintained, but the acoustic cues which are associated with particular speech sounds are exaggerated.
This objective is achieved by a novel form of multi-band non-linear processing, in which the signal generated by the microphone of the hearing aid is compressed, then the intensity range of signals in each of a plurality of separate frequency bands within the hearer's frequency range is expanded, each frequency band is amplified separately, the amplified signals are combined, and the combined signal is modified by a signal derived from the envelope of the originally received signal.
Preferably the signal received by the microphone of the hearing aid is first passed through a filter which has a response that is the inverse of the long term average spectrum of speech, to prevent the low frequency channels in the amplification stage from almost always having a higher gain than the high frequency channels.
The division of the received and compressed signal into different frequency channels is effected using band pass filters, and the amplification is a non-linear amplification, which is achieved by a combination of an expander and an attenuator.
The combining of the amplified signal is preferably effected using an adder and the modification of the output signal from the adder is conveniently effected using a multiplier which (a) reinserts variations of the received signal into the output signal and (b) acts as a limiter to prevent output signals from having an intensity which is sufficient to cause loudness discomfort in the user of the hearing aid.
Thus, according to the present invention, a method of amplifying a signal received by the microphone of a hearing aid comprises the steps of
(a) compressing the output signal from the microphone;
(b) dividing the compressed signal into a plurality of separate frequency bands within the hearer's frequency range;
(c) expanding the signal within each separate frequency band;
(d) combining the expanded signals of each separate frequency band; and
(e) modifying the combined signal by a signal which is derived from the envelope of the output signal from the microphone.
Also according to the present invention, an amplification circuit for performing multi-band amplification of the signal from a microphone of a hearing aid comprises:
(i) a plurality of band pass filters, each adapted to pass a respective frequency band wit
REFERENCES:
patent: 3764745 (1973-10-01), Bottcher et al.
patent: 3818149 (1974-06-01), Stearns et al.
patent: 3848091 (1974-11-01), Stearns et al.
patent: 4025723 (1977-05-01), Blackledge
patent: 4061875 (1977-12-01), Freifeld et al.
patent: 4099035 (1978-07-01), Yarick
patent: 4118604 (1978-10-01), Yanick
patent: 4366349 (1982-12-01), Adelman
patent: 4454609 (1984-06-01), Kates
patent: 4517415 (1985-05-01), Laurence
Villchur E., "Signal Processing To Improve Speech Intelligibility in Perceptive Deafness", Journal of the Acoustical Society of America, vol. 53, No. 6, pp. 1642-1657.
WO 83/02212, "Method and Apparatus for Adapting the Transfer Function in a Hearing Aid", Peter Biegaard et al., Jun. 23, 1986.
Byrd Danita R.
Ng Jin F.
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