Electrical audio signal processing systems and devices – Sound effects – Reverberators
Patent
1996-04-12
1997-11-11
Kuntz, Curtis
Electrical audio signal processing systems and devices
Sound effects
Reverberators
381 682, H04R 2500
Patent
active
056872410
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a circuit arrangement for automatic control of the gain and/or frequency response of the transmission characteristic of an open-loop-controllable and closed-loop-controllable signal processing circuit of a hearing aid.
Severely deaf persons or persons suffering from impaired hearing typically possess a raised hearing threshold, while the influence on the upper threshold, the so-called pain threshold, is significantly less pronounced. For this reason, the dynamic range of the hearing-impaired individual is often significantly reduced compared with normal hearing perception. A certain difficulty arises immediately if it is attempted to counteract this impediment by means of a hearing aid. If linear gain is applied, the restricted dynamic range of the hearing perception of a hearing-impaired individual unavoidably results in amplified signals that conflict with the restricted dynamic range of the person using the hearing aid in certain cases, i.e. for certain combinations of speech and background noise. On the other hand, clinical tests on the use of hearing aids in a noisy environment and in a quiet environment have shown that linear gain with well-matched frequency-dependent gain is clearly superior to every system with automatic gain control for single-channel or multi-channel devices with respect to speech comprehension.
Various devices are already known which allow adaptation of the output signal of a hearing aid to the reduced hearing ability of a hearing-impaired individual. This is the case both for single-channel and for multi-channel hearing aids with automatic control.
In a hearing aid with single-channel gain control, the gain is the parameter of the hearing aid that has to be controlled. The temporal characteristic of this control process is of very great significance as regards the quality of the processed signal. Normally, the time characteristic and temporal behavior of a circuit for automatic gain control are determined by the response time and release time. Typically, response times of a few milliseconds and release times of a few hundred milliseconds are considered optimum for hearing aids. However, operation of a conventional automatic gain control circuit is clearly audible even with these response and release times, and this may possibly have a detrimental effect on speech comprehension and hearing comfort. In order to avoid this, a .number of different and somewhat complex circuits for automatic gain control have already been described. Such a system with two different release times is described in U.S. Pat. No. 4,718,099. A further such system uses two input circuits for automatic gain control, namely a quick-action circuit and a slow-action circuit. Both of these systems are less sensitive to loud pulse-type disturbances compared with conventional automatic gain control systems, and both can be called parametric control systems.
Multi-channel technology offers the possibility of independent gain control in a number of channels, thereby permitting control of the output spectrum in greater detail. If such an automatic control facility were thus to be provided in each channel, the relatively stationary response of the automatic control facility would affect the overall frequency response of the hearing aid, since said frequency response is determined by the gain in the respective channels.
A further method is described in U.S. Pat. No. 4,852,175 which utilizes the amplitude distribution in a large number of channels or frequency bands to adjust the frequency response of a multi-channel hearing aid. For this purpose, the complete histogram is calculated and stored for each channel and the gain is then controlled in each channel such that any masking extending into a higher channel does not impair the information in the higher channels.
If, however, non-parametric control is used instead of parametric control, it is possible to obtain a different time characteristic, and this characteristic appears considerably more suited for the type
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Chang Vivian
Kuntz Curtis
Topholm & Westermann ApS
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