Hearing aid and a method of detecting and attenuating transients
Active Publication Date: 2011-11-03
WIDEX AS
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[0023]In a digital hearing aid, the signal is sampled according to the Nyquist criterion, i.e. the sample rate is twice the highest frequency in the signal to be reproduced. If, for instance, the signal is sampled at 32 kHz, the highest frequency that may be reproduced will be 16 kHz. Experience has shown that transient peaks having a slope higher than 160.000 dB/second are practically absent in speech sounds, but frequently occurring in other sounds. Thus a slope level detector in a transient detection system in a digital hearing aid performing at a sample rate of 32 kHz may beneficially be configured to react on slopes higher than 5 dB/sample.
[0024]If the slope level detector were allowed to react to any slope above 5 dB/sample, too many fast transients at very low levels would be detected. Therefore a sound level limit is introduced in the transient detection system, below which limit transients are not detected.
Problems solved by technology
This is especially true for transient noises, which may both be very loud, and of very short duration.
Speech signals in noise are particularly difficult to understand by a hearing impaired person, and the optimization process thus takes this factor into account when the hearing aid is fitted to the user.
Transient noises pose a special problem, as they may be too short for the compressor to react upon and attenuate.
A slow attack-time, i.e. a low speed in reducing the gain, may result in too much gain when the input level changes from soft to loud.
A slow release-time, i.e. a low speed in ramping the gain from the reduced level back up to the normal level, may result in too little gain being applied to soft sounds immediately following loud sounds.
Speech signals in noise are particularly difficult to understand by a hearing impaired person, and the optimization process thus takes this factor into account when the hearing aid is fitted to the user.
It is well known that sharp transient noises do not agree well with users of hearing aids.
The sounds of cutlery and dishes banging together, glasses clinking, paper crumbling, slamming doors or other loud, transient noises not only transcend the threshold of the so-called upper comfort level but are also next to impossible to eliminate by the compressors as generally used, and may be perceived as unbearable noises by the hearing aid user.
This method has the drawback of having to analyze and extract five different parameters of the input signal, an envelope curve, a slope, a signal peak, an edge presence and a zero transition.
The method also has the added complexity of having to calculate the precise attenuation level for each signal peak to be dampened.
Furthermore, a zero crossing rule is no guarantee that clicks are not produced by the system, as a change in the slope at the zero crossing of the waveform due to attenuation may still introduce clicks and artefacts into the reproduced sound.
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[0034]In FIG. 1 is shown a block schematic of a prior art transient detection and attenuation device 1 for a hearing aid. The input is connected to a band-split filter block 2, splitting the usable frequency range of the input signal into a plurality of frequency bands. Each frequency band of the plurality of frequency bands has its own detection and attenuation means each comprising an envelope calculation block 3, a non-speech slope detection block 4, an RMS calculation block 5, a speech characteristics input block 6, and a gain factor calculation block 7. Only one exemplified, arbitrary frequency band is shown in FIG. 1, the remaining frequency bands being suggested for clarity. The output of the detection and attenuation means 3, 4, 5, 6, 7, is a gain value, which is used to amplify the signal of the particular frequency band in an amplifier 8. The result of this amplification in the plurality of frequency bands is recombined in a band summation block 9, which is common to the plur
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A hearing aid has means (15) for detecting fast transients in the input signal and means (16, 12, 13) for attenuating the detected transients prior to presenting the signal with the attenuated transients to a user. Detection is performed by measuring the peak difference of the signal upstream of a band split filter bank (11) and comparing the peak difference against at least one peak difference limit. Then, if a transient is detected, a state machine (20) analyzes the peak level and the absolute average level of the signal and engages a gain calculator (12) to follow either the peak level or the absolute average level of the input signal for at least the duration of the transient in order to attenuate the transient. The engagement of the gain calculator (12) is performed in each frequency band dependent of the detected transient. The invention further provides a method for detecting fast transients.
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