Abstract
A multichannel dynamic-range compressor system using digital frequency warping is described. A frequency-warped filter is realized by replacing the filter unit delays with all-pass filters. The appropriate design of the frequency warping gives a nonuniform frequency representation very close to the auditory Bark scale. The warped compressor is shown to have substantially reduced group delay in comparison with a conventional design having comparable frequency resolution. The warped compressor, however, has more delay at low than at high frequencies, which can lead to perceptible changes in the signal. The detection threshold for the compressor group delay was determined as a function of the number of all-pass filter sections in cascade needed for a detectible change in signal quality. The test signals included clicks, vowels, and speech, and results are presented for both normal-hearing and hearing-impaired subjects. Thresholds for clicks are lower than thresholds for vowels, and hearing-impaired subjects have higher thresholds than normal-hearing listeners. A frequency-warped compressor using a cascade of 31 all-pass filter sections offers a combination of low overall delay, good frequency resolution, and imperceptible frequency-dependent delay effects for most listening conditions.
Highlights
Multichannel dynamic-range compression is an important feature in hearing aids (Kates [1])
The purpose of this paper is to describe a dynamic-range compression system based on digital frequency warping, and to determine the detection threshold for the frequencydependent group delay inherent in the warped compression system
Using a compression filter having even symmetry guarantees that the group delay does not depend on the instantaneous compression gains, and removes any phase modulation that could occur as the gains change in response to the incoming signal and ensures that localization phase cues are preserved in a binaural fitting
Summary
Multichannel dynamic-range compression is an important feature in hearing aids (Kates [1]). Compared to normal listeners, hearing-impaired listeners typically have elevated auditory thresholds which interfere with the perception of lowintensity sounds. The perception of loudness for intense sounds is often similar to that of normal-hearing listeners. An objective of a hearing aid is to fit the dynamic range of speech and everyday sounds into the restricted dynamic range of the impaired ear. Hearing losses are typically frequency-dependent, so the compressor should provide different amounts of dynamic-range compression in different frequency regions. The solution to this design problem is generally a multichannel system, such as a filter
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