Multichannel Dynamic-Range Compression Using Digital Frequency Warping

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

Effects of Sound-Induced Hearing Loss and Hearing Aids on the Perception of Music

“…Some hearing aids produce a delay that is frequency dependent; usually the low frequencies are delayed more than the high frequencies [94]. This can make transient sounds appear to be smeared in time.…”

“…Also, delays of 15 ms or more had a significant deleterious effect on the ability to identify nonsense syllables. Kates and Arehart [94] measured the smallest detectable across-frequency delay for various types of signals, using both normal-hearing and hearing-impaired subjects. The lowest threshold of about 2 ms was found for the normal-hearing subjects and a click stimulus.…”

“…The thresholds for speech signals were higher. The across-frequency delays in current hearing aids are usually less than the smallest detectable delay for speech stimuli [94].…”

Uniform and warped low delay filter-banks for speech enhancement

“…Especially the system delay is often an important design criterion. An example are speech processing systems for hearing aids where a high system delay can cause 'coloration effects', e.g., [5]. These distortions occur when the processed speech signal interacts with the original speech signal which reaches the cochlea with minimal delay via bone conduction or through the hearing aid vent.…”

“…The FBE performs time-domain filtering with coefficient update in the frequency-domain. A similar approach has been proposed independently in [5] for dynamic range compression in hearing aids. For acoustic echo cancel-2 lation and active noise control applications, a related time-domain filtering approach can be found in [19] where the coefficients are adapted in the uniform frequency-domain.…”