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Second Biennial Hearing Aid Research and Development Conference

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September 22-24, 1997
National Institutes of Health
Bethesda, Maryland

A Filterbank-based Digital Multichannel Compression Hearing Aid

Robert Brennan and Todd Schneider, Unitron Industries Ltd., Fort Huron, Michigan

Our research seeks to develop an architecture for a flexible, multichannel compression hearing aid that can be used to map, with minimally audible distortion, the wide dynamic range of speech into the reduced dynamic range of a hearing impaired listeners. The design is based on an oversampled, weighted overlap-add filterbank that achieves a balance between the conflicting demands of good frequency selectivity, low delay, reduced memory usage and a wide range of gain adjustment (to support fitting flexibility). A transform- based implementation, which results in uniform band spacing, is used to minimize computational requirements. A selectable even/odd stacking feature provides improved fittings for low frequency and precipitous losses by doubling the number of potential band edges. Using this filterbank, we have developed a sixteen-channel filterbank hearing aid that can provide compression in up to sixteen independent bands. A feedforward topology is used with each of the sixteen band gains controlled by an adjustable combination of overall level and the individual channel level. The overall level is measured using a fuzzy detector that provides a dual time constant level measurement. Using an adjustable combination of overall level and individual channel level for channel gain control allows the hearing aid to behave as a single- channel system (with sixteen bands used for fitting), a system with sixteen independent bands of compression or a combination of these. The filterbank and prototype compression system have been implemented in real-time on a Motorola DSP56301-based body worn hearing aid.

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