Signal-Processing Hearing Aids for the Totally and Profoundly Deaf

We have developed a family of single-channel signal-processing aids for the profoundly and totally deaf. Common to them all are the analysis of speech into the components most important to the deaf lipreader; the synthesis of stimuli which make the best use of the patient's sensory abilities; and facilities to ensure accurate matching of the aid to the patient. The totally deaf are electrically stimulated by electrodes on the promontory or on the round window of the cochlea using charge-balanced controlled current square waves automatically adjusted to be at a comfortable level. Many potential candidates for electrocochlear stimulation have significant low frequency residual hearing, but do not find conventional hearing aids to be useful. We have found that they can often make very effective use of the voice fundamental frequency presented as an acoustic sinusoid. Our approach to these patients avoids the need for implant surgery but preserves that option should total loss of hearing occur in the future. Both electrocochlear and acoustic methods of signal presentation are implemented with similar hardware. The speech signal from a microphone or other source is analyzed by a voice fundamental frequency extractor and a voiceless sound detector. Their outputs are processed by a single chip microcomputer that synthesizes the output waveform. In both devices the aid is tailored to the patient using a desktop computer that stores amplitude-frequency characteristics and frequency mapping tables into a read-only memory.