STUDY ON A NEW COCHLEAR IMPLANT SYSTEM BASED ON PHYSIOLOGICAL MODEL OF AUDITORY PERIPHERY SYSTEM

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This two year project consists of six parts. (1) A real-time processor of the auditory filter and the hair-cell model had been implemented on multiple digital signal processors. (2) A new speech processor was designed using an auditory model to improve the quality of consonants. (3) A new acoustic simulation was designed to cause perceptually equivalent sensation of loudness in accordance with the measured T-level. C-level of electric current. (4) The FIR filters of low frequency were redesigned to have higher resolution for the characteristic frequencies. (5) Compound speech processing was examined with the auditory model for the voiceless region and the LPC model for the voiced region. (6) Trigger pulse was extracted based on speech coding technology.Results for this project were as follows. Fixed point signal processing realized with sufficient precision on a microprocessor. The new speech processor extracts the fundamental frequency from lower channels of the auditory model. Within a pitch interval, four most firing channels were selected to stimulate in sequence. Perceptual test by acoustic simulation had shown improved quality of voiced stops, voiceless fricatives, voiceless africatives, voiced africatives, and nasals. Vowel quality was not as good as the current practical speech processor. The compound processing was designed to improve vowel part of speech by LPC model. The LPC model could improve voice quality of vowels, however, had degraded consonants. Problem remained is that consonant part of speech was not necessarily processed with the auditory model but LPC instead. We examined different approach based on wave coding technology that extracts sequence of driving impulse to minimize squared error of synthesized wave. Extraction algorithm took into account the constraints of cochlear implant system. Speech perception test by analysis and synthesis had shown good quality of vowels and fair quality of consonants. This seems worth continuing further study.