Evaluation of digital speech coding strategies for the Nucleus 22-electrode cochlear implant.

Abstract

Two new processing strategies have been implemented on a cochlear implant digital speech processor (CIDSP) for the Nucleus 22-electrode cochlear prosthesis using a single chip digital signal processor (TMS320C25, Texas Instruments). The first approach, pitch excited sampler (PES), is based on the classical channel vocoder concept whereby the spectral energy of a number of logarithmically spaced frequency bands is transformed into appropriate electrical stimulation parameters for up to 22 electrodes using a voice pitch synchronous pulse rate at any electrode. The second approach, continuous interleaved sampler (CIS), uses a maximally high pitch-independent stimulation pulse rate on a selected number of electrodes. Optimization and fine-tuning of processing parameters for these coding strategies is based on a number of psychophysical experiments and some additional constraints. Evaluation experiments with seven cochlear implant users resulted in significantly improved performance in consonant identification tests with the new processing strategies as compared with the subjects own wearable speech processors whereas improvements in vowel identification tasks were rarely observed. Scores were strongly dependent on carefully selected modifications of the basic PES- and CIS-strategies. [Work supported by the Swiss National Research Foundation.]