Asynchronous interleaved stimulation (AIS): a new speech coding strategy for cochlear implant.

Cochlear implants use a fixed or F0 dependent stimulation rate, whatever the sound analysed (vowel or consonant) or procedure (fixed filters or FFT). We present a procedure which varies as a function of the input signal. This method is based on FFT analysis using a variable width analysis window. Short windows are applied on the transient part of the signal, providing poor frequency resolution but good time resolution; they permit us to accurately follow the speech signal in time during its fast temporal variations. Large windows are applied on the stationary parts of the signal, providing better frequency resolution but poor time resolution. Transient parts of the speech order this window switching; they are detected using the statistical properties of the FFT; moreover, the narrow windowing is coupled to an increase of the stimulation frequency. This strategy has been implemented using the DIGISONIC cochlear implant software, and clinically assessed in 6 regular cochlear implant users, on the basis of a consonant-vowel-consonant test. This Asynchronous-Interleaved-Stimulation (AIS) strategy provides the patients with better discrimination than fixed window FFT analysis. The coding protocol will be described and results presented.