SPIRAL – a vocoder with a spiral ganglion for cochlear implant simulation
Vocoders for cochlear implant simulation use a filterbank to simulate the filter channels of the processor. Temporal envelopes extracted from these channels are then directed to the positions in the cochlear of a simulated electrode array by modulating narrowband carrier signals. Traditionally, there is one carrier signal for each processor channel/electrode. The SPIRAL vocoder breaks with this tradition by using many carrier signals. Each may be considered to represent a sub-population of ganglion cells. The modulation of each carrier is a combination of all the channel envelopes mixed according to an exponentially decaying spread of current along the electrode array, using formulae from Oxenham and Kreft (2014). Carrier signals can be noises or tones, but noise introduces intrinsic modulations that are independent of the signal envelope. Consequently, we have found that tonal carriers provide lower SRTs even with up to 160 tones. With the electrodes, in this sense, decoupled from the ganglion cells, the two can be independently manipulated. The positions of the electrodes can be changed without changing the set of carriers used. Dead regions can be simulated by omitting a range of carrier frequencies. Any degree of current spread can be simulated. The vocoder has been configured so that the user can specify the electrode number and spacing, as well as the depth of insertion, and different mappings of frequencies to electrodes can be explored.
Oxenham, A. J., & Kreft, H. A. (2014). Speech Perception in Tones and Noise via Cochlear Implants Reveals Influence of Spectral Resolution on Temporal Processing. Trends in Hearing, 18:1–14.