The frequency-to-channel mapping for Cochlear implant (CI) signal processors was originally designed to optimize\r\nspeech perception and generally does not preserve the harmonic structure of music sounds. An algorithm aimed\r\nat restoring the harmonic relationship of frequency components based on semitone mapping is presented in this\r\narticle. Two semitone (Smt) based mappings in different frequency ranges were investigated. The first, Smt-LF,\r\ncovers a range from 130 to 1502 Hz which encompasses the fundamental frequency of most musical instruments.\r\nThe second, Smt-MF, covers a range from 440 to 5040 Hz, allocating frequency bands of sounds close to their\r\ncharacteristic tonotopical sites according to Greenwood�s function. Smt-LF, in contrast, transposes the input\r\nfrequencies onto locations with higher characteristic frequencies. A sequence of 36 synthetic complex tones (C3 to\r\nB5), each consisting of a fundamental and 4 harmonic overtones, was processed using the standard (Std), Smt-LF\r\nand Smt-MF mappings. The analysis of output signals showed that the harmonic structure between overtones of\r\nall complex tones was preserved using Smt mapping. Semitone mapping preserves the harmonic structure and\r\nmay in turn improve music representation for Nucleus cochlear implants. The proposed semitone mappings\r\nincorporate the use of virtual channels to allow frequencies spanning three and a half octaves to be mapped to 43\r\nstimulation channels. A pitch difference limen test was done with normal hearing subjects discriminating pairs of\r\npure tones with different semitone intervals which were processed by a vocoder type simulator of CI sound\r\nprocessing. The results showed better performance with wider semitone intervals. However, no significant\r\ndifference was found between 22 and 43 channels maps.
Loading....