Abstract
The frequency-to-channel mapping for Cochlear implant (CI) signal processors was originally designed to optimize speech perception and generally does not preserve the harmonic structure of music sounds. An algorithm aimed at restoring the harmonic relationship of frequency components based on semitone mapping is presented in this article. Two semitone (Smt) based mappings in different frequency ranges were investigated. The first, Smt-LF, covers a range from 130 to 1502 Hz which encompasses the fundamental frequency of most musical instruments. The second, Smt-MF, covers a range from 440 to 5040 Hz, allocating frequency bands of sounds close to their characteristic tonotopical sites according to Greenwood's function. Smt-LF, in contrast, transposes the input frequencies onto locations with higher characteristic frequencies. A sequence of 36 synthetic complex tones (C3 to B5), each consisting of a fundamental and 4 harmonic overtones, was processed using the standard (Std), Smt-LF and Smt-MF mappings. The analysis of output signals showed that the harmonic structure between overtones of all complex tones was preserved using Smt mapping. Semitone mapping preserves the harmonic structure and may in turn improve music representation for Nucleus cochlear implants. The proposed semitone mappings incorporate the use of virtual channels to allow frequencies spanning three and a half octaves to be mapped to 43 stimulation channels. A pitch difference limen test was done with normal hearing subjects discriminating pairs of pure tones with different semitone intervals which were processed by a vocoder type simulator of CI sound processing. The results showed better performance with wider semitone intervals. However, no significant difference was found between 22 and 43 channels maps.
Highlights
Music can be described as a series of complex acoustic sounds composed of tones with fundamentals and overtones that are harmonically related to each other [1]
An important aspect of music is melody [3] which can be defined as a sequence of individual tones that are perceived as a single entity [4]
This article investigates the preservation of the harmonic structure by employing semitone mapping with two new ranges together with the use of virtual channels (VCs)
Summary
Music can be described as a series of complex acoustic sounds composed of tones with fundamentals and overtones that are harmonically related to each other [1]. The majority of musical instruments generate fundamental frequencies below 1 kHz [2]. An important aspect of music is melody [3] which can be defined as a sequence of individual tones that are perceived as a single entity [4]. Preserving the harmonic structure of individual tones is important for preserving the melody perception. Cochlear Implants (CIs) were originally designed to restore speech perception for patients with profound hearing loss [5,6].
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