Role of carrier signal types in perception of Thai phonemes: Implications for cochlear implant recipients

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It is well known that segmental (consonants and vowels) and suprasegmental (e.g., tone and stress) speech sounds provide intrinsically different kinds of perceptual information. These differences suggest that phoneme perception might be improved in cochlear implant strategy by employing a carrier signal that is most compatible to each type of speech sounds. However, all speech vocoder strategies for cochlear implant (CI) available in most of works use one carrier signal due to their simplicity to implement. In this paper, we investigated the role of five different carrier signals for Thai speech perception on different types of phonemes, i.e., sine-carrier with/without band-pass filter (TF/TNF), noise-carrier with/without band-pass filter (NF/NNF), and temporal fine structure carrier (TFS). Each type of carrier signal was used to synthesize speech stimuli using the continuous interleaved sampling (CIS) strategy as basic and commonly used for CI. Four different psychoacoustic tests for initials, finals, vowels were performed on 20 Thai normal hearing (NH) listeners, while 13 NH listeners were used for lexical tones. Experimental results showed that sine-carriers provided the highest intelligibility scores for initials, finals, and vowels, while NNF provided the highest intelligibility scores for lexical tones. The results suggested that the way to provide an optimal speech perception for tonal language is to use two parallel carriers in speech vocoder strategy for CI.