Obstruent acoustic landmark enhancement for cochlear implants

Cochlear implant users are typically able to achieve high levels of speech recognition in quietbut struggle to follow speech even in moderate levels of background noise. It may be possibleto improve cochlear implant users’ speech perception scores in noise, by making moreefficient use of the limited bandwidth available for transmitting important speech information,rather than increasing the amount of information transmitted. Acoustic landmarks arelocations in the speech signal which are rich in information and allow a listener to identify aparticular speech sound as a vowel, sonorant consonant or obstruent consonant; it is aroundthese regions that important speech cues tend to be concentrated. Obstruent consonants aresignalled by sudden amplitude and spectral changes and the onset/offset of a period of noise.It has been shown that the auditory system is particularly responsive to rapid spectral changes,manifested as increased firing rates of auditory nerve fibres, particularly at onsets of signals.Cochlear implant users commonly confuse speech sounds with rapidly changing spectralpatterns, possibly due to the poor transmission of obstruent landmark information.The aim of the present work was to develop an obstruent landmark enhancement strategywhich could be integrated into current cochlear implant processing. The first stage of thisprocess required the identification of obstruent landmarks from the noise-mixed speechstimuli. An existing automatic landmark detection algorithm did not achieve the high levelsof accuracy required for use in the present study and so a set of hand-generated labels werecreated, and used to guide the proposed obstruent landmark enhancement strategy. A series ofcochlear implant simulation experiments were conducted to help evaluate the strategy andguide further developments. Results from the simulation studies suggest that the proposedmethod of obstruent landmark enhancement does not help to improve speech recognition innoise for normal hearing listeners listening to a cochlear implant simulation. It is likely thatthe strategy outlined in this thesis did not help to improve the saliency of obstruent landmarkevents as the enhancement was applied to the noise as well as the target speech signal,making it difficult for listeners to resolve the boosted landmark information. However, theresults also highlight the limitations of using cochlear implant simulations to evaluate thestrategy and so the findings are not necessarily a predictor of actual cochlear implant userperformance