Speech Coding Thesis

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This outcome suggests that having access to the acoustic landmarks provided by the obstruent consonants enables listeners to integrate effectively pieces of the message glimpsed over temporal gaps into one coherent speech stream.

The same hypothesis was also tested with CI listeners.

In these methods, the speech signal is first enhanced using the noise reduction method and the enhanced signal is then processed using the speech processor.

A better and more efficient approach is to integrate the noise reduction mechanism into the cochlear implant signal processing.

Since their inception in early 1970s cochlear implants have gradually gained popularity and consequently considerable research has been done to advance and improve the cochlear implant technology.

Most of the research conducted so far in the field of cochlear implants has been primarily focused on improving speech perception in quiet.A selective compression algorithm, with estimated landmarks, was incorporated into the CI strategy, and tested by presenting the processed stimuli to CI users.Significant benefits were observed compared to performance obtained with the unprocessed noisy speech.S-shaped compression technique divides the compression curve into two regions based on the noise estimate.This method applies a different type of compression for the noise portion and the speech portion and hence better suppresses the noise compared to the regular power-law compression.In our hypothesis, when listening to speech in fluctuating maskers (e.g., competing talkers), CI users cannot fuse the pieces of the message over temporal gaps because they are unable to perceive reliably the acoustic landmarks introduced by obstruent consonants (e.g., stops).These landmarks, often blurred in noisy conditions, are evident in spectral discontinuities associated with consonant closures and releases, and are posited to aid listeners in determining word/syllable boundaries.In the current work we investigate the use of new filter spacing techniques called ‘Semitone filter spacing techniques’ in which filter bandwidths are varied in correspondence to the musical semitone steps.Noise reduction methods investigated so far for use with cochlear implants are mostly pre-processing methods.A second experiment was conducted to examine the reasons contributing to the CI users’ inability to perceive reliably the acoustic landmarks embedded in the signal.It is hypothesized that the envelope compression smears the acoustic landmarks that signify syllable/word boundaries.

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