Speech Coding Thesis

Speech Coding Thesis-83
In this dissertation, we propose a new hypothesis for the observed absence of release from masking by CI users.

In this dissertation we investigate the use of two such embedded noise reduction methods namely, ‘SNR weighting method’ and ‘S-shaped compression’ to improve speech perception in noisy listening conditions.

SNR weighting noise reduction method is an exponential weighting method that uses the instantaneous signal to noise ratio (SNR) estimate to perform noise reduction in each frequency band that corresponds to a particular electrode in the cochlear implant.

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.

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.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.Music perception and speech perception in noisy listening conditions with cochlear implants are still highly challenging problems.Many research studies have reported low recognition scores in the task of simple melody recognition.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.Most of the cochlear implant devices use envelope cues to provide electric stimulation.Understanding the effect of various factors on melody recognition in the context of cochlear implants is important to improve the existing coding strategies.

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