Analysis of a Digital Technique for Frequency Transposition of Speech

摘要

Frequency transposition is the process of raising or lowering the frequency content (pitch) of an audio signal. The hearing impaired community has the greatest interest in the applications of frequency transposing. Though several analog and digital frequency transposing hearing aid systems have been built and tested, this thesis investigates a possible digital processing alternative. Pole shifting, in the z-domain, of an autoregressive (all pole) model of speech was proven to be a viable theory for changing frequency content. Since linear predictive coding (LPC) techniques are used to code, analyze and synthesize speech, with the resulting LPC coefficients related to the coefficients of an equivalent autoregressive model, a linear relationship between LPC coefficients and frequency tranposition is explored. This theoretical relationship is first established using a pure sine wave and then is extended into processing speech. The resulting speech synthesis experiments failed to substantiate the conjectures of this thesis. However, future research avenues are suggested that may lead toward a viable approach to transpose speech. (Author)