Date of Award

Summer 1981

Document Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Electrical and Computer Engineering

First Advisor

Niederjohn, Russell J.

Second Advisor

Heinen, James A.

Third Advisor

Moeller, Arthur C.

Abstract

In the development of any speech processing system, one is ultimately concerned with the attributes of the speech waveform which are essential for intelligible speech. Infinite amplitude clipping is a process which removes all amplitude information except polarity from a signal, preserving only the zero crossing locations. Earlier experiments have shown that highpass filtered speech, subjected to infinite clipping, is highly intelligible. Other studies have shown this form of processed speech to be highly immune to noise and one study even suggests filtering and clipping as a method for the enhancement of speech already degraded by additive random noise. These findings suggest that zero crossings, or similarly, the sequence of zero crossing intervals, are important information-bearing attributes of the speech waveform. This thesis investigates the phenomenon of infinitely clipped speech and the nature of the information carried by the zero crossing interval sequence. With the aid of a digital computer, the effects of processes which reduce or distort the zero crossing information are studied. Also studied is the influence of filtering and clipping on already noisy speech. Intelligibility test results show that zero crossing information is very intolerant to further information reduction. Preservation of the correct order of the zero crossing intervals is very important as is the accurate representation of each zero crossing interval length, especially those lengths representing frequencies in the second formant range. Regardless of the preclipping filter used, intelligibility test results also show that the zero crossing information must result from sufficiently "clean" speech in order to yield intelligible clipped speech. This causes filtering and clipping to be unsuccessful for enhancing speech degraded by additive random noise.

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