Time Encoded Speech (TES) transmits information concerning the duration between zero-crossings, shape and the amplitude of the signal between successive zero-crossings. This thesis examines a number of aspects of TES with the view of achieving data reductions to enable the transmission of speech, with acceptable quality and intelligibility, at low bit rates and a practical system delay. This thesis presents: (i) A study of techniques for signalling amplitude information in a TES coder. It was indicated that a minimum of the order of 1 bit per epoch is required. Diagnostic Rhyme Tests (DRT) yielded intelligibility scores of the order of 88% for algorithms employing 1 and 2 bits of amplitude information per epoch. (ii) Investigations into Median and Moving Average filtering for preprocessing the epoch duration sequences. It has been shown that such applications, which involve simple numerical smoothing, are of little value for they degrade the quality of the synthesised speech. (iii) Studies of Extremal Coding and Orthogonal Transformations for achieving data reductions in the signalling of epoch duration and, in some instances, the peak magnitude sequences. Each technique yielded a useful data reduction. The technique using Hadamard Transformations yielded the greatest data reduction, a ratio of 2:1 for the representation of the epoch duration sequences. The Hadamard Transformation also proved to be of low complexity in its implementation. (iv) A real-time simplex digital voice channel, developed during the course of this thesis, and a study of the implementation of TES and TES related coders. It is reported that speech of acceptable quality and intelligibility is achieved for a transmission rate of 10 or 15kb/s with a transmission delay of 300ms.
|Date of Award