An intelligibility evaluation of several linear prediction vocoder modifications

Several linear prediction vocoder modifications and an evaluation of their effects on intelligibility are presented. Diagnostic rhyme test (DRT) comparisons among 1) the fixed filter order, fixed analysis frame rate vocoder, 2) various modified vocoders, and 3) the input speech, are implemented using two speakers, seven listeners, and a selected set of word pairs reflecting six phonemic attribute contrasts. Reducing the filter order from ten to four for unvoiced speech frames analyzed at a rate of 44.4 frames/s produces no significant decrease in the scores for all of the six features tested by the DRT. Updating the unvoiced analysis frames with shorter windows at twice the frame rate (88.8 frames/s) leads to a significant improvement in the scores for the sustention feature. Lower order coefficient representation and faster analysis update during unvoiced speech improves the sustention feature with little degradation to the other features, and almost no increase to transmission rate. Results of the DRT evaluation and considerations for implementing the test within ordinary laboratory facilities are discussed.