Richard Vandervoort Cox

Enhancing speech intelligibility using variable-rate time-scale modification

The method and preprocessor enhances the intelligibility of narrowband speech without essentially lengthening the overall time duration of the signal. Both spectral enhancements and variable-rate time-scaling procedures are implemented to improve the salience of initial consonants, particularly the perceptually important formant transitions. Emphasis is transferred from the dominating vowel to the preceding consonant through adaptation of the phoneme timing structure. In a further embodiment, the technique is applied as a preprocessor to a speech coder.

A sub-band coder designed for combined source and channel coding (speech coding)

The coder is a dynamic bit allocation sub-band coder of the type first proposed by Ramstad (1982). It is structured in such a way that the relative importance of all bits is established as a byproduct of the dynamic bit allocation. It is shown that there is a difference in error sensitivity of four orders of magnitude between the most and the least important bits of the bit stream on average. A very flexible unequal error protection scheme is used to match the error sensitivity of the sub-band coder bit stream. This is accomplished using the concept of rate compatible punctured convolutional coding. The resulting coder gives robust performance over a simulated noisy channel. Its nominal bit rate is 16 kb/s, with 12 kb/s assigned for speech coding and 4 kb/s assigned for error correction. The entire encoder and decoder have been implemented on a single AT&T DSP-32 digital signal processor.<<ETX>>

An intelligibility enhancement for the mixed excitation linear prediction speech coder

We recently proposed a technique to increase the robustness of perceptually important acoustic cues in speech, based on modification of the phoneme timing structure. We now apply the technique as a preprocessor to improve the intelligibility of the mixed excitation linear prediction (MELP) coder. The enhancement strategy, however, requires adaptations to minimize the introduction of unnatural speech characteristics that are introduced and intensified by parametric coding schemes. The enhanced 2.4 kb/s fixed-point MELP coder achieves a level of speech intelligibility comparable to the 8 kb/s G.729 Annex A coder.