A harmonic deviations linear prediction vocoder for improved narrowband speech transmission

This paper presents the development, study, and experimental results of a narrowband speech coder called Harmonic Deviations (HDV) Vocoder. The HDV coder is based on the LPC vocoder, and it transmits additional information over and above the data transmitted by the LPC vocoder, in the form of deviations between the speech spectrum and the LPC all-pole model spectrum at a selected set of frequencies. At the receiver, the spectral deviations are used to generate the excitation signal for the all-pole synthesis filter. The paper describes and compares several methods for extracting the spectral deviations from the speech signal and for encoding them. To limit the bit-rate of the HDV coder to 2.4 kb/s the paper discusses several methods including orthogonal transformation and minimum-mean-square-error scalar quantization of log area ratios, two-stage vector-scalar quantization, and variable frame rate transmission. The paper also presents the results of speech-quality optimization of the HDV coder at 2.4 kb/s. Both noisefree and noisy channel applications are considered. The resulting HDV coders yield noticeable improvement in speech quality over LPC vocoders.