Digital Speech: Coding for Low Bit Rate Communication Systems, 2nd Edition

8

Harmonic Speech Coding

8.1 Introduction

A general sinusoidal analysis and synthesis concept was introduced by McAulay [1] when he developed the Sinusoidal Transform Coder (STC) [2] to demonstrate the applicability of the technique in low bit-rate speech coding. Sinusoidal coding does not restrict the component sinusoids of the synthesized speech to be harmonics of the fundamental frequency. The frequency tracks of the sinusoids may vary independently of each other. However in harmonic coding the higher frequency sinusoids are restricted to be integer multiples of the fundamental frequency [3]. Therefore harmonic coding can be seen as a subset of a generalized sinusoidal transform coding. At low bit-rates, STC also restricts the frequency tracks to be harmonics of the fundamental frequency, and deduces the harmonic phases at the decoder, simply because the available bits are not sufficient to encode the large number of parameters of the general sinusoidal representation.

The STC was introduced as an alternative to the source filter model, and its analysis and synthesis was directly applied to the original speech signal. The binary voicing decision of the source filter model is one of its major limitations. The STC employs a more general mixed-voicing scheme by separating the speech spectrum into voiced and unvoiced components, using a voicing transition frequency above which the spectrum is declared unvoiced. However, one of the most recent harmonic coders operates in the LPC ...