Chapter 10 Transmission and Storage 10.1 Overview

Until the late seventies, research in speech compression followed two di erent directions: vocoders (abbreviation of voice coders) and waveform coders. The two approaches substantially di er in their underlying principles and performance. Whereas the rst explore our knowledge of speech production, attempting to represent the signal spectral envelope in terms of a small number of slowly varying parameters, the latter aim at a faithful reproduction of the signal either in the time or frequency domains. They also represent two opposite choices in terms of the interleaving of the four main dimensions of the performance of speech coding: bit rate, speech quality, algorithm complexity and communication delay. Vocoders achieve considerable bit rate savings at the cost of