Speech and Audio Signal Processing: Processing and Perception of Speech and Music, Second Edition

CHAPTER 33

LOW-RATE VOCODERS

33.1 INTRODUCTION

To a first approximation, a teletype system transmitting at 75 bps can transmit textual information at almost the same rate as a person speaking the same text. Of course, the speech has much more information than the text. The speaker's identity, emotional state, and prosodic nuances are all information, though not all of this information may be necessary for speech communication per se. For this chapter, it is assumed that a good 2400-bps vocoder contains all of the relevant information. Given this assumption, we will consider low-rate vocoders to encompass bit rates between 75 and 2400 bps.

In Chapter 32 we examined two methods of bit-rate reduction: efficient quantization schemes and linear transformations. In this chapter we extend this discussion to report on several other bit-saving methods. First, we describe the benefits obtainable by taking advantage of the time correlation of the spectral and excitation components. When the sampling rates of these components are lowered, bit saving automatically takes place; the trick will be to find interpolation algorithms that do not inordinately degrade the output speech.

A different approach to bit-rate reduction comes from the original work by C. P. Smith [18], [19] on channel vocoders, which was later applied to LPC vocoders by Buzo et al. [3]. This approach was called pattern matching ...