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

Preface

Speech has remained the most desirable medium of communication between humans. Nevertheless, analogue telecommunication of speech is a cumbersome and inflexible process when transmission power and spectral utilization, the foremost resources in any communication system, are considered. Digital transmission of speech is more versatile, providing the opportunity of achieving lower costs, consistent quality, security and spectral efficiency in the systems that exploit it. The first stage in the digitization of speech involves sampling and quantizations. While the minimum sampling frequency is limited by the Nyquist criterion, the number of quantifier levels is generally determined by the degree of faithful reconstruction (quality) of the signal required at the receiver. For speech transmission systems, these two limitations lead to an initial bit rate of 64kb/s – the PCM system. Such a high bit rate restricts the much desired spectral efficiency.

The last decade has witnessed the emergence of new fixed and mobile telecommunication systems for which spectral efficiency is a prime mover. This has fuelled the need to reduce the PCM bit rate of speech signals. Digital coding of speech and the bit rate reduction process has thus emerged as an important area of research. This research largely addresses the following problems:

• Although it is very attractive to reduce the PCM bit rate as much as possible, it becomes increasingly difficult to maintain acceptable speech quality as ...