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

2

Coding Strategies and Standards

2.1 Introduction

The invention of Pulse Code Modulation (PCM) in 1938 by Alec H. Reeves was the beginning of digital speech communications. Unlike the analogue systems, PCM systems allow perfect signal reconstruction at the repeaters of the communication systems, which compensate for the attenuation provided that the channel noise level is insufficient to corrupt the transmitted bit stream. In the early 1960s, as digital system components became widely available, PCM was implemented in private and public switched telephone networks. Today, nearly all of the public switched telephone networks (PSTN) are based upon PCM, much of it using fibre optic technology which is particularly suited to the transmission of digital data. The additional advantages of PCM over analogue transmission include the availability of sophisticated digital hardware for various other processing, error correction, encryption, multiplexing, switching, and compression.

The main disadvantage of PCM is that the transmission bandwidth is greater than that required by the original analogue signal. This is not desirable when using expensive and bandwidth-restricted channels such as satellite and cellular mobile radio systems. This has prompted extensive research into the area of speech coding during the last two decades and as a result of this intense activity many strategies and approaches have been developed for speech coding. As these strategies and techniques matured, standardization ...