Coding Strategies and Standards - Digital Speech: Coding for Low Bit Rate Communication Systems - page 4

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
followedwith specific application targets. This chapter presents a brief review
of speech coding techniques. Also, the requirements of the current generation
of speech coding standards are discussed. The motivation behind the review
is to highlight the advantages and disadvantages of various techniques. The
success of the different coding techniques is revealed in the description of the