ce called a codec performs both functions. Sometimes the term codec is used to re- fer to the coding scheme as well as to the physical device. In this chapter we will explore the engineering issues of digital encoding. There are several fundamental concepts that will come into play as we look at the various coding schemes that have been used in telephony and other audio technologies: There is an inverse relationship between the quality of reproduction of audio sound and the bandwidth (bits per second) of the signal. In a given coding scheme, better sound takes more bits. Different coding schemes have different amounts of intelligence built into them. A scheme that takes into account how human beings speak or hear can be more efficient, getting the same quality at lower bandwidth than a less intelligent coding scheme. 1 1This may seem like an odd use of the word intelligence. We would suggest that even a type- writer keyboard has intelligence built in. Knowledge of the frequency of letters in the English al- phabet was used in choosing where to place the letters. That is intelligence. In fact, the standard keyboard was designed to be slow SO that fast typists would not jam the keyboards of mechanical typewriters. Electronic keyboards and other higher-speed devices can use a different layout, such as the Dvorak keyboard, which is optimized for speed, allowing for faster typing. Similarly, audio coding schemes can be more intelligent if they use our knowledge of how people speak and hear in order to optimize quality or maintain acceptable quality with reduced bandwidth. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Speech Coding Key Data Concepts Once a signal is encoded, it is a digital bit stream. It can have error correction added. It can be packaged, stored, and transported in any number of ways. Whatever is done to it, if the bits presen