Variable Rate Coding of Speech

In the design of digital speech coders it is often assumed that the coder and channel operate at fixed bit rates. In reality, however, it is known that speech is an intermittent and nonstationary process, and that in many applications the user demand on a communication system is a variable process. In practice, these intermittent properties can be utilized to make the design of a communication system more efficient. For example, the first property, that of an intermittent source, is utilized in communication systems such as T A S I (Time Assignment Speech Interpolation).1"1 The second property, that of a variable demand on the system, is being explored by various authors for use in packet transmission systems4,5 and results in a variable rate channel from the point of view of the user. In both of the above systems, an important element of the system is a variable-rate coder. In its simplest form, it may amount to a trivial transmit/no transmit decision as was used in the initial T A S I systems. More generally, we might characterize a variable-rate coder according to a configuration shown in Fig. 1 where both the source activity and the channel rate are assumed to be variable. 577