ion 4.78. output vary with time. PCM and DPCM encoders are designed on the basis that the source output is stationary, and the performance of (4.78) these encoders can be improved by using adaptive methods. One such method being the use of an adaptive quantiser. PCM, DPCM and the adaptive counterparts are source encoding For example, a 7-bit quantiser gives a quantisation noise power of techniques that attempt to represent the output waveforms. Conse- -52.8 dB. quently these methods are known as waveform encoding tech- A uniform quantiser provides the same spacing between suc- niques. In contrast to these methods, Linear Predictive Coding cessive levels throughout the entire dynamic range of the signal. In models the source as a linear system, which when excited by an practice this can cause problems since for many signals, speech for appropriate input signal gives rise to an observed source output. example, small signal amplitudes occur more frequently than large Instead of transmitting the samples of the source waveform to the signal amplitudes, and a better approach would have more closely receiver, the model parameters of the linear system are transmitted spaced levels at low signal amplitudes, and vice versa. The desired together with the appropriate excitation signal. References 4/13 References Jaynes, E.T. (1989) Papers on probability, statistics and statistical physics (ed. R.D. Rosenkranz) Kluwer Academic Press, 1989. Cox, R.T. (1946) Am.J.Phys 17.1.1946. Leff, H.S. and Rex, A.F. (1990) Maxwell's Demon, Entropy, Infor- Gull, S. (1988) Developments in Maximum Entropy data analysis. mation, Computing. Adam Hilger Press. In Maximum Entropy and Bayesian Methods (ed. J. Skilling). Proakis, J. (1987) Digital Communications. McGraw-Hill. Kluwer Academic Press. Shannon, C. (1949) The Mathematical Theory of Communication, Hartley, R.V.L. (1928) Transmission of Information. Bell Systems University of Illinois Press. Technical Journal, July. Skilling, J. (ed) (1988) Maximum Entropy and Bayesian Methods. Jaynes, E.T