me opposite ends of the range. For example, it can transmit a signal of maximum amplitude at the maximum baseband fre- quency. Such rapidly changing signals rarely occur, so it should be From Equation 19.59, the slope of the characteristic is as in possible to convey the baseband signal with adequate fidelity by Equation 19.61. transmitting fewer bits per second than is needed by conventional PCM. Differential pulse code modulation (DPCM) does this by (19.61) encoding the difference between each sample and the previous one, instead of encoding the actual value of each sample (Peebles, 1976; Taub and Schilling, 1986). Equation 19.61 shows that, when u=255, the SQNR changes by The block diagram of a basic DPCM system is shown in Figure less than 3dB over an input level change of 40dB. For small signals 19.21. Each sample presented to the coder is the difference between Equation 19.62 is obtained. the present value of the baseband input voltage and the integrated value of previous transmitted samples. At the receiver, the output of (19.62) the decoder is fed to the baseband output channel via a similar integrator. Thus, the coder at the sending end is constantly updating the output of the receiver's integrator by comparing the output of its This corresponds to the companding advantage; thus, for 255, own integrator with the voltage received from the baseband input this is 33dB. channel and transmitting the difference. Usually, the difference will Early PCM systems use non-linear networks in conjunction with be much smaller than the full amplitude of the input signal, SO it can uniform encoding. Later systems used piecewise linear approxima- be encoded with fewer digits. Typically, 4-digit coding can be used tions to the compression law, since these are easy to implement instead of 8-digit coding, thereby halving the transmitted digit rate digitally. Two such logarithmic quantising schemes for 8-bit encod- and the bandwidth required. ing have been defined by CCITT Recommendation G.711. They Occasionally, the input si