e sequence to reproduce the com- pressed analog signal. We then expand this analog signal by amplifying low levels less than high levels to cancel the distortion that was produced by the compressor in the encoder. After linear decoding in the receiver, the noise level is the same at any sample level. In expansion a low-level signal is reduced to its original value and quantizing noise is attenuated. This makes the noise level lower at low signal levels than at high signal levels and improves the S/N at low signal levels. The integrated codec (encoder/decoder) chips that are available for PCM coding include both encoder and decoder circuits. They use signal processing technology to perform companding and we cannot find separate analog nonlinear amplifiers in real-life chips. An example of a PCM compressor curve for positive analog signal val- ues is presented in Figure 3.12. The horizontal axis represents the original value of an analog voice signal, and the vertical axis gives the output value of the compressor. Uniformly spaced levels of the linear quantizer are shown on the left-hand side. At a certain sampling instant, an analog signal value X is quantized according to the curve into one of the quantum levels of Z(x) and this level is then transmitted as a digital word unique to that level. When a high signal value changes (see change "b" in Figure 3.12), only a couple of quantizing levels are involved. This is adequate because the quan- tizing noise does not disturb the listener very much if the signal level is high Compressor "Compressed" Input Expander Output digital signal signal code words Figure 3.11 Nonuniform quantizing. Signals Carried over the Network Compressed signal Z (x) Uniformly spaced quantum levels Original signal Figure 3.12 Compressor characteristics. as well. At low levels (see change "a" in Figure 3.12), a small change of signal level uses many quantizing levels; this results in a smaller quantizing error or noise. This improvement of the average S/N at low analog signal levels is essenti