Data Communications Networking Devices - 4th Ed.pdf

6.2 T1/ E1 MULTIPLEXERS _________________________________________________________ 653QuantizationSince it would be impossible to encode PAM samples into a ®xed number of bitpositions, quantization is used as a mechanism to convert each PAM sample into adiscrete value. Thus the second step in the PCM process is quantization.One of the problems engineers faced when they were developing quantizationmethods was the type of interval to use. To illustrate this problem, consider Figure6.21 which shows a uniform series of quantization intervals used to provide adiscrete value to each PAM sample. Note that when a PAM sample falls into aparticular quantization interval, its value is based on the discrete value assigned tothe center of the quantization interval. Also note that the difference between thevalue of the interval used to denote the sample and the actual height of the samplerepresents a quantization error referred to as quantization `noise'. Although anobvious method to reduce quantization noise is to increase the number of quantizationlevels, this would also increase the number of bits required to encode eachPAM signal. Thus, other methods were investigated as a mechanism to reducequantization noise while minimizing the number of bits per sample.Figure 6.21Using uniform quantization intervals to assign values to each PAM signalTwo techniques used to reduce the effect of quantization noise while minimizingthe number of bits per sample are companding and non-linear quantization.Companding is a term used to represent the compression and expansion of speechenergy. When a voice signal is compressed, its high-energy components are loweredwhile its low-energy components are raised in value, resulting in a lower dynamicrange for the signal to be quantized and encoded.The compression and latter expansion functions are logarithmic and follow oneof two `laws'Ðthe A law which is used in European PCM systems and the pronounced mu) law used in North America. The curve for the A law can be