 given at the website. Telephony Chapter 3 Pulse Code Modulation (PCM) Thanks to the work performed by Harry Nyquist at Bell Laboratories in the 1920s, we know that to optimally represent an analog signal as a dig- itally encoded bitstream, the analog signal must be sampled at a rate that is equal to twice the bandwidth of the channel over which the signal is to be transmitted. Since each analog voice channel is allocated 4 KHz of bandwidth, it follows that each voice signal must be sampled at twice that rate, or 8,000 samples per second. In fact, that is precisely what hap- pens in T-Carrier systems, which we now use to illustrate our example. The standard T-Carrier multiplexer accepts inputs from 24 analog chan- nels, as shown in Figure 3-66. Each channel is sampled, in turn, every one eight-thousandth of a second in round-robin fashion, resulting in the generation of 8,000 pulse amplitude samples from each channel every second. The sampling rate is important. If the sampling rate is too high, too much information is transmitted and bandwidth is wasted; if the sampling rate is too low, then we run the risk of aliasing. "Aliasing" is the interpretation of the sample points as a false waveform, due to the paucity of samples. This PAM process represents the first stage of PCM, the process by which an analog baseband signal is converted to a digital signal for transmission across the T-Carrier network. This first step is shown in Figure 3-67. The second stage of PCM, shown in Figure 3-68, is called quantization. In quantization, we assign values to each sample within a constrained range. For illustration purposes, imagine what we now have before us. We have "replaced" the continuous analog waveform of the signal with a series of amplitude samples which are close enough together that we can Figure 3-66 Time-division multiplexing (TDM) Broadband Facilly Multiplexer Inbound Traffic Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. 