The first step in analog-to-digital encoding is called pulse amplitude modulation (PAM). This technique takes analog information, samples it, and generates a series of pulses based on the results of the sampling. The term sampling means measuring the amplitude of the signal at equal intervals.
The method of sampling used in PAM is more useful to other areas of engineering than it is to data communication. However, PAM is the foundation of an important analog-to-digital encoding method called pulse code modulation (PCM).
In PAM, the original signal is sampled at equal intervals as shown in Figure 4.2-2. PAM uses a technique called sample and hold. At a given moment the signal level is read, then held briefly. The sampled value occurs only instantaneously in the actual waveform, but is generalized over a still short but measurable period in the PAM result.
The reason PAM is not useful to data communications is that, although it translates the original waveform to a series of pulses, these pulses are still of any amplitude (still an analog signal, not digital). To make them digital, we must modify them by using pulse code modulation (PCM).
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