Pulse Modulation

In continuous-wave modulation, which we studied in Chapter 2, some parameter of a sinusoidal carrier wave is varied continuously in accordance with the message signal. This is in direct contrast to pulse modulation, which we studied in this chapter.

In pulse modulation, some parameter of a pulse train is varied in accordance with the message signal. We may distinguish two families of pulse modulation: analog pulse modulation and digital pulse modulation. In analog pulse modulation, a periodic pulse train is used as the carrier wave, and some characteristic feature of each pulse (amplitude, duration or position) is varied in a continous manner in accordance with the corresponding sample value of the message signal. Thus in analog pulse modulation, information is transmitted basically in analog form, but the transmission takes place at discrete times. In digital pulse modulation, on the other hand, the message signal is represented in a form that is discrete in both time and amplitude, thereby permitting its transmission in digital form as a sequence of coded pulse; this form of signal transmission has no continuous-wave counterpart.