In continuous-wave (CW) modulation, which was studied briefly 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 study in this chapter. In pulse modulation, some parameter of a pulse train is varied in accordance with the message signal. On this basis, we may distinguish two families of pulse modulation:
1. Analog pulse modulation, in which a periodic pulse train is used as the carrier wave and some characteristic feature of each pulse (e.g., amplitude, duration, or position) is varied in a continuous 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.
2. Digital pulse modulation, in which the message signal is represented in a form that is discrete in both time and amplitude, thereby permitting transmission of the message in digital form as a sequence of coded pulses; this form of signal transmission has no CW counterpart.
The use of coded pulses for the transmission of analog information-bearing signals represents a basic ingredient in digital communications. In this chapter, we focus attention on digital pulse modulation, which, in basic terms, is described as the conversion of analog waveforms into coded pulses ...