ABSTRACT
A signal is a time-varying function that carries information representing a certain phenomenon. Thus, it is analog by nature. A typical example of a signal is a telephone call, where speech (audio signal), in the form of acoustical (mechanical pressure) waves, is changed into electrical variations through the handset-microphone. These propagate via a cable to the receiver, where they are turned back into their original (acoustical) form through the built-in loudspeaker in the receiver’s handset. Figure 1.1 illustrates examples of some signals in practice. The signal shown in Figure 1.1a is part of a speech signal, where low-amplitude variations are the periods of silence. In Figure 1.1b, a radio frequency signal is modulated by an audio frequency one. Such types of signals are common in wireless communications and radio and TV broadcasting. The composite TV signal depicted in Figure 1.1c represents one line out of the 625 lines that are used to represent one frame. Each line carries information representing variations in the illumination together with other control pulses (synchronizing and blanking pulses), as we shall see in Chapter 10.
