What Is a Signal?
Any complete definition of the word signal must be, by necessity, somewhat vague. For example, some people define a signal as any representation of information conveyed to a receiver. Rather than discussing the meanings of those defining words, let’s clarify what the word signal means to us by considering examples of signals that we’ve experienced in our daily lives. For example, when we listen to music produced by a loudspeaker, we’re experiencing a signal in the form of sound waves traveling through the air that stimulates our eardrums. When we drive our cars to a traffic intersection, a light signal radiated by a red or green traffic light tells us whether we should stop or proceed. And if we ignore the red light, we find another red light following us down the road with a siren to signal us to pull over!
When you want to make a cell phone call, the symbol on your phone’s screen, shown in Figure 1-1, is a visual indicator that your phone is receiving a sufficiently strong radio signal from a local cell phone tower. The height of a thermometer’s mercury column is a visual signal indicating temperature. When we receive a kiss on the cheek, that’s a tactile signal of affection. All of these examples are instances of receiving a signal that conveys information.
Figure 1-1 Cell phone signal-strength indicator.