A signal is any waveform that serves as a means of communication. It represents a fluctuating
electric quantity, such as voltage, current, electric or magnetic field strength, sound, image, or
any message transmitted or received in telegraphy, telephony, radio, television, or radar. A typical
signal which varies with time is shown in figure 4.1 where f(t) can be any physical quantity such
as voltage, current, temperature, pressure, and so on.