ANALOG AND DIGITAL SIGNALS , examples , what is definition of ANALOG AND DIGITAL SIGNAL

what is definition of ANALOG AND DIGITAL SIGNAL , ANALOG AND DIGITAL SIGNALS , examples :-
ANALOG AND DIGITAL SIGNALS
          Before we discuss analog and digital communications, let us have a quick view of analog and digital signals.
The analog signal is that type of signal which varies smoothly and continuously with time. This means that analog signals are defined for every value of time and they take on continuous values in a given time interval. Thus, we can say that analog messages are characterized by data whose values vary over a continuous range. The signal depicted in figure 1.4 is an analog signal. In fact, for analog signal, the name derives from the fact that such a signal is analogous to the physical signal that it represents. The vast majority of signals in the world around us are analog. For example, the temperature or the atmospheric pressure of a certain location may vary over a continuous range and
may assume an infinite number of possible values. Similarly, a speech waveform is an analog signal since it has amplitudes that vary over a continuous range.
FIGURE 1.4 An analog signal.
An alternative form of signal representation is that of a sequence of numbers, each number representing the signal magnitude at an instant of time. The resulting signal is called a digital signal. Digital messages are constructed with a finite number of symbols. For example, the printed language consists of 26 letters, 10 numbers, a space and several punctuation marks. Therefore, any text is a digital message constructed from about 50 symbols.
FIGURE 1.5 Variation of a binary digital signal with time.
Now, since a digital signal is represented only by digits, therefore, we can use any number system to represent a digital signal. However, in practice, we generally use binary number system to represent a digital signal. In a binary system, each digit in the number takes on one of only two possible values, denoted 0 and 1. Correspondingly, the digital signals in binary systems need have only two voltage levels which may be labeled low and high. Figure 1.5 shows a digital signal. Observe that the waveform is a pulse train with 0 V representing a ‘0’ signal or logic ‘0’ and + 5 V representing logic ‘1’.