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| Advantages of Digital Communication Systems over Analog Communication Systems |
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| The advantages associate with digital communication systems include |
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| (i) A common format for encoding different kinds of message signals (e.g., speech signal, video signal, computer data, etc.) for the purpose of transmission. |
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| (ii) An improved security of message. |
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| (iii) Increased immunity to noise and external interference. |
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| (iv) Flexibility in configuring digital communication systems. |
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| The disadvantages associate with digital communication systems include |
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| (i) Increased transmission |
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| (ii) Increased system complexity |
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| Although digital communication is becoming increasingly attractive for practical use, it is a fact that analog communication systems are still in existence. Most of the broadcasting systems and a large part of telephone networks in use today are analog in nature. |
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| There are numerous ways of classifying signals depending on the requirement of study. A signal is usually described by a function of one or more independent variable. The value of the signal may be real or complex. We would consider the signal to be a function of a signal independent variable i.e. time t. Analog signals or continuous-time signals are defined for every value of time and they take on continuous values in a given time interval. |
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| Mathematically these signals can be described by function of a continuous variable. For example, the signals. |
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| s1 (t) = cos (500 pt) |
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| are analog or continuous-time signals. |
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| Most of the naturally occurring signals are also analog in nature. Speech, electrocardiogram(ECG), electroencephalogram(EEG) signals are some of the examples of naturally occurring analog signals. |
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 Sine wave; example of an analog signal |
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| An analog signal is best fitted illustrated by a sine wave as shown in above figure. Notice that the sine wave is continuous. The value of the analog signal at any given instant can be anywhere within the range of the signal's extremes. |
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| Digital signals or Discrete-time signals are defined at discrete values of time. |
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| These time instants need not be equidistant. In practice, however, for mathematical convenience these time intervals are taken to be
equispaced. The signal x(tn) = exp (-|tn|), where n =
0 ± 1,
± 2, ...............is an example of a discreate - time signal. |
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