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Analog and Digital Signals

In communication system, a signal means a time, varying electrical signal containing informations.

Analog signals

It is a continuous wave form which changes smoothly over time.
  • Such signals can be easily generated from the source of information by using an appropriate transducer, e.g., pressure variations in the sound waves can be converted into corresponding current or voltage pulses with the help of a microphone.
  • A simple analog signal is represented by a sine wave (Fig. 4).
Fig. 4
  • The frequency of analog signals associated with speed or music varies over a range between 20 Hz to 20 kHz.
Fig. 5
  • The range over which the frequencies of a signal vary is called bandwidth.
  • The term base band designates the band of frequencies representing the signal supplied by the source of information.
  • A signal consists of two or more waves of different frequencies known as a complex analog signal (Fig. 6).
Fig. 6

Digital signals

A digital signal is a discontinuous function of time. It has only two voltage levels, i.e., either low (0) or high (1) (Fig. 7). Either of 0 and 1 is known as bit. A group of bit is called byte. A byte comprising 2 bits can give four code combinations, i.e., 00, 01, 10, and 11.
Fig. 7
The number of code combination increases with number of bits in a byte is given by N = 2x, where x = number of bits in a byte. The number of binary digits (bits) per second, which describe a digital signal is called its bit rate. Bit rate is expressed in bits per second (bps).

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