Communication System Model

The purpose of a communication system is to communicate data / information as video, audio, (voice, picture, music) etc., from one point called source of generation to distant point called destination of end user. The communication channel or media connects source as well as destination over telephone lines, microwave links, optical fibre cable, etc.

As shown in the figure above, if the data/message generated is not electrical, then an input transducer is used to convert the message signal to be transmitted as a time – varying electrical quantity called as message signal. At the receiver or destination end, another transducer is used to convert the electrical waveform to the appropriate audio / video information like message.

The transmitter comprises amplifier, modulator, oscillator, filter circuits to perform necessary signal processing operation such that message or information signal gets modulated in a form ready to be transmitted over long distance transmission lines. The communication channel comprises transmission lines as telephone cables, microwave links, satellite, links, optical fibre cable etc., through which modulated signal travels to the destination.

The receiver at the destination consists of demodulator circuit, rectifier and filter circuits. Thus receiver extracts the message signal from the carrier by demodulator process. Thus useful message signal as voice / data is conveyed to destination of the end user.

Functions of Transmitter

The Transmitter performed the function of modulating the audio signal and rediate the same in the form of electromagnetic wave. The Transmitter performs signal processing operation and thereby it couples the input message signal to the communication channel. Transmitter functions pertaining to the signal processing are (i) Amplifications (ii) Filtering and (iii) Modulation.

Thus resultant modulated wave could be transmitted into air through antenna or transmitting aerial, which transmits these signals uniformly in all direction in the form of electro magnetic waves. In wireless communication we see that for every transmitter there are a number of receivers.

Functions of Receiver

Receiver in a communication system performs the function of receiving and recovering the original information (for eg. Audio signal) transmitted by the transmitter. The receiver should be capable of selecting the desired frequencies while rejecting all the unwanted frequencies i.e., carrier frequencies and should demodulate receiver signal.

The selection of a receiver to perform the above function depends on those factors such as frequency of operation, type of modulation employed, type of modulating signal etc., The classification of receiver is based on type of modulation employed.

Modulation

Modulation is used in communication systems for matching signal characteristics to channel characteristics, for reducing noise and interference, for simultaneously trans-mitting several signal over a single channel and for overcoming a few limitations of the communication devices.

In particular, Modulation is necessary in communication systems due to the following reason :

Antenna length : In order to transmit a signal effectively, the length of the transmitting antenna should be approximately equal to the wave length of the signal. Therefore, for transmitting audio waves (whose wavelength is very high) we need to have very long antennas, which is impractical.

Operating range : The energy of a wave depends upon its frequency. The greater the frequency of the wave, the greater the energy possessed by it. As the audio signal frequencies are small they cannot be transmitted over long distances.

Wireless Communication : One desirable feature of audio transmission is that it should be carried without wires, that is radiated into space. AT audio frequencies, radiation is not practical because the efficiency of radiation is poor at low frequencies.

The only practical solution for the above problems is to modulate a high frequency carrier wave with the audio signal and permit the transmission to occur at this high frequencies ( carrier frequency).

Demodulation

To recover the conversion (information signal or audio wave) at the receiving end of the channel, the signal is demodulated. Usually, while sending, audio signal are modulated so as to travel over a long distances. This modulated signal is picked up by the receiver, which contains carrier and side band frequencies which are in the radio frequency angle. The process of demodulation is done by multiplying the received signal by the carrier and by rejecting higher frequencies using an electrical circuit called a filter. In other words, the signal are recovered at the receiving end by demodulation them with the respective carrier frequencies.

The essential requirements of the demodulation process are it should rectify the modulated wave which is in the form of a.c. and then it has to separate the audio signal from the carrier. The rectification is achieved by the employing a diode circuit and separation is achieved by means of a filter circuit which removes the carrier frequency and allows audio signal to reach the speaker.

Thus, the process of recovering the audio signal from the modulated wave is known as Demodulation.

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