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| Need for Modulation |
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| The velocity of electromagnetic waves is 3 x 108ms-1. On the other hand, the velocity of sound waves cannot be used to transmit intelligence to far off places. Only electromagnetic waves can be made to do this. |
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| Modulation is extremely necessary in communication systems due to the following reasons: |
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| When free space is the communication channel, antennas radiate and receiver the signal. Theory shows that the antennas operate effective only when their dimensions are of the order of the magnitude of wavelength of the signal being transmitted. |
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| The audio frequencies range from 20 Hz to 20 kHz. Suppose a frequency of 20 kHz is to be radiated directly into space. For this, |
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| This is too long antenna to be constructed practically. So, it is impracticable to radiate audio signal directly into space. |
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| Let us now calculate the length of the antenna if a carrier wave of say, 1000kHz is used to carry the signal. |
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| An antenna of 300m length can be easily constructed. |
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| One desirable feature of radio transmission is that it should be carried without wires (i.e.,) radiated into space. At audio frequencies, radiation is not practicable because the efficiency of radiation is poor. However, efficient radiation of electrical energy is possible at high frequencies (>20kHz). For this reason, modulation is always done in communication systems. |
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| The energy of a wave depends upon its frequency. The greater the frequency of the wave, the greater is the energy possessed by it. As the audio signal frequencies are small, these cannot be transmitted over large distances if radiated directly into space. The only practical solution is to modulate a high frequency carrier wave with audio signal and permit the transmission to occur at this high frequency (carrier frequency). |
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