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Define modulation and explain any two need of modulation.
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In the modulation process, two signals are used namely the modulating signal and the carrier, the modulating signal is nothing but the base-band signal or information signal while carrier is a high frequency sinusoidal signal.

In the modulation process some parameter of the carrier wave (such as amplitude, frequency or phase) is varied in accordance with the modulating signal, this modulated signal is then transmitted by the transmitter, in the process of modulation, the carrier wave actually acts as a carrier which carries the information signal from the transmitter to receiver.

The main need/advantages of modulation are:

1] Reduction in the height of antenna.

2] Increases the range of communication.

3] Avoids mixing of signals.

4] Multiplexing is possible.

5] Improves quality of reception.

1] Reduction in the height of antenna : For the transmission of radio signals, the antenna height must be a multiple of ($\lambda$/4) where $\lambda$ is the wave length ($\lambda$ = c/f), the minimum antenna height required to transmit a base-band signal of f = 10KHz is calculated as follows:

Minimum antenna height = $\frac{\lambda}{4} = \frac{C}{4f} = \frac{ 3 \times 10^8}{4 \times 10 \times 10^3}$

= 7500 mtr i.e. 7.5 km

Now consider a modulated signal at f = 1MHz

The minimum height is given by

$= \frac{\lambda}{4} = \frac{c}{4f} = \frac{3 \times 10^8}{4 \times 1 \times 10^6} = 75 \ mtr$

This size antenna can be installed practically.

2] Avoid mixing of signals : If the base-band sound signal are transmitted without using modulation by more than one transmitter then all the signal will be in the same frequency range, therefore all the signal get mixed together and a receiver cannot separate them from each other, modulation avoids such mixing of signals.