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As bandied before, a high frequence carrier surge is used to carry the audio signal.




The question arises how the audio signal should be ‘‘ added ’’ to the carrier surge.

The result lies in changing some specific of carrier surge in agreement with the signal.

Under similar conditions, the audio signal will be contain in the attendant surge.

This process is call modulation and may be define as under.
The process of changing some characteristic (e.g. breadth, frequence or phase) of a carrier surge in agreement with the intensity of the signal is know as modulation.

Modulation means to “ change”. In modulation, some characteristic of carrier surge is change in.

Agreement with the intensity ( i.e. breadth) of the signal.

The attendant surge is call modulate surge or radio surge and contains the audio signal.

Thus, modulation permits the transmission to
. do at high frequence while it contemporaneously allows the carrying of the audio signal.

Need for modulation

Modulation is extremely necessary in communication system due to the
following reasons

(i) Practical antenna length

Proposition shows that in order to transmit a surge effectively, the length of the transmitting antenna should be roughly equal to the wavelength of the surge.

As the audio frequentness range from 20 Hz to 20 kHz, thus, if they're transmit directly into space,

the length of the transmitting antenna needed would be extremely large.

For case, to radiate a frequence of 20 kHz directly into space, we'd need an antenna length of 3 × 108/ 20 × 103= metres.

This is too long antenna to be construct virtually. For this reason, it's inoperable to radiate audio signal directly into space. On the other hand, if a carrier surge say of 1000
.kHz is use to carry the signal, we need an antenna length of 300 metres only and this size can be
. fluently constructed.

(ii) Operating range

The energy of a surge depends upon its frequence.

The lesser the frequence of the surge, the lesser the energy held by it. As the audio signal frequentness are small.

Thus, these can not be transmit over large distances if radiated directly into space.

The only
practical Result is to modulate a high frequence carrier surge with audio signal and permit the
transmission to do at this high frequence ( i.e. carrier frequence).

(iii) Wireless communication

One desirable point of radio transmission is that it should be carry without cables i.e. radiate into space.

At audio frequentness, radiation isn't practicable.

Because the effectiveness of radiation is poor.

Still, effective radiation of electrical energy is possible at high frequentness (> 20 kHz).

For this reason, modulation is always do in communication systems.

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