5.6 Modulation

As mentioned earlier, an audio signal has low frequency (< 20 KHz).

Low frequency signals can not be transmitted over large distances.

Because of this, a high frequency wave, called a carrier wave, is used.

Some characteristic (e.g. amplitude, frequency or phase) of this wave is changed in accordance with the amplitude of the signal.

This process is known as modulation.

Modulation also helps avoid mixing up of signals from different transmitters as different carrier wave frequencies can be allotted to different transmitters.

Without the use of these waves, the audio signals, if transmitted directly by different transmitters, would have got mixed up.

Modulation can be done by modifying the

(i) amplitude (amplitude modulation)

(ii) frequency (frequency modulation), and

(iii) phase (phase modulation) of the carrier wave in proportion to the amplitude or intensity of the signal wave keeping the other two properties same.

The carrier wave is a high frequency wave while the signal is a low frequency wave.

Amplitude modulation (AM) is simple to implement and has large range.

It is also cheaper.

Its disadvantages are that

(i) it is not very efficient as far as power usage is concerned

(ii) it is prone to noise and

(iii) the reproduced signal may not exactly match the original signal.

In spite of this, these are used for commercial broadcasting in the long, medium and short wave bands.

Frequency modulation (FM) is more complex as compared to amplitude modulation and, therefore is more difficult to implement.

However, its main advantage is that it reproduces the original signal closely and is less susceptible to noise.

This modulation is used for high quality broadcast transmission Phase modulation (PM) is easier than frequency modulation.

It is used in determining the velocity of a moving target which cannot be done using frequency modulation.