Question

Explain the term modulation

Let us see how radio broadcasting stations broadcast speech or music etc. from their broadcasting studios.

First, the speech or music which consists of a series of compressions and rarefactions is translated into a tiny varying electric current with the help of a crystal microphone.

The frequency of variations of this current lies in the audio-range, hence it is known as audio frequency signal.

The audio-frequency signal cannot be radiated out from the antenna directly because transmission at audio-frequencies is not practical. For this purpose, oscillations of very high frequency or radio-frequency are produced with the help of any one of the oscillators discussed above.

These electromagnetic waves so produced are of constant amplitude but of extremely high frequency.

These waves, which are neither seen nor heard, travel through space with the velocity of light i.e. 3xl08m/s (approx).

The audio-frequency signal which is to be broadcast, is then superimposed on the R-F waves, which are known as carrier waves (because they carry the A.F. signal through space to distant places).

In a way, the carrier waves can be likened to a horse and the audio-frequency signal to a rider.

The process by which A.F. signal or information is impressed on the carrier wave is known as modulation.

The horse and rider travel through space.

At the receiving end, they strike the receiving aerial and enter the receiver which separates the horse from the rider.

The horse i.e. carrier wave is returned and the rider re. audio-frequency signal is converted back into sound.

This process by which the R.F. waves and A.F. waves are separated is known as detection or demodulation (because it is the reverse of modulation).