(introduction...)

Let us first sum up, what we know till here:

1. Our ears are able to receive frequencies within the so called audio-frequency spectrum which starts at about 30 Hz and ends below 20 000 Hz.

2. Only frequencies of a minimum of about 30 000 Hz can be transmitted in form of electromagnetic waves.

3. Therefore it is easy to understand that we will hear nothing if a radio receiver is picking up a very strong transmission of a certain radiostation unless....

4. ... there is an audio signal transmitted as well.

HOW TO TRANSMIT THEN THE WANTED AUDIOSIGNAL BY MEANS OF RADIOWAVES?

The technical solution for this problem is: to let the radio frequency signal “carry” the audiosignal.

The process of charging the “lorry” (putting the audiosignal on the carrierwave) is called MODULATION.

fig. 26

Modulation means actually “varying”. So modulation of the carrierwave means to change the carrier-wave in one of its aspects, proportionally to the amplitude of the audiosignal.

The carrierwave has two main characteristics which determine its shape: the FREQUENCY and the AMPLITUDE.

To ENCODE a message (here to encode the audiosignal on the carrierwave) it is enough to change one of these two factors. Considering this fact, it is easy to understand why there are two methods of modulation.