When we have to transmit a message with electromagnetic waves, first of all, it must be transformed into numerical form, for example with the use of the binary code, with a sequence of 0 and 1. Each number is then associated with two different frequencies, generating a variable electromagnetic wave, which has variable frequencies.
When received, this frequence variation is detected and, using again the binary code, is converted back into the original message.
Today, electromagnetic waves are used in many fields: mobile phones, Internet, television, radio and remote controls. Unfortunately the frequencies used, which are not infinite, have to be divided between the various transmitters, in order to avoid signals' overlap. Therefore international rules establish the distribution of frequencies between the various television and radio broadcasters. To each of them, however, it is not assigned only a single frequency, but a continuous range which extends between two limits.
The width of this interval, and so also the number of different signals broadcastable, is determined both by the complexity of the transmitted signal (a written message is less complex than sound, the sound less than a still image; this much less of a moving image), both by the need not to disturb the channels immediately close (there must be some 'free zones'). If for the telephone signal, which just needs to ensure the recognizability of the human voice, just a few thousand Hertz are enough (between 300 and 3400 Hz); the radio, which transmits voice, sounds and Hi-fi music, requires a wider band: from 15 to 20,000 Hz; a television broadcaster PAL (which needs to broadcast sound and moving images) needs a bandwidth of approximately 5.5 MHz.