Modulation is the periodic variation of one or more properties of a signal or wave. A principle application of modulation is communications - the transmission of information. In the case of the telephone, for example, waves of air that have been modulated in intensity and frequency by the speaker are picked up by a microphone.
The microphone converts the waves into an electrical signal the intensity and frequency of which follow that of the audible signal. This modulated electrical signal is then transmitted through a conducting wire to a receiver, in which the modulated electrical signal is demodulated - that is converted back into the audible signal.
Information may be conveniently transmitted by means of electromagnetic radiation. The wave has several properties that may be altered, or modulated, the most important of which are its amplitude and frequency. The unit for measuring the frequency of a wave, formerly called a cycle per second, is now called a hertz (hz).
In the field of communication, when the goal is the transmission of information, modulation may be more precisely defined as the process of superimposing the amplitude or frequency of an incoming wave (the signal) upon a transmitted wave (the carrier). Several forms of modulation are in common use.
The transmission of information by varying the amplitude of a fixed frequency carrier wave in proportion to the amplitude of the signal is called amplitude modulation (AM). The method is employed by commercial radio broadcast station that transmit worldwide over the frequency band between 540 000 and 1 600 000 Hz (540 - 1.600 kHz). Amplitude modulation is simple to implement but is susceptible to various type of electromagnetic noise.