Modulation refers to altering an oscillating, fixed-frequency carrier wave (radio wave) in order to transmit data, audio or video from one location to another, either wired or wireless. The data are merged into the carrier.
At the receiving end, a tuner latches onto the particular carrier frequency and a demodulator circuit isolates the data from the carrier. See carrier
In traditional analog telephony, modulation alters a DC current. In such a conversation, both parties are modulating the current simultaneously.
Amplitude, Frequency and Phase
There are three basic types of modulation: amplitude modulation (AM) varies the voltage; frequency modulation (FM) varies the frequency, and phase modulation (PM) varies the angle of the wave. In quadrature amplitude modulation (QAM), both amplitude and phase are modified. See amplitude modulation
, frequency modulation
, phase modulation
In AM/FM radio, the radio station's carrier frequency has been modulated by the analog sound waves.