A CDE Definition
Typically refers to the altering of a fixed-frequency carrier wave (radio or TV channel) in order to transmit data, audio or video from one location to another (see carrier). At the receiving end, a tuner latches onto the particular carrier frequency and a demodulator circuit isolates the data from the 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 (see amplitude modulation, frequency modulation and phase modulation). In quadrature amplitude modulation (see QAM), both the amplitude and phase of the carrier are modified.
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