Definition of:amplitude modulation
amplitude modulation

Varying the voltage of a carrier or a direct current in order to transmit analog or digital data. Amplitude modulation (AM) is the oldest method of transmitting human voice electronically. In an analog telephone conversation, the voice waves on both sides are modulating the voltage of the direct current loop connected to them by the telephone company.

Modulate a Carrier
Amplitude modulation (AM) is also widely used to alter a carrier wave to transmit data. For example, in AM radio, the voltage (amplitude) of a carrier with a fixed center frequency (the station's channel) is varied (modulated) by the analog audio signal.

AM is also used for digital data. In quadrature amplitude modulation (QAM), both amplitude and phase modulation are used to create different binary states for transmission (see QAM). AM is also used to modulate light waves in optical fibers. See modulation and carrier.

Vary the Amplitude
In AM modulation, the voltage (amplitude) of the carrier is varied by the incoming signal. In this example, the modulating wave implies an analog signal.

Digital Amplitude Shift Keying (ASK)
For digital signals, amplitude shift keying (ASK) uses two voltage levels for 0 and 1 as in this example.