/ˌeɪ ˈɛm/
noun … “sending sound by stretching and shrinking a carrier wave.”
AM, short for Amplitude Modulation, is a method of encoding information onto a carrier wave by varying its amplitude while keeping the frequency and phase constant. Unlike FM or digital modulation schemes such as QPSK, AM directly scales the voltage of the carrier signal in proportion to the instantaneous value of the message signal, typically audio or telemetry data.
The core characteristics of AM include a carrier signal, sidebands (upper and lower), and susceptibility to noise, since amplitude variations caused by interference affect the transmitted signal. The modulated signal can be represented mathematically as V(t) = [1 + m(t)] * cos(2πfct), where m(t) is the message signal normalized to 1, and fc is the carrier frequency.
In practice, AM is used in broadcasting (e.g., AM radio), aviation communications, and legacy telemetry systems. A transmitter takes the audio signal, modulates a high-frequency carrier, and sends it through an antenna. The receiver demodulates by extracting amplitude variations to reconstruct the original message.
Conceptually, think of AM as a flexible canvas: the carrier wave is a steady brushstroke, and the message signal paints over it with varying intensity, creating a waveform that can travel long distances but is sensitive to smudges—i.e., noise.