Amplitude modulation is a modulation technique that varies the strength of a carrier wave. The carrier is a steady high-frequency signal. The information — audio, data — changes the carrier's amplitude. The result is a wave that rises and falls in strength, carrying the original signal.

AM radio is the classic example. A station broadcasts at a fixed frequency, say 1010 kHz. The amplitude of that wave changes with the sound. A simple receiver detects those changes and recovers the audio. AM is simple and travels long distances. But it is susceptible to noise from lightning, motors, and electrical equipment.

Common forms

• AM radio: medium wave broadcast
• QAM: quadrature amplitude modulation, used in digital TV and modems
• ASK: amplitude shift keying, digital on-off
• SSB: single sideband, efficient voice transmission

AM is not just for radio. QAM carries digital data in cable modems, Wi-Fi, and digital TV. It combines amplitude and phase changes to encode many bits per symbol. The old technique still powers modern networks.