Amplitude modulation (AM)

Amplitude modulation is the oldest and simplest way to put a message on a carrier: you let the message swell and shrink the carrier's height. Picture a steady drumbeat (the carrier) whose volume rises and falls in time with someone's voice — the loudness envelope traces out exactly the speech waveform. Mathematically the transmitted signal is the carrier multiplied by (1 + the message), so the message rides as a slowly breathing outline on top of the rapid carrier oscillation.

AM's charm is that a receiver can recover it with almost nothing — a diode and a capacitor (an 'envelope detector') will do, which is why crystal radios with no battery work. Its weakness is that this amplitude envelope is exactly what atmospheric noise, lightning, and engine sparks also disturb, so AM hisses and crackles. AM also wastes power: most of it sits in the carrier itself, which carries no information. Commercial AM broadcast lives at 530–1700 kHz; the same idea, stripped of its wasteful carrier, becomes SSB used by ham radio and aircraft.