Frequency modulation (FM)

Frequency modulation keeps the carrier's height rock-steady and instead lets the message speed up or slow down its wiggling. Loud or high parts of the music push the instantaneous frequency a little above the carrier; quiet parts pull it below. Because the information now lives in the timing of the zero-crossings rather than the amplitude, a receiver can simply clip off every amplitude variation — and with it most of the noise — before reading the frequency. That single trick is why FM radio sounds clean and hi-fi while AM hisses.

FM 'buys' its quietness with bandwidth: a wide frequency swing gives better noise immunity but eats more spectrum, which is why an FM broadcast channel is 200 kHz wide versus AM's 10 kHz. This is a beautiful illustration of the bandwidth-versus-fidelity trade that runs through all of communications. FM also has a 'capture effect' — the strongest station wins outright instead of two signals blending — and it underlies everything from walkie-talkies to the data tones in older modems.