modulation

Radio and many wires can carry a steady humming wave — a carrier — beautifully, but a flat carrier says nothing on its own. To send data, you nudge some feature of that wave in time with your bits, like waving a flashlight beam: the beam is the carrier, and you encode your message by changing its brightness, colour, or how you wiggle it. Modulation is exactly this: making your digital bits ride on an analog carrier wave by varying one of its three knobs — amplitude (height), frequency (pitch), or phase (timing).

A sine-wave carrier has three things you can change, giving three basic digital schemes. ASK (amplitude-shift keying) varies the height: a tall wave for 1, a short wave for 0 — simple but easily wrecked by noise that changes amplitude. FSK (frequency-shift keying) varies the pitch: one frequency means 0, another means 1 — robust, used by early modems making those screechy tones. PSK (phase-shift keying) varies the timing of the wave's cycles: shifting the phase by 180 degrees flips a 0 to a 1 — very noise-resistant and the basis of most modern systems. You can even change two knobs at once, which is how QAM (next entry) combines amplitude and phase to pack many bits per symbol.

Modulation matters because it lets digital data cross media and distances that a raw baseband signal cannot. By choosing a high carrier frequency you can send through the air as radio, fit many independent signals side by side on one wire (frequency-division multiplexing), or push light down a fiber. It is the difference between baseband transmission (the bare signal on the wire, as in Ethernet over a short cable) and broadband/passband transmission (bits modulated onto a carrier, as in Wi-Fi, cellular, and cable internet). Without it there would be no wireless networking at all.