Digital modulation (ASK / FSK / PSK)

When the message is a stream of 1s and 0s instead of a continuous voice, modulation becomes 'keying': you switch one of the carrier's three knobs between discrete states. Amplitude-shift keying (ASK) turns the carrier loud for a 1 and soft (or off) for a 0 — Morse code's on/off tone is the ancestor. Frequency-shift keying (FSK) sends a 1 as one tone and a 0 as another, the two-note warble you hear from a fax machine. Phase-shift keying (PSK) keeps amplitude and frequency fixed and flips the carrier's phase — in BPSK a 0° start means '1' and a 180° start means '0'.

Each scheme trades simplicity against efficiency and robustness. ASK is cheapest but most vulnerable to noise, since fading directly mimics the bit. FSK is sturdy and easy to detect, used in pagers, Bluetooth, and key fobs. PSK packs more bits into the same bandwidth and resists noise well, which is why it and its richer cousin QAM dominate Wi-Fi, cellular, and satellite links. A useful picture is the constellation diagram: each possible symbol is a dot in a 2-D plane, and the receiver decides which dot the received point is closest to.