PSK31 is a keyboard-to-keyboard chat mode: you type, the other operator sees it appear almost as you type it. It sends text on a single, very narrow signal, so it works well with low power and simple antennas, and many conversations fit in a tiny slice of a band. The name means Phase Shift Keying at about 31 baud.
How it works
CW: Morse by switching the carrier. PSK: data by flipping the wave's phase.
CW switches a carrier on and off. PSK31 leaves the tone on and flips its phase: a reversal of phase is one value of a bit, no reversal is the other. A sharp reversal would splatter energy across the band, so PSK31 fades the amplitude smoothly down to zero at each reversal and back up.
Smooth changes make narrow signals. Abrupt changes make wide ones.
That smooth shaping is why the signal stays about 31 Hz wide. It also means a clean transmitter matters: an overdriven or distorted PSK31 signal grows extra traces on the waterfall and wastes its narrow-band advantage.
Varicode
Variable length = fewer bits for common characters = faster typing for the same baud.
Instead of giving every character the same number of bits, Varicode gives common letters short codes and rare ones long codes. Lower-case text therefore goes faster than text full of capitals, digits and symbols, because those need longer bit sequences. Capitals are not sent with more power, only with more bits.
In practice
The signal is a thin line on the waterfall: click on it to tune. The software follows small drifts, but the narrow signal needs a stable radio.
It uses an SSB radio with computer audio, normally upper sideband. Run modest power and keep audio levels low enough that the radio's ALC is not working.
With no error correction, a weak or fading signal turns into gibberish. The basic mode is best at moderate strength with a clean path; modes built for the weakest signals appear in FT8 and FT4 and MFSK, Olivia and Contestia.
Contacts are conversational: call CQ, then trade names, locations and signal reports as in Ragchewing.