PSK31

PSK31 or "Phase Shift Keying, 31 Baud" is a digital mode of radio communication, used primarily in the amateur radio service.

PSK31 mode was developed by English amateur radio operator Peter Martinez (G3PLX). Martinez initially called his creation "varicode", because it uses variable length encodings to represent characters. It was designed so that the more frequently occurring characters had shorter encodings and the rarer characters used the longer encodings. One uses computer software to generate and decode an audio signal containing the encodings; the encoded audio signal is used to modulate the RF carrier of a transmitter.

PSK31 audio bandwidth is very narrow (about 100 Hz), making it highly suitable for low power and crowded band operation.