PSK31

Several adjacent PSK31 signals, at least six, taken from the waterfall display of DigiPan. Frequency is plotted horizontally, and time is plotted vertically, with the earliest at the bottom. Strength is indicated by colour, increasing from blue to yellow.

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, a coding scheme similar to Morse code. 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 (31.25 Hz), making it highly suitable for low power and crowded band operation.