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. Technically, varicode is the encoding, and PSK31 the transmitting method. Varicode 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. The 31.25 Hz bandwidth was chosen because transmitting Varicode at a normal typing speed of about 50 words per minute needs a bit-rate of about 32 per second. The bit-rate of 31.25 was chosen because it could easily be derived from the 8 KHz sample-rate used in many DSP systems such as those used in computer sound cards commonly used for PSK31 operation.