PSK31

[1][2] The 31 baud BPSK modulation system used in PSK31 was introduced by Pawel Jalocha (SP9VRC) in his SLOWBPSK program written for Motorola's EVM radio.

PSK31 was enthusiastically received, and its usage spread rapidly worldwide, lending a new popularity and tone to the on-air conduct of digital communications.

[citation needed] Due to the efficiency of the mode, it became, and still remains, especially popular with operators whose circumstances do not permit the installation of large antenna systems, the use of high power, or both.

When the operator enters a message for transmission, the software produces an audio tone that sounds, to the human ear, like a continuous whistle with a slight warble.

However, the software rapidly shifts the phase of the audio signal between two states (hence the name "phase-shift keying"), forming the character codes.

[3] Because PSK31 was developed for use through a computer's sound card, many programs have since been created to use the same technology for other modes, such as RTTY, Hellschreiber, and Olivia MFSK.

Sound card to radio interfaces typically use isolation transformers on both the send and receive audio paths to eliminate hum caused by ground-loops.

Like other narrow band digital modes, PSK31 can often overcome interference and poor propagation conditions in situations where voice or other methods of communication fail.

However, it can be adversely affected by propagation modes—such as transpolar paths—where auroral "flutter" or multipathing can disrupt the signal phase continuity.

PSK31 is typically created by software that generates an amplitude- and phase-modulated waveform that is converted to an audio frequency analog signal by a sound card.

[4] As shown in the figure, a cosine filter is used to smooth the rise and fall times of the audio waveform and eliminate key clicks.

Colloquial usage of the term 'PSK31' in amateur radio usually implies the use of the most commonly used variant of PSK31: binary phase shift keying (BPSK).

In a coherent receiver, the bit error probability of QPSK is the same as for BPSK operating at the same power, making QPSK31 the generally preferable mode from a robustness, and thus reach, point of view.

This is because (a) signals then spread upwards into the digimode section from the "base" frequency, and (b) using QPSK requires both stations to use the same sideband.