PSK31 or "Phase Shift Keying, 31 Baud", also BPSK31 and QPSK31, is a popular computer-sound card-generated radioteletype mode, used primarily by amateur radio operators to conduct real-time keyboard-to-keyboard chat, most often using frequencies in the high frequency amateur radio bands (shortwave). PSK31 is distinguished from other digital modes in that it is specifically tuned to have a data rate close to typing speed, and has an extremely narrow bandwidth, allowing many conversations in the same bandwidth as a single voice channel. This narrow bandwidth makes better use of the radio frequency (RF) energy in a very narrow space thus allowing relatively low-power equipment (5 watts) to communicate globally using the same skywave propagation used by shortwave radio stations. The International Telecommunication Union designates this type of emission as 60H0J2B, often abbreviated as J2B.
History PSK31 was developed and named by English amateur radio operator Peter Martinez (call sign G3PLX) and introduced to the wider amateur radio community in December 1998. 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. Instead of the traditional frequency-shift keying, the information is transmitted by patterns of polarity-reversals (sometimes called 180-degree phase shifts). PSK31 was enthusiastically received, and its usage spread rapidly worldwide, lending a new popularity and tone to the on-air conduct of digital communications. 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.
Use and implementation
A PSK31 operator typically uses a single-sideband (SSB) transceiver connected to the sound card of a computer running PSK31 software. 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. This sound is then fed through either a microphone jack (using an intermediate resistive attenuator to reduce the sound card's output power to microphone levels) or an auxiliary connection into the transceiver, from which it is transmitted.
From the perspective of the transmitter, the sound amounts to little more than somebody whistling into the microphone. However, the software rapidly shifts the phase of the audio signal between two states (hence the name "phase-shift keying"), forming the character codes. These phase shifts serve the same function as the two tones used in traditional RTTY and similar systems. To decode PSK31, the audio whistle received from the transceiver's headphone output is fed into a computer sound card's audio input, and software decodes it. The software displays the decoded text. 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. So, once it has been set up to run PSK31, a computer can be used for a variety of digital message transmission modes. Aside from a standard radio transceiver and a computer with a sound card, very little equipment is required to use PSK31. Normally, an older computer and a few cables will suffice, and many PSK31 software applications are free and open source. Many operators now use a commercially available interface/modem device (or "nomic" ) between their computers and radios. These devices incorporate the necessary impedance matching and sound level adjustment to permit the sound card output to be injected into the microphone input, send the radio's audio output to the sound card input, and handle the radio's transmit-receive switching. Sound card to radio interfaces typically use isolation transformers on both the send and receive audio paths to eliminate hum caused by ground-loops. Many interfaces also incorporate their own sound card and can be powered and run from the computer via a single USB connection. Some modern transceivers have these interfaces built in, requiring only a USB connection from the computer to the radio.
Resistance to interference 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, PSK31 was designed only for leisure use by amateurs, and due to its relatively slow speed and limited error control, is not suitable for transmitting large blocks of data or text, or critical data requiring high immunity from errors. PSK31 works well over propagation paths that preserve phase, and resists fading (QSB) well. However, it can be adversely affected by propagation modes—such as transpolar paths—where auroral "flutter" or multipathing can disrupt the signal phase continuity. In such cases the use of QPSK (see below) is often beneficial. Some software supports PSK10 and PSK05 variants, running at 10 baud and 5 baud, respectively. These slower speeds sacrifice throughput to provide greater resistance to noise and other interference. Conversely, PSK63 is increasingly used for faster exchanges, especially during amateur radio contest operating.
Technical information
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