PSK31
PSK31 (Phase Shift Keying, 31 Baud [symbols per second]) is a computer sound card-generated keyboard-to-keyboard digital mode, used for real-time chat, most often using frequencies in the HF amateur radio bands. 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 more efficient use of the radio energy in a very narrow space thus allowing relatively low-power equipment to communicate globally using the same skywave propagation used by single-sideband (SSB, the most popular HF voice mode) operators and shortwave broadcast radio stations.
Unlike FT8 but similar to JS8, PSK31 allows for conversations, rather than simple automated exchanges.
Hardware and software
A PSK31 operator typically uses an 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. It is common to use a prebuilt interface such as a SignaLink, or to use an interface built in to the radio.
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.
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 (FOSS). Fldigi is a popular program that supports PSK31 as well as many other digital modes. Many operators now use a commercially available interface/modem device 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 (such as FT8), 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, it 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 multipath propagation can disrupt the signal phase continuity. In such cases the use of QPSK is often beneficial.
BPSK31 and QPSK31 variants
Typically, usage of the term "PSK31" implies the use of the most commonly used variant of PSK31: binary phase shift keying (BPSK31). The BPSK variant of PSK31 uses no error control. QPSK31, the variant based on quadrature phase shift keying (QPSK), uses four phases instead of two. It is simple to switch from BPSK31 to QPSK31 if difficulties arise during a contact; QPSK31 has the same number of symbols per second, and hence the same bandwidth as the BPSK variant. 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.
Using four instead of two phases provides twice the physical layer bit rate, which allows addition of redundant information to provide a degree of forward error correction. When QPSK is used, after the text is converted to a sequence of bits, a special code is applied which doubles the length of the sequence by adding redundant information. This code is designed so the receiver can often perfectly reconstruct the original text, even if the bit sequence contained some errors.
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