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
(near-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 also concentrates the RF energy in a very narrow
space thus allowing relatively low-power equipment (25 watts) to
communicate globally using the same skywave propagation used by
shortwave radio stations.
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.[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. 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 grew like wildfire
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 mount of large antenna systems, the
use of high power, or both.
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. [3]
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.[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. So, once
it has been set up to run PSK31, a computer can be used to explore 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. 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. Soundcard to radio interfaces typically
use isolation transformers on both the send and receive audio paths
to eliminate hum caused by ground-loops. Recently introduced
interfaces also incorporate their own sound card and can be powered
and run from the computer via a single USB connection.
