PSK31
| PSK31 | |
|---|---|
| Mode type | Digital text, binary phase shift keying (BPSK) |
| Developed by | Peter Martinez G3PLX |
| Introduced | Late 1990s |
| Speed | 31.25 baud, about 50 words per minute |
| Technical | |
| Signal bandwidth | Very narrow; usually quoted as about 60 Hz |
| Code | Varicode, with upper and lowercase letters |
| Common frequencies | 3.580 MHz (80 m), 7.070 MHz (40 m), 14.070 MHz (20 m), 21.070 MHz (15 m) |
| Needs | An SSB radio, a computer and a sound-card interface |
PSK31 is a digital mode for live, keyboard-to-keyboard conversation, designed by the British amateur Peter Martinez G3PLX and introduced in the late 1990s. It sends text at about typing speed in a signal so narrow that dozens of conversations fit side by side within the passband of one SSB receiver. Because it needed nothing more than a computer sound card and free software, PSK31 brought many hams into digital operating for the first time, and through the 2000s it was one of the most popular digital modes on HF. Its activity has fallen sharply since FT8 arrived in 2017, but it is still heard every day.
How it works
A PSK31 signal is a single audio tone, sent through an SSB transmitter, whose phase is flipped by 180 degrees to carry information. The mode is called binary phase shift keying (BPSK): a reversal of phase means a 0, and no reversal means a 1. Symbols are sent at 31.25 baud, which gives the mode its name.
Each reversal is smoothed so the signal's amplitude dips gently to zero and back instead of switching abruptly, which keeps the signal narrow. When the operator is not typing, the transmitter sends continuous reversals, the soft warbling idle sound that identifies PSK31 on the air.
Because the amplitude varies, a PSK31 signal must pass through a linear transmitter without distortion. Too much audio drive, or a speech processor left on, causes splatter, extra signals on either side that interfere with neighbors. Most PSK31 software measures the IMD (intermodulation distortion) of a received idle signal, so other stations can say whether a signal is clean.
Varicode
PSK31 uses its own character code, Varicode. Like Morse code, it gives common letters short codes and rare ones long codes: a lowercase "e" is just two bits. No character contains two 0s in a row, so a pair of 0s marks the gap between characters, and the receiver needs no separate start and stop bits. Unlike the 5-bit Baudot code of RTTY, Varicode includes both upper and lowercase letters and punctuation; lowercase is quicker to send because those codes are shorter. The result is about 50 words per minute, faster than most people type.
The basic mode has no error correction. A variant called QPSK31, which uses four phases and adds error correction, is part of the original design but is seldom used. Faster versions such as PSK63 and PSK125 work the same way at double and four times the speed and width.
On the air
PSK31 activity gathers just above a few well-known dial frequencies, the best known being 14.070 MHz on the 20-meter band. With the radio set to upper sideband on that dial frequency, the signals appear as a column of thin traces in the software's waterfall, a scrolling display of the receiver's audio passband. Clicking a trace tunes the decoder to it, and the text appears in a window as it is typed at the other end.
Contacts are conversational. A typical exchange starts with a CQ, moves on to names, locations and equipment, and can run as long as a voice ragchew. Most programs let operators store macros, prepared lines of text such as a CQ call or a station description, sent with a single key. Software that handles PSK31 includes fldigi, DigiPan and the DM780 module of Ham Radio Deluxe.
Since FT8 took over much of the digital activity on HF, the PSK31 frequencies are often quiet, and a CQ may go unanswered. Activity tends to pick up on weekends and during PSK events and contests, and the mode keeps a loyal following among hams who want an actual conversation rather than an exchange of reports. The PSK31 and FT8 frequencies are only a few kilohertz apart on most bands, so a PSK31 station should take care to keep clear of the FT8 signals.