Field Guide · protocol

Also known as: PSK31

PSK31 is a narrowband amateur-radio keyboard chat mode that carries live, conversational text in a signal only about 31 Hz wide. It sends data as differential BPSK at 31.25 baud and encodes text with a variable-length Varicode alphabet — short bit patterns for common letters, longer ones for rare characters — so ordinary English flows at a comfortable typing speed.1 Introduced in 1998, PSK31 revitalized HF digital operation by pairing sound-card decoding with a waveform far narrower and more sensitive than RTTY, and it remains a staple of the HF digital sub-bands.

phase 180° phase 0° 180° reversal = bit "0" → phase reversal"1" → no reversal31.25 baud · Varicode text
PSK31 differentially encodes each bit as the presence (0) or absence (1) of a 180° carrier phase reversal, spelling out Varicode-encoded text at 31.25 baud.

Overview

PSK31 is built for casual real-time chat between two or more stations. Because it is differential — each bit is decided by comparing one symbol’s phase to the previous one — the receiver does not need an absolute phase reference, only to detect whether the carrier flipped 180°. Idle time is filled with a steady stream of phase reversals, which also gives the decoder a clean timing signal to lock onto. Raised-cosine pulse shaping keeps the occupied bandwidth tight and the spectrum clean.

Variants

A phase-only signal is vulnerable at very low SNR, so QPSK31 adds a convolutional code across a four-phase constellation for error correction at the cost of a more demanding receiver. Faster relatives (PSK63, PSK125) simply scale the symbol rate up for better throughput on stronger paths, and multi-carrier variants stack several PSK streams side by side.

Technical characteristics

Property Value
Modulation Differential BPSK (QPSK variant optional)
Symbol rate 31.25 baud
Occupied bandwidth ~31 Hz per signal
Text coding Varicode (variable-length, self-synchronizing)
Pulse shaping Raised-cosine
Error control None (BPSK); convolutional code (QPSK variant)

History

PSK31 was developed by Peter Martinez (G3PLX) and released in 1998. It arrived just as personal computers with sound cards made software modems practical, and its combination of tiny bandwidth, good sensitivity, and free decoding software made it an immediate hit, displacing much everyday RTTY chat on HF.2 The Varicode alphabet — inspired by the efficiency ideas behind Morse code — was a key part of that success.

Deployment

PSK31 lives on the HF amateur bands, clustered in well-known digital watering holes such as 14.070 MHz on 20 m, where many narrow signals pack into a single waterfall display. It is used for conversational contacts, DXing, and casual nets worldwide.

Decoding it with GopherTrunk

GopherTrunk does not decode PSK31 — it is a trunked land-mobile scanner, not an HF keyboard-mode decoder. PSK31 is received with an SSB receiver or SDR feeding audio into a multimode program such as fldigi, Digital Master, or MultiPSK, which recovers the differential phase and decodes Varicode. Its extreme narrowness means the front end mainly needs frequency stability and a clean audio slice.

Sources

  1. PSK31 — Wikipedia, for the differential BPSK waveform, 31.25-baud rate, Varicode alphabet, and the mode’s origin and use. ↩

  2. PSK31 Specification — ARRL / G3PLX, the authoritative description of PSK31’s modulation, Varicode coding, and idle-signaling behavior. ↩

See also