U.S. Naval Cryptologic Veterans Association

PSK31

Phase Shift Keying 31.25 baud

PSK31 is a very popular form of digital communications used by amateur radio operators around the world. In reality they use “Binary Phase-Shift Keying” (BPSK) or “Quadrature Phase-Shift Keying” (QPSK).

PSK31 was designed by Peter Martinez [G3PLX]. It is well suited for low-power (stations typically run less than 30 watts), narrow bandwidth (approximately 60Hz), and live keyboard to keyboard operation. It works well for net operations. It is one of the more common sound card based digital modes of operation used by amateur radio operators.

PSK31 does not offer error free links. It is not suitable for transmitting files or accessing mailboxes. While it does not offer error-free links, it is particularly useful in poor propagation conditions and low Signal-Noise Ration (SNR) situations.

PSK31 uses Varicode character coding. Different characters are represented by a variable-length combination of bits. It is a self-synchronizing code using the presence of at least two consecutive 0's to represent the break between characters. No character can begin with or end with a zero in its coding. It supports upper and lower case characters and the shortest codes are assigned to the most commonly used lower cased characters of the English language. It approaches 50 WPM communications speed.

BPSK

BPSK is the simplest mode to implement. It is sideband INDEPENDENT. That is the stations communicating with each other can be using different sidebands (LSB/USB).

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QPSK

QPSK can be used either to double the data rate compared with a BPSK system while maintaining the same bandwidth of the signal, or to maintain the data-rate of BPSK but halving the bandwidth needed. The amateur radio implementation maintains the bandwidth and transmits convolutional encoded characters for error reduction. The convolutional coding used by PSK31 doubles the number of data bits used to represent a character. The receiving station uses Viterbi decoding to decode the characters.

QPSK is sideband DEPENDENT. Stations communicating with each other must use the same sideband (either LSB or USB). By convention most amateurs use USB. (There's no particular advantage or disadvantage to either sideband.) In common practice most amateur radio contacts begin by using BPSK and then may switch to QPSK if all stations engaged in the contact agree to the switch.

Minimum Requirements

  • A HF transmitter capable of operating Single Side-Band (SSB) that can be adjusted to run less than 30 Watts.
  • A computer with a sound card.
  • Software:
  • An antenna system!

Highly Recommended Additions

A rig-to-computer interface that provides isolation and its own sound card. (You don’t want your computer sending extraneous sounds to your radio). In alphabetical order:

PSK31 Demos

Some very basic demonstrations showing how easy PSK31 is to use.

This video shows how to receive PSK31 using just a microphone connected to your computer's sound card to acoustically couple the output of your receiver's speaker and the free DigiPan software.

Note: The video doesn’t mention using the F12 function of DigiPan to allow you to simultaneously monitor the text of each of the signals being decoded.

Additional Reading

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Naval Security Group Command  Cryptology - Information WarfareCenter for Information Dominance