PSK31

A waterfall display depicting several PSK31 transmissions at around 14,070 kHz.

PSK31 or "Phase Shift Keying, 31 Baud" is a digital radio modulation mode, used primarily in the amateur radio field to conduct real-time keyboard-to keyboard informal text chat between amateur radio operators.

History

PSK31 was developed by English amateur radio operator Peter Martinez (G3PLX) and introduced to the wider amateur radio community in December 1998. Martinez initially called his creation "varicode", because it uses his own arrangement of variable length encodings (Huffman codes) to represent characters.

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 remains) especially popular with operators whose circumstances do not permit the erection of large antenna systems and/or the use of high power.

Use and implementation

Very little equipment – in addition to a standard radio transceiver – is required to use PSK31. Normally an old PC and a few cables will suffice; the software is both free to download and runs happily on old, slow computers from the early Pentium era or even earlier.

The operator typically uses a single sideband transceiver connected to the sound card of the PC. When the operator enters a message for transmission, the software produces an audio tone, which sounds to the human ear like a continuous whistle with a slight warble. This is then fed through either a microphone jack (using an intermediate resistor to reduce the sound card's output power to microphone levels) or an auxiliary connection into the transceiver, where it is transmitted. (PSK31 Station Setup)

From the perspective of the transmitter, this amounts to little more than somebody whistling into the microphone. However, the software rapidly shifts phase of the audio signal between two states (hence "phase-shift keying"), forming the varicode. These phase shifts serve the same function as the two tones used in traditional RTTY and similar systems.

To decode PSK31, the received audio whistle from the transceiver's headphone output is fed into the sound card's audio input, and the software decodes it. The software also includes a user interface on the PC, which is used to display the decoded text and manage the software configuration. (PSK31 Station Operation)

The use of PSK31 does not require exclusive use of a dedicated computer. When not running the PSK31 software, the computer can still be used exactly as before. Also, once set-up to run PSK31, the same computer and cables can then also be used to explore other interesting modes such as RTTY, Hellschreiber, Olivia MFSK etc., and perform useful tasks such as automatically monitoring radio propagation beacons.

Resistance to interference

PSK31 can often overcome interference and poor propagation conditions in situations where voice or other data methods of communication fail. However, PSK31 was only designed for leisure use by amateurs, and due to its relatively slow speed and minimal or no error control, is not intended for the transmission of large blocks of data or text, or critical data requiring high immunity from errors.

PSK31 works well with propagation paths that preserve phase, and can be adversely affected by those that do not, such as transpolar paths, where auroral influence can disrupt the signal phase continuity.

Some software supports PSK10 and PSK05 variants, running at 10 baud and 5 baud, respectively. These slower speeds sacrifice throughput to provide even greater resistance to noise and other interference.

Technical information

PSK31 is a half-duplex mode of communication. Thus only one participant in the conversation can transmit at a time.

Technically, varicode is the encoding method, and PSK31 the transmitting method. Martinez arranged his varicode alphabet so that, as in Morse code, the more frequently occurring characters would have the shortest encodings, while rarer characters used longer encodings.

PSK31's bandwidth of 31.25 Hz was chosen because a normal typing speed of about 50 words per minute requires a bit rate of about 32 bits per second, and specifically because 31.25 Hz could easily be derived from the 8 kHz sample rate used in many DSP systems, including those used in the computer sound cards commonly used for PSK31 operation (31.25 Hz is 8 kHz divided by 256, and so can be derived from 8 kHz by halving the frequency eight times).

Colloquial usage of the term 'PSK31' in amateur radio usually implies the use of the most commonly-used variant of PSK31: binary phase shift keying. BPSK uses no error control, but an allied mode, QPSK31, uses four phases instead of two, to provide a degree of forward error correction. It is very simple to switch from BPSK to QPSK if difficulties arise during a contact.

Spectrum efficiency compared to other modes

PSK31's efficiency and narrow bandwidth make it highly suitable for low-power and crowded-band operation. PSK31 contacts can be conducted at less than 100Hz separation, so with disciplined operation at least twenty simultaneous PSK31 contacts can be carried out side-by-side in the bandwidth required for just one SSB voice contact.

Common Frequencies

The following amateur radio frequencies are commonly used for transmitting and receiving PSK31 signals.

PSK31 Frequencies

Frequency	Amateur Band
1.838 MHz	160 meters
3.580 MHz	80 meters
7.035 MHz	40 meters (regions 1 and 3)
*7.080 MHz	40 meters (region 2)
10.142 MHz	30 meters
14.070 MHz	20 meters
18.100 MHz	17 meters
*21.080 MHz	15 meters
24.920 MHz	12 meters
28.120 MHz	10 meters
50.290 MHz	6 meters

* Current usage as of 2010, based on observation, is centered on 7,070.15 and 21,070.15. There is no authoritative list as the frequencies are determined by common convention.

References

• Peter Martinez. PSK31: A new radio-teletype mode with a traditional philosophy. http://det.bi.ehu.es/~jtpjatae/pdf/p31g3plx.pdf
• Meltz, Steve "The New HF Digital Modes - PSK31", QST, April, 1999, pp. 50-51
• Martinez, Peter. [1] "PSK31: A New Radio-Teletype Mode". RadCom, December 1998, updated February 1999

External links

• The Official PSK31 Page
• How to operate PSK31 | PSK31 Guide
• PSK31 Frequencies A listing of frequencies for PSK31 operation.
• Varicode Explained Layman's guide to varicode.