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.
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PSK31 was developed and named by English amateur radio operator Peter Martinez (G3PLX) and introduced to the wider amateur radio community in December 1998.[1][2]
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 erection of large antenna systems and/or the use of high power.
A PSK31 operator typically uses a single sideband transceiver connected to the sound card of a PC running PSK31 software. 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.[3]
From the perspective of the transmitter, this 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 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.[3]
The use of PSK31 does not require exclusive use of a dedicated computer. When it is not running the PSK31 program, the station 'utility' computer can still be used exactly as before. PSK31 is a 'soundcard' mode, and many programs have since been created to utilise the same technology for other interesting modes such as RTTY, Hellschreiber, Olivia MFSK etc. So, once it has been set-up to run PSK31, the same set-up can also be used to explore a variety of these other modes at the click of a mouse.
In addition to a standard radio transceiver, very little equipment is required to use PSK31. Normally, an older 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. 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 soundcard's output to be injected into the microphone input, the radio's audio output to be sent to the soundcard's input, and also handle the radio's transmit-receive switching. Recently introduced interfaces also incorporate their own soundcard, and can therefore be powered and run from the PC via one single USB connection.
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. After encoding into varicode, the binary signal is further transformed into a quaternary set of phase shifts. A sliding window of five bits is used to select one of the four possible phase shifts, providing a means of error correction by spreading each bit of data across adjacent bits. To successfully decode an input bit requires a large number of phase shift sequences to be received, causing a 20 bit, 640 millisecond latency in the output of the decoder.[4] The idle sequence is a continuous sequence of zeros, since no varicode word may contain two adjacent zeros, which results in a continuous reversals of phase as in BPSK.[2]
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.
PSK31 is created by switching the polarity of the signal used to key the computer's sound card. In the most-commonly-used variant (BPSK31), binary information is transmitted by either imparting a 180-degree polarity shift (a binary 'zero') or no polarity shift (a binary 'one') in each 32ms symbol interval. As shown in the figure above, a "raised cosine" filter is used to limit the occupied bandwidth, resulting in both amplitude and phase modulation of the carrier. The 180 degree phase shift for a "zero" bit code occurs during at an amplitude null.[5] Subsequent amplification of the signal must be linear, preserving the modulation waveform, to ensure minimum occupied bandwidth.
The boundaries between character codes are marked by two or more consecutive zeros. Since no character code contains more than one consecutive zero, the software can therefore instantly detect the 'space' between characters. Martinez arranged the character alphabet so that, as in Morse code, the more frequently occurring characters would have the shortest encodings, while rarer characters used longer encodings. He gave the name 'varicode' to this encoding scheme.
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.
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.
The following amateur radio frequencies are commonly used for transmitting and receiving PSK31 signals.
| Frequency | Amateur Band |
|---|---|
| 1.838 MHz | 160 meters |
| 3.580 MHz | 80 meters |
| *7.040 MHz | 40 meters (regions 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.
* 7.035 MHz not allowed in Region 1 Bandplan is changed on 03.2009 (7.000 to 7040 is CW Only.) Digimodes is from 7040 to 7060. See on: http://www.iaru-r1.org/index.php?option=com_remository&Itemid=173&func=startdown&id=67
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