PSK31 or “Phase Shift Key­ing, 31 Baud” is a pop­u­lar com­put­er-gen­er­at­ed Radio Tele­type mode, used pri­mar­i­ly by ama­teur radio oper­a­tors to con­duct real-time key­board-to-key­board infor­mal text chat over the air.

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.

PSK31 is cre­at­ed by switch­ing the polar­i­ty of the sig­nal used to key the com­put­er’s sound card, result­ing in both ampli­tude and phase mod­u­la­tion of the car­ri­er. 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. The 180 degree phase shift for a “zero” bit code occurs at an ampli­tude null.

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 100 Hz 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. They nor­mal­ly occu­py the low­er edge of each band’s ‘digi­modes’ sec­tion. PSK31 Oper­a­tors gen­er­al­ly use Upper Side­band (USB) even on fre­quen­cies below 10 MHz where the nor­mal con­ven­tion requires Low­er Side­band. This is because (a) sig­nals then spread upwards into the digi­mode sec­tion from the ‘base’ fre­quen­cy, and (b) using QPSK requires both sta­tions to use the same side­band.

PSK31 Fre­quen­cies
Fre­quen­cy Ama­teur Band
1.838 MHz 160 meters
3.580 MHz 80 meters
7.040 MHz 40 meters (region 1)
7.035 MHz 40 meters (regions 2,3)
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

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