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.
| 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 |