How to use it
PSK31 is a keyboard-to-keyboard text mode used by radio amateurs. On the air it sounds like a thin, slightly warbling whistle. This page listens to the audio of your receiver and prints the text. To try it without a radio, press Play demo signal.
Each station shows up in the waterfall as a narrow bright trace. Click the trace and the text appears below. Receiver settings and example frequencies follow in the next section.
Decoding continues while this tab is in the background, since you will usually be looking at the radio program. Only the drawing pauses.
Where to find it and how to tune
Getting the audio in
There are four ways to get the signal in. Nothing is uploaded: all decoding happens in this tab.
- No radio needed — a web receiver in another tab. Open a public web-based SDR receiver in a second tab (universities and radio clubs run many of them), tune it as described below and start its audio. Here, press Use computer audio, choose that tab in the sharing dialog and switch on sharing of the tab’s audio. Keep the receiver tab playing; this page can stay in the background.
- Your own receiver or SDR program on this computer. Press Use computer audio and share the program’s window or the entire screen together with system audio, or send the program’s output to a virtual audio cable and pick that cable after pressing Use microphone.
- A radio with a speaker. Hold it near the microphone and press Use microphone. It works, but room noise hurts weak signals; a cable from the headphone socket to the line or microphone input is better.
- A recording. Load a WAV or MP3 file with Open audio file.
Receiver setup
| Setting | Value |
|---|---|
| Mode | USB on every band, even below 10 MHz where voice uses LSB. This page decodes either sideband. |
| Filter width | The normal voice filter, 2.4–3 kHz. A PSK31 signal is only about 31 Hz wide, so many fit side by side. |
| Tuning | Put the dial on the listed frequency. A pitch of 1000–1500 Hz works well; click a trace in the waterfall to decode it. |
| AGC / note | Medium AGC. Switch off noise reduction and other audio effects. |
Example frequencies
| Frequency | What | Receiver setting | When / note |
|---|---|---|---|
| 14.070 MHz | 20 m, the best-known PSK31 frequency (digital segment 14.070–14.089 MHz). | USB | Beacons start at 14.099 MHz. |
| 3.580 MHz | 80 m, narrow-band digital segment 3.580–3.600 MHz. | USB | Voice uses LSB here; PSK31 stays on USB. |
| 7.040 MHz | 40 m, narrow-band digital segment 7.040–7.050 MHz. | USB | In the Americas the 40 m frequency is 7.070 MHz. |
| 10.140–10.142 MHz | 30 m. | USB | Narrow band: digital modes and Morse only, no voice. |
| 21.070–21.080 MHz | 15 m, narrow-band digital segment. | USB | Depends on the ionosphere. |
| 28.120 MHz | 10 m, narrow-band digital segment 28.120–28.150 MHz. | USB | Depends on the ionosphere. |
Segments and schedules change from time to time. Check the current IARU band plan for the digital segments.
Controls
- Waterfall: click, tap or drag to place the carrier marker. A click snaps to the centre of a nearby signal.
- Keyboard: focus the waterfall; left and right arrows move 1 Hz, up and down (or Shift with left and right) move 10 Hz, Page Up and Page Down 50 Hz.
- Auto tune: jumps to the strongest signal in the audio band.
- AFC: automatic frequency control follows the carrier when it is a few hertz off or drifts slowly.
- Squelch: text is printed only while the signal quality is above this level, so plain noise prints nothing.
- Text: switch Auto-scroll off to read back; Copy, Save .txt and Clear act on everything decoded so far.
How it works
PSK31 was introduced by Peter Martinez, G3PLX, in December 1998. It sends 31.25 symbols per second on a single audio tone. The information is not in the tone’s pitch or loudness but in its phase: at each symbol boundary the carrier either keeps its phase, which means a 1, or flips by 180°, which means a 0. During a flip the amplitude fades to zero and back with a cosine shape, which keeps the signal only a few tens of hertz wide. BPSK63 is the same signal at twice the speed and twice the width.
The decoder multiplies the audio x by a complex oscillator at the carrier frequency f, which shifts the chosen signal down to 0 Hz. A low-pass filter matched to the cosine pulse shape removes everything else, including neighbours 100 Hz away. The rhythm of the amplitude dips tells the decoder where each symbol is centred. It then compares each symbol zk with the previous one: the product dk points one way when the phase stayed and the opposite way when it flipped, so the absolute phase never has to be known. A tilted product means the tuning is slightly off, and AFC corrects the oscillator, up to about a quarter of the symbol rate (close to 8 Hz for PSK31).
Bits are turned into characters with Varicode. Frequent characters have short codes (a space is 1, “e” is 11), rare ones have up to ten bits, and no code contains two zeros in a row. Two zeros therefore mark the gap between characters. An operator who is not typing sends only zeros, a continuous run of reversals, which lights the Idle lamp instead of printing anything.
Things to try
- Play the demo, switch AFC off and detune a few hertz with the arrow keys. The phase scope tilts, and at about 8 Hz off the text turns to garbage.
- Add the second signal and click each trace in turn: two transmissions 100 Hz apart, decoded separately.
- Choose heavy noise. The tone is hard to hear, yet most of the text still comes through.
- Switch the mode to BPSK63 and compare the width of the trace.
- Set the waterfall span to 500 Hz and look at the idle signal: it splits into two lines 31 Hz apart.
Frequently asked questions
I see a trace but only garbage or nothing is printed. Why?
Check that the red marker sits in the middle of the trace and that the mode matches. If the quality stays low, the signal may be another mode: RTTY, for example, shows two separate lines about 170 Hz apart.
Does it matter whether I use USB or LSB?
Not for this decoder. A two-phase signal looks the same mirrored, so both sidebands decode. USB is the convention, and the four-phase QPSK31 variant, which this page does not decode, does depend on it.
What does the S/N number mean?
It is estimated from the scatter of the phase points around their two ideal positions, within the narrow bandwidth of the signal itself. It is an indication, not a calibrated measurement.
Is my audio uploaded anywhere?
No. All processing runs in this page on your device, and the page works offline once loaded.