How to use it
Slow-scan television (SSTV) sends one still colour picture as a sound that lasts from half a minute to a few minutes. Radio amateurs use it on shortwave. Tune a receiver to an SSTV signal and bring its audio in as described under “Getting the audio in” below. When a transmission starts, the page reads its header, picks the mode and paints the picture line by line. Finished pictures go to the gallery.
No radio? Pick a mode and press Play demo: the page turns a test picture into SSTV tones and decodes them again. Live audio keeps being decoded while the tab is hidden, so you can watch the radio program instead; 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 20, 15 and 10 m; LSB on 80 and 40 m, where amateurs use the lower sideband below 10 MHz. FM for the space station. |
| Filter width | At least 2.7 kHz on shortwave. For the space station use a wide FM filter. |
| Tuning | Put the dial on the listed frequency, then fine-tune until the sync pulses sit on the 1200 Hz mark. The page corrects a mistuning of up to 150 Hz. |
| AGC / note | Medium AGC, noise reduction off. Keep the tuning still during a picture. |
Example frequencies
| Frequency | What | Receiver setting | When / note |
|---|---|---|---|
| 14.230 MHz | 20 m, the busiest calling frequency (image centre of activity in the IARU Region 1 band plan). | USB, 2.7 kHz or wider | Robot, Martin, Scottie and PD modes are common. |
| 21.340 and 28.680 MHz | 15 m and 10 m image centres of activity. | USB, 2.7 kHz or wider | Open only when the ionosphere supports these bands. |
| 7.165 MHz and 3.735 MHz | 40 m and 80 m image centres of activity. | LSB, 2.7 kHz or wider | In the USA, amateurs also use 7.171 MHz. |
| 145.800 MHz | International Space Station during ARISS SSTV events, usually PD120. | FM, wide filter | A few events a year, announced by ARISS. Doppler shift about ±3.5 kHz. |
| 437.550 MHz | Space station, UHF downlink, in Robot 36. | FM, wide filter | Frequency and mode differ by event. |
Segments and schedules change from time to time. ARISS announces each space-station event.
Controls
- Sources: computer audio, microphone (with an input chooser when there are several), an audio file, or the demo. Stop ends the current source and keeps the partial picture.
- Speed: real time plays the demo or file through the speakers; the faster settings decode silently.
- Start now: if you joined late and missed the header, choose the mode in the list beside the button and press it. When a header is read from the VIS code, the list switches to that mode, so the next Start now uses the last mode you received. Finish ends a picture early.
- Start on the VIS code: automatic start. Follow sync pulses: automatic line timing and slant correction.
- Slant and Shift: fine adjustment by hand. The picture is redrawn from the stored signal, so nothing is lost.
- Volume and Mute: for the demo and files only. Captured computer or microphone audio is never played back.
- Save PNG stores the picture at its native size; Clear empties the gallery.
How it works
SSTV is frequency modulation of an audio tone. Black is 1500 Hz, white is 2300 Hz, and a brightness value v from 0 to 255 becomes
Short bursts of 1200 Hz, below black, are the sync pulses that mark each line. A transmission opens with a calibration header: 300 ms of 1900 Hz, a 10 ms break at 1200 Hz and another 300 ms of 1900 Hz. Then comes the VIS code in 30 ms bits: a 1200 Hz start bit, seven data bits with the least significant first (1100 Hz is a one, 1300 Hz a zero), an even parity bit and a 1200 Hz stop bit. Code 44 means Martin M1, 60 Scottie S1, 8 Robot 36 and 95 PD120.
To measure the tone, the decoder multiplies the audio by a 1700 Hz complex oscillator, low-pass filters the result and looks at how far the phase turns from one sample to the next. That rotation is the instantaneous frequency. Averaging it over the time of one pixel gives the brightness. Martin and Scottie modes send green, blue and red for every line. Robot and PD modes send brightness plus two colour-difference signals, and Robot 36 and the PD modes share the colour between two neighbouring lines.
No two sound cards run at exactly the same rate. An error of 0.1 % moves the last line of a Martin M1 picture about 250 pixels sideways, which is why uncorrected pictures lean. The page fits a straight line through the arrival times of the sync pulses and uses it as its clock. Noise cannot shake single lines, and when the sync fades the timing simply runs on.
Supported modes: Martin M1 and M2, Scottie S1, S2 and DX, Robot 36 and Robot 72, PD50, PD90, PD120 and PD180.
Things to try
- Play the Robot 36 demo in real time and watch the waterfall: the dots at 1200 Hz are the line sync pulses.
- Switch off “Follow sync pulses”, move Slant to 0.3 % and see the finished picture lean. Then switch it on again.
- Compare Martin M1 with PD120 at the fastest speed. PD120 has four times as many pixels and takes only a little longer.
- Start a demo, wait ten seconds, press Finish and then Start now with the right mode. The picture locks on from the middle.
- Play the demo through a loudspeaker on another device and decode it with the microphone.
Frequently asked questions
Why is my picture slanted?
The sender’s and your sound card clocks differ slightly. Keep “Follow sync pulses” on, or correct the rest with the Slant slider.
Why are the colours wrong or the picture green?
Usually the receiver is off tune, so every tone is shifted, or the wrong mode was chosen by hand. The Tuning offset readout shows the measured shift; the decoder corrects up to 150 Hz.
Nothing starts. What should I check?
Look at the level readout and the waterfall. If there is a signal but the header was missed or damaged, choose the mode and press Start now.
Is my audio uploaded anywhere?
No. Everything is computed in this page, which works offline once it is loaded.