Tutorial SSTV

Tutorial: SSTV (slow-scan television)

What it is

SSTV sends still pictures over a radio channel, line by line, taking from eight seconds to a few minutes per picture. Amateurs use it on HF, and the International Space Station sends pictures on VHF during special events. OnAir decodes the common formats: Martin, Scottie, Robot and PD.

What you need

• An HF-capable receiver. An SDRplay or an Airspy HF+ tunes the HF bands directly. An RTL-SDR reaches HF only in direct-sampling mode (Radio settings, see Radio-RTL-SDR; the RTL-SDR Blog V4 uses its built-in up-converter). A HackRF or another radio that starts above the HF bands needs an up-converter such as Ham It Up or SpyVerter, which you pick in Radio settings so that the whole app shows the true frequency (see Supported-radios). The mode asks for 2 Msps and keeps working down to 250 ksps, so even a small radio is enough: the signals sit within about 3 kHz of the dial frequency. For the ISS on 145.800 MHz any VHF radio works, including an RTL-SDR.
• An HF antenna: a long wire of 10 m or more as far from household electronics as you can get it, a dipole cut for the band, or a magnetic loop for a noisy site. Keep the feed line away from switching power supplies and LED lighting, which are the usual source of S9 noise on a city balcony. For the ISS, a simple vertical or a turnstile antenna and a satellite pass prediction.
• Frequencies (USB dial on HF): SSTV 3.845, 7.171, 14.23 (the default), 21.34 and 28.68 MHz. The ISS SSTV downlink is 145.800 MHz in FM during events (in the database as the ISS FM entry; use narrow FM there).

Before you start

Lower the gain until the ADC level in the status line is well below full scale: strong broadcast stations next to an amateur band overload a direct-sampling RTL-SDR easily, and the result is spurs that look like signals. Use Auto-tune or Optimise gain (see Your-first-reception) rather than guessing. Choose upper sideband for all dial frequencies in the lists below; the decoders listen from the dial frequency upwards. A DC spike in the middle of the spectrum is normal for some radios and is not a signal: leave a few kHz of offset if you can. SSTV is easy to ruin with noise: a picture of 128 lines suffers from lightning crashes. Try a quiet moment.

Step by step

1. Connect the radio and antenna and start OnAir. Open the group Amateur: digital and choose Pictures (the entry reads Pictures, 14.230 MHz).
2. Press Start. The panel says waiting for a picture (VIS header, or the line sync pulses). OnAir recognises the VIS header at the start of a picture or, if that was missed, the line sync pulses.
3. When a picture starts, the panel shows its mode, VIS code, slant and tuning offset, and the picture builds line by line: %d / %d lines.
4. When it ends it says picture done, waiting for the next.
5. Press Save PNG to save the picture. Tick Save every picture to save automatically (the pictures go into a folder named sstv under the OnAir data folder; the status text says saved ...).
6. Press Clear to wipe the screen. Last pictures is a strip of recent pictures; click one to see it.

What you should see

A picture that appears from top to bottom. Colour formats (Martin, Scottie, PD) show colours; Robot has a different look. A slanted picture means the sound-card or radio clock is slightly off; OnAir shows the slant in ppm and corrects it. An incomplete picture is saved with (incomplete).

Reading the screen

The status text above the picture gives the mode, VIS code, slant and the tuning offset. Live is the picture being received. Last pictures is the gallery. The status line has sample rate and level.

Options and settings

• Save every picture (automatic).
• Save PNG, Clear, Last pictures.
• Band row: presets for the SSTV activity frequencies (the panel marks the busiest one and the ISS FM frequency 145.800 MHz).
• Listen (HF digital panel): hear the audio.

Troubleshooting

• No picture starts. Check that the dial is right (OnAir demodulates the sideband shown in the panel's hint, upper sideband) and that a transmission is on. Pictures are sent in bursts.
• Slanted or sheared picture. A clock error; OnAir estimates the slant, but a weak signal prevents it.
• Cut off at the bottom. The signal faded; the picture is saved as incomplete.
• Noisy stripes. Impulse noise. Raise the antenna or move it away from electronics.
• ISS picture is all noise. FM mode and the Doppler shift of a pass: tune in narrow FM and expect a drift.

Tips

• The ISS events (ARISS SSTV) are announced by amateur radio news; a pass lasts about ten minutes.
• Keep a recording (Tutorial-Record-IQ-then-the-Signal-workbench) of a good opening and replay it to try the settings.

Legal and privacy notes

Receiving amateur signals is allowed in most countries; the text is public by design. Transmit works in OnAir and has been used on a real radio by the maintainer and in loop-back tests; you need an amateur licence to transmit (see Transmit and Legal). Pictures can show faces and private images; do not publish pictures of private persons.

Under the hood

SSTV frequency-modulates an audio sub-carrier: 1500 Hz is black, 2300 Hz is white, 1200 Hz is the sync pulse, with a VIS header of 1900 Hz and 1200 Hz tones that identifies the format. Martin, Scottie, Robot and PD differ in line time and colour order. OnAir demodulates the audio, detects the sync pulses and builds the picture, with slant correction.

Related

Mode-SSTV, Tutorial-Weather-satellite-images, Tutorial-HF-digital-FT8-and-WSPR, Radio-RTL-SDR, Live-map, FAQ