Signal guide

Slow-scan TV (SSTV)

Still pictures sent as sound, one line at a time. A picture takes from about half a minute to a few minutes.

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Slow-scan TV sounds like a warbling, whistling tone that sweeps up and down in pitch, punctuated by a regular tick about two to seven times a second, and runs from about half a minute to a few minutes.

What does it sound like?

Plate 59Picture

A steady, sweeping, whistling sound with a regular rhythm, starting with a short musical tone sequence.

0 Hz1,0002,0003,000 Hz

12-second example. Slow-scan TV made by Cohera in Robot 36, the format the Space Station uses: the start tones, then the picture line by line.

Field marksA steady ticking sync pulse between linesTones sweeping about 1,500-2,300 HzA picture takes from 36 s to a few minutes

Hear it, step by step

This slow-scan TV clip is the first 12 seconds of a Robot 36 picture: the header, then the top of a practice picture, line by line.

0 Hz1,0002,0003,000 Hz

Tap a step to hear that moment. The box shows where it is on the waterfall.

0.0 sThe opening leader tonesA steady 1,900 Hz tone for 0.3 seconds, a 10-millisecond blip at 1,200 Hz, then another 0.3 seconds at 1,900 Hz. This fixed opening starts the header that comes before the picture.0.6 sA code names the modeNext come ten 30-millisecond steps; the data steps sit at 1,300 Hz for a 0 and 1,100 Hz for a 1. Here they spell the number 8, the code for Robot 36, so the receiver knows what format follows.0.9 sA tick starts every lineFrom 0.91 seconds, each picture line begins with a short pulse at 1,200 Hz, lower than black. The pulses come every 0.15 seconds, which is 400 lines a minute, the Robot 36 rate.0.9 sDark sky, low pitchPitch is brightness: 1,500 Hz is black and 2,300 Hz is white. The top of this picture is dark blue sky, so each line's brightness part sits near 1,590 Hz and creeps up to about 1,650 Hz as the sky lightens.4.8 sWhite letters, high pitchHere the white word PRACTICE is being sent, and parts of each line leap to about 2,300 Hz, the white end of the scale. The word PICTURE starts the same way at 9.3 seconds.

Make your own: type a message and hear it as Slow-scan TV (SSTV) in the Signal Maker.

Who uses it?

Amateur radio operators, and special events such as picture broadcasts from the International Space Station.

Where and when can I hear it?

Around 14.230 MHz on shortwave. Space station picture events: 437.550 MHz in 2026 (earlier events used 145.800 MHz); dates are announced on ariss.org.

The science

SSTV sends a picture as an audio tone whose pitch sets the brightness: 1,500 Hz means black and 2,300 Hz means white, and short 1,200 Hz pulses, blacker than black, mark the start of each line. Color modes send each line in pieces; the popular Martin M1 mode sends green, blue and red in turn after a 4.862-millisecond sync pulse and takes about 114 seconds for a 320 by 256 picture. A short digital header at the start (the VIS code) tells the receiver which mode is coming.

Why use it?

Because the whole signal fits in a voice channel, SSTV works with an ordinary SSB or FM receiver, and space station events can be received with a simple handheld radio or scanner that covers the event frequency, fed into free decoding software.

Lore

Copthorne Macdonald, then a University of Kentucky student, began developing SSTV in 1957; the first image crossed the Atlantic on December 20, 1959, and the FCC formally authorized SSTV in 1968. Space station events have used the PD120 mode on 145.800 MHz, and ARISS lists 2026 events in Robot 36 on 437.550 MHz.

What do the decoded parts mean?

• Image: The picture, built up line by line.
• Mode: The format used (for example Scottie 1, Martin 1, Robot 36, PD120), which sets the size, colors and speed.

Easily confused with

• Weather fax (radiofax): fax is slower and black and white
• Weather satellite pictures (APT): APT was a steady tick-tock at 137 MHz (it ended in 2025)

How to recognize it

An SSTV picture opens with a short, recognizable header: two steady tones with a brief break between them, followed by a quick burst that tells the receiver which format is coming. After that the picture itself is sent as a continuous tone whose pitch follows the brightness of the image, from about 1500 Hz for black to 2300 Hz for white.

Between image lines (every line in most formats, every other line in the PD formats) you hear a short, lower sync pulse at 1200 Hz. That repeating pulse gives SSTV its steady "tick... tick..." rhythm, and the pitch in between sounds busier over detailed parts of the picture and smoother over plain sky.

Easily confused with:

• Weather fax: a similar line-by-line scan, but slower and harsher, about two lines per second for many minutes, and with no musical header.
• NOAA weather satellites (APT): a steady "tick-tock" of two lines per second, heard during satellite passes on 137 MHz until NOAA switched off the last APT satellite in August 2025.
• Fax-like data modes: steady rhythms with no pitch sweeping to the picture content.

Where and when to hear it

On shortwave, the classic meeting place is 14.230 MHz (upper sideband), with 14.233 as an overflow; you will also find SSTV around 7.171, 3.845, 21.340 and 28.680 MHz. Weekends are busiest. Operators often send picture replies, test cards or their station photo.

The International Space Station runs occasional SSTV events through the ARISS program, sending a themed series of pictures for several days. These are announced ahead of time on ariss.org, and all you need to receive them is a simple VHF or UHF receiver and patience for the station's passes overhead.

A little history

Copthorne Macdonald developed slow-scan television in the late 1950s as a way to send pictures through a voice-sized radio channel. Early stations used long-persistence radar screens to view each image as it slowly built up. In the 1970s, scan converters replaced those screens, color pictures became practical in the 1980s, and from the 1990s personal computers with sound cards replaced most dedicated hardware.

Today dozens of formats exist. The most common include Martin 1 and Scottie 1 (each just under two minutes for a color picture), Robot 36 (36 seconds) and PD120 (about two minutes), which space station events used through 2025; the 2026 events use Robot 36.

Common questions

Sources

• ARISS SSTV events
• ARRL: SSTV (slow-scan television)
• MMSSTV
• SSTV Handbook ch. 2: Slow-scan television (M. Bruchanov)
• SSTV Handbook ch. 4: Formats of SSTV transmission
• ARISS: Upcoming SSTV events
• ARRL: ARISS announces SSTV event (2019)
• SSTV Handbook ch. 3: Modern SSTV features (VIS code)
• IARU Region 1 HF band plan (effective October 16, 2020)
• ARRL band plan