SSTV
| SSTV (slow-scan television) | |
|---|---|
| Mode type | Analog still images sent as audio tones |
| Bandwidth | Fits within an ordinary voice channel, under 3 kHz |
| Picture time | From about 36 seconds to several minutes, depending on mode |
| Popular modes | Scottie, Martin, Robot 36, PD |
| On the air | |
| Main HF frequency | 14.230 MHz (20 m), upper sideband |
| Other frequencies | 21.340 MHz (15 m), 28.680 MHz (10 m) |
| From space | ARISS SSTV events from the ISS on 437.550 MHz FM (2026; earlier on 145.800 MHz) |
| Software | MMSSTV, QSSTV, phone apps such as Robot36 |
SSTV, or slow-scan television, is a way of sending still pictures over an ordinary voice radio channel. The picture is converted into a sequence of audio tones, sent through a single sideband or FM transmitter, and rebuilt line by line by the receiving station, a process that takes from about half a minute to a few minutes per image. Because it needs nothing more than a voice transceiver and a computer or smartphone, SSTV is one of the easiest ways to send images by radio, and pictures received from the International Space Station have introduced many newcomers to amateur radio.
How it works
An SSTV transmission scans the image one line at a time. The brightness along each line is sent as an audio tone that varies in frequency, from 1,500 Hz for black to 2,300 Hz for white, and a short pulse at 1,200 Hz marks the start of each line. Color images are sent as separate scans of each color component. Because the information is carried by frequency rather than by signal strength, a moderately noisy picture is still recognizable, though fading and interference show up as streaks and smudges.
Each transmission begins with a short digital header called the VIS code, which identifies the mode so that the receiving software can switch to it automatically. If the start of a picture is missed, most programs can still pick up and display the rest, sometimes slanted until the timing is corrected.
SSTV was developed in the late 1950s by Copthorne Macdonald, and early systems sent small black-and-white pictures, a few seconds each, viewed on long-persistence radar-style display tubes. Dedicated scan converters later added color, and today the job is done by computer software.
Modes
Many SSTV modes exist, and both stations must use the same one. A few dominate:
| Mode | Approximate time per picture | Notes |
|---|---|---|
| Robot 36 | 36 seconds | Quick color mode, lower resolution; used by ARISS for ISS SSTV events since 2026 |
| Scottie 1 | 110 seconds | The most common mode in North America |
| Martin 1 | 114 seconds | Popular in Europe |
| PD120 | 126 seconds | Used by ARISS for ISS images until 2025 |
Slower modes give sharper pictures and better color; faster modes tolerate less noise. There are also digital SSTV programs, which send an image file with error correction, so a picture is received either perfectly or not at all.
On the air
On HF, SSTV is an image mode, permitted in the US in the same segments as voice. The busiest frequency is 14.230 MHz on the 20-meter band, with activity spreading a few kilohertz either side, and SSTV is also heard around 21.340 MHz on 15 meters and 28.680 MHz on 10 meters. HF SSTV is sent on upper sideband.
Operators often coordinate by voice on the same frequency, taking turns to send pictures. A typical exchange is an image showing the sender's call sign, name and location, often over a photo of the station or the scenery, followed by a reply that includes a signal report or the received picture sent back. Stations must still identify as usual; showing the call sign in the picture is customary, but US rules accept identification by image only for fast-scan television that meets broadcast standards, so SSTV operators in the US also give their call by voice or CW.
An SSTV picture holds the transmitter at full output for a minute or two, unlike speech, so many operators reduce power well below the radio's rating to keep it cool.
Images from the ISS
From time to time, ARISS (Amateur Radio on the International Space Station) runs SSTV events, in which the space station transmits a series of commemorative images. In 2026 these events used Robot 36 mode on 437.550 MHz FM; earlier events used PD120 on 145.800 MHz. Because the 437 MHz signal shifts with Doppler by up to about 10 kHz, listeners may need to retune during a pass. The frequency and mode can change from one event to the next, so check the ARISS SSTV events page beforehand. The signal is strong enough to be received on a handheld with a simple antenna, or a scanner, with the audio fed to a computer or held next to a phone running an SSTV app. Receiving is open to anyone, licensed or not, and ARISS often offers certificates to those who submit pictures.
Software
Popular programs include MMSSTV for Windows, by Makoto Mori JE3HHT, and QSSTV for Linux. Phone apps, such as Robot36 for Android, decode SSTV from a phone's microphone, which makes receiving possible with no cables at all. Transmitting needs an interface between the radio and the computer, as for other sound-card modes such as FT8.