Slow-scan television (SSTV) sends a still picture as audio tones, one line at a time, in tens of seconds to a few minutes. Because the picture is just sound, it fits through an ordinary SSB voice channel on HF, or an FM voice channel on VHF, with no special transmitter.
From picture to tones
The sending software scans the picture in rows. For each pixel it picks a tone whose pitch is the brightness: low tone for dark, high tone for bright. A short sync pulse of a different pitch starts each row, so the receiver knows when a new line begins.
Each pixel in the chosen row becomes one steady tone. The receiver reverses the mapping and paints the row.
Highest tone in this row2300Hz
The receiver listens, measures the pitch, and paints the pixel. Brightness is carried by frequency, not by volume, which is why SSTV survives fading and level changes better than you might expect.
Colour and the start of a picture
For colour, each row is sent as several scans, one per colour, one after another, and the receiver recombines them. A header of tones called the VIS code comes before the first line and tells the software which SSTV mode follows, so it configures itself.
Frequency, not amplitude, carries brightness. Specific tones mark the line start.
Modes, time and quality
An SSTV mode is a recipe for how many lines, how long per line and in what order the colours go. Families such as Robot, Martin, Scottie and PD exist, and they trade time for quality: a longer mode usually gives a sharper picture. A faster mode is easier to fit into a short fading-limited contact but looks rougher. This is the same trade as for all television, see Fast-scan TV.
On the air
It runs on voice frequencies, generally in the phone segments of the band plan. Check Band plans.
Software on a computer or phone listens through a sound-card connection or even through the speaker, and the sending side feeds audio into the microphone input.
Keep the audio level modest. Overdriving a SSB transmitter distorts the tones and widens the signal; see Mic gain, compression and ALC.
A picture that leans or looks sheared often means the sender's and receiver's sound-card clocks differ slightly. Most programs have a slant correction.
Digital SSTV modes also exist. They too produce audio, so an SSB receiver can carry them.