An analogue method of transmitting still pictures via amateur
radio. (TV proper is what on the amateur bands is called FSTV.) Each
scanline of the image is converted into an audio tone of varying
pitch, followed by a sync pulse to keep the scanlines lined up at the
receiving end. There are many different protocols, most of them
incompatible, but the most common one, Scottie, encodes varying
shades of gray on a scale where a low pitch means black, and a high
pitch means white. Colour transmissions are usually achieved by
sending the same scanline three times, one with the red portion, one
with the green, and one with the blue.
Nowadays, computer software is by far the most common means of sending
and receiving SSTV, but before the advent of cheap, good sound cards,
people had to use specialized electronics, including cameras and
so-called slow scan monitors that would stay illuminated for a long
time after the cathode ray had swept over it. The HF bands don't
have a lot of available bandwidth, so it takes a couple of minutes
to transmit a single image. Radio amateurs who were active on SSTV in
elder days often made a habit of turning off the
lights in the room so that the image would be visible on the screen a
little longer before it disappeared forever.
Sometimes, radio amateurs have entire QSOs entirely on SSTV. People
use paint software to write "CQ CQ de AB1XYZ PSE K" on top of images, or neutral
backgrounds. Sometimes they are answered by another image, saying things
such as "AB1XYZ de AB1UVW TNX CALL UR 579".
Hams also use SSTV to spice up or illustrate conversations they're
having on SSB (telephony).
Images that amateurs send on SSTV range from pictures of themselves and
their dog, their radio equipment, and the impressive antennas at the
nearby MW transmitter site, to outright pornography. That last
part is one of the reasons that some people criticize SSTV operators of
being lids -- ie. people who make a habit of not following proper
etiquette on the air.