It is a much slower method of still picture transmission, usually taking from about eight seconds to a couple of minutes, depending on the mode used, to transmit one image frame.

Vostok was based on an earlier videophone project which used two cameras, with persistent LI-23 iconoscope tubes.

A similar concept, also named SSTV, was used on Faith 7,[4] as well as on the early years of the NASA Apollo program.

[5] Commercial systems started appearing in the United States in 1970, after the FCC had legalized the use of SSTV for advanced level amateur radio operators in 1968.

Usually there was a scanner or camera, a modem to create and receive the characteristic audio howl, and a cathode-ray tube from a surplus radar set.

The special cathode-ray tube would have "long persistence" phosphors that would keep a picture visible for about ten seconds.

A modern system, having gained ground since the early 1990s, uses a personal computer and special software in place of much of the custom equipment.

There are a number of different modes of transmission, but the most common ones are Martin M1 (popular in Europe) and Scottie S1 (used mostly in the USA).

The Scottie and Martin modes were originally implemented as ROM enhancements for the Robot Research Corporation SSTV unit.

AVT is an abbreviation of "Amiga Video Transceiver", software and hardware modem originally developed by "Black Belt Systems" (USA) around 1990 for the Amiga home computer popular all over the world before the IBM PC family gained sufficient audio quality with the help of special sound cards.

Full frame images can be reconstructed with reduced resolution even if as much as 1/2 of the received signal was lost in a solid block of interference or fade because of the interlace feature.

This is a significant visual improvement over losing a non-recoverable contiguous block of lines in a non-interlaced transmission mode.

Using a receiver capable of demodulating single-sideband modulation, SSTV transmissions can be heard on the following frequencies: In Valve's 2007 video game Portal, there was an internet update of the program files on 3 March 2010.

The hidden signals became part of an ARG-style analysis by fans of the game hinting at a sequel of the game – some sounds were of Morse code strings that implied the restarting of a computer system, while others could be decoded as purposefully low-quality SSTV images.

According to a hidden commentary node SSTV image from Portal 2, the BBS is running from a Linux-based computer and is linked to a 2,400 bit/s modem from 1987.

These 3 images are slides with bullet points on how the ARG was done, and what the outcome was, such as how long it took the combined internet to solve the puzzle (the average completion time was 71⁄2 hours).

[13] In another video game, Kerbal Space Program, there is a small hill in the southern hemisphere on the planet "Duna", which transmits a color SSTV image in Robot 24 format.

[citation needed] As of the latest version of the game (1.12), the hill no longer transmits the signal.