Slow-scan television
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Slow-scan television (SSTV) is used by amateur radio operators, to transmit and receive static pictures in monochrome (black & white) or colour.
SSTV was used extensively during the early years of the NASA Apollo program to transmit images to Earth, and the first images from Apollo 11 on the Moon were SSTV. (See also Apollo moon landing hoax accusations.)
Broadcast television requires huge 6 MHz wide channels, because it transmits thirty full pictures per second (in the NTSC system). SSTV is less greedy, usually taking up only 3 kHz bandwidth. SSTV can transmit data much more slowly because it sends still pictures, usually one every ten to thirty seconds. The technical term for this is narrowband.
SSTV originally required quite a bit of specialized equipment. 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 a minute or so.
The modem would generate audio from pictures, and pictures from audio. The audio would be attached to a radio receiver and transmitter.
The modern system uses a personal computer and special software in place of much of the custom equipment. The audio system of a PC, with special processing software, acts as a modem. The computer screen provides the output. A small digital camera or digital photos provide the input.
Since SSTV produces and generates audio, amateurs use it on shortwave, VHF and UHF radio.
References
- Glidden, Ramon (September 1997). "Getting Started With Slow Scan Televion." (http://www.arrl.org/tis/info/pdf/99753.pdf) QST. Accessed Apr. 28, 2005.
- "Slow scan definition." (http://cancerweb.ncl.ac.uk/cgi-bin/omd?slow+scan) On-line Medical Dictionary. Accessed Apr. 28, 2005.
- Turner, Jeremy (December 2003). "07: Interview With Tav Falco About Early Telematic Art at Televista in Memphis, New Center for Art Activities in New York and Open Space Gallery in Victoria, Canada." (http://www.openspace.ca/outerspace/TavFalcoInterview2003.html) Outer Space: The Past, Present and Future of Telematic Art. Accessed Apr. 28, 2005.