How the Slow Scan Television Works

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PHOTO: MOTHER EARTH NEWS STAFF
The Robot Research Model 400 slow-scan/fast-scan scan-converter permits a bright, non-fading display of SSTV pictures on a conventional television monitor or modified TV set. In addition, the unit allows ""stop motion"" transmission of video images from a standard closed-circuit TV camera.

Copthorne Macdonald — the inventor of slow scan television — concludes the two part discussion of SSTV which began in MOTHER’s article, “Slow-Scan Television: Alternative TV.”

Alternative TV

Last time — you’ll recall — we talked at some length about slow-scan TV (SSTV for short), which we said was a means of converting a picture into sound frequencies, transmitting that “sound” via telephone, audio tape, or ham radio, and converting that same signal back into a picture on the receiving end.

We also mentioned the fact that because SSTV pictures are sent at the rate of one frame every eight seconds (in contrast to “regular” TV pictures, which are broadcast at 30 frames per second), the arriving SSTV image must be stored for viewing … either; In the afterglow of the special long-persistence phosphors used in SSTV viewing screens, or in the digital memory chips of a device called a scan-converter.

Basically, a scan-converter works like this: First, a standard 525-line, 30-frames-per-second TV signal is fed into the scan-converter unit. (The signal would normally originate from a small closed-circuit TV camera, but it could just as easily be a cable TV — or off-the-air broadcast TV — transmission, or the output of a video tape recorder.) Every eight seconds, the converter “snatches” a single frame from the stream of incoming video information and freezes that image in its digital memory. This stored picture; Is then slowly transmitted out over the next eight seconds as a slow-scan signal (which can — in turn — be sent via ham radio).

  • Published on Mar 1, 1977
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