DIY





How the Slow Scan Television Works

Continue to learn about Copthorne MacDonald's slow scan television.

| March/April 1977

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).



In its receiving mode, the scan-converter "writes" incoming slow-scan pictures into digital storage during the eight seconds it takes each frame to arrive, then reads this picture information out of memory non-destructively at 30 frames per second into an ordinary TV monitor, where it's displayed as a bright, non-fading picture for as long as one cares to view it. (Scan-converters also have "continuous update" capability, wherein each newly arriving image replaces the old one line by line. Here again, the picture you see is as bright as anything you're likely to get on your home TV, and doesn't fade or flicker.)

Amateur radio buffs who want their own scan-converters have two options: build or buy. George Steber (WB9LVI) — one of those who've chosen to go the first route — has designed a unit that performs well and that has been copied successfully by many other hams. To help other folks get Into SSTV, a group headed by Ed Arvonio (W3LY) Is making available at cost some of the key components used in the construction of George's converter. Ed can supply a set of the 15 bare printed circuit boards you'll need, plus about 50 sheets of construction information ... and for an extra cost, you can have a set of the 76 required Integrated-circuit memory chips. All other parts, however, must be rounded up by the builder him/her self.






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