NEW DIRECTIONS radio
(Page 2 of 2)
January/February 1977
By Copthorne MacDonald
Since ham radio operators can avail themselves of long-range short-wave frequencies for their voice transmissions, they're able?by coupling this strange new "sound" (SSTV signals) into their transmitters to send a picture wherever their voices will travel. (A ham station's regular single side band voice transmitter and receiver can be used to generate and receive the on-air signals.)
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Of course, some way must be found (at the receiving end) to store each incoming SSTV picture as it arrives, bit by bit, over the eight?second period during which it is received. This storage is normally provided by a special TV picture tube made with a type-P7 long-persistence phosphor. What you see when you watch such a screen is a bright, horizontal "writing line" that moves across the picture and works its way slowly from top to bottom of the screen over an eight-second interval. The visual image, then, is "stored" right in the tube's yellow phosphorescent afterglow . . . much like the images on a radar screen. (And, since the phosphors in the picture tube continue to glow for about eight seconds, a new image is always wiped onto the screen just as the old one fades out.)
One further point: Because the "glow in the dark" material in a picture tube isn't particularly bright, SSTV normally must be viewed in a dimly lit room or through a viewing hood.
The boom in digital computer technology has recently brought beneficial developments to SSTV. The cost of digital memory chips, for instance, has fallen to the point where it's now feasible (economically and technically) to use such chips to provide slow-scan picture storage. We'll talk about devices (scan converters) that employ digital memory to store SSTV pictures in the next installment of this column.
NDR SCHEDULES
From now on, we will be relying on The New Directions Roundtable Newsletter?put out by Randy Brink (WA7BKR) and Burton Bogardus (W6HSE)?for current on?the?air schedule information. (Space no longer permits me to print that info here.) To get on the mailing list, send stamps?one for each issue of the newsletter you'd like to receive?to Randy at 1174 S.E. Fern Rd., Port Orchard, Wash. 98366. (While you're at it, you might send along a small donation, too.)
Peace,
Cop Macdonald (VE1 BFL)
99 Fitzroy St.
Charlottetown
Prince Edward Island
Canada C1A 1R6
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