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Getting Started in SSTV
by Sumner, W1VIV
[email protected]
Amateur Radio Slow-scan television (SSTV) and I go way back. This mode has always fascinated me, because, instead of a 4.5MHz video bandwidth to send good pictures, all you need is a 2KHz audio bandwidth! I built my first SSTV monitor about 25 years ago. Instead of building it from scratch, I modified an old vacuum tube Eico 5 inch oscilloscope. That way, I already had the low voltage and high voltage power supplies, horizontal and vertical sweep circuits, and a chassis to mount everything on, saving a lot of work. I built up a solid state circuit board with all the decoding circuits on it, and replaced the original CRT which had a P1 phosphor with a surplus unit with P11 phosphor, a very long persistence. This came from an old radar PPI scope. As I recall, the old SSTV mode had a horizontal sweep rate of 8 seconds, and the long persistence was needed to keep the beginning of the picture from fading while the system was still writing the end of the picture. In a slightly darkened room, you could see pretty good picture. That is, if you didn't mind the fact that the entire picture was amber or green, depending on the phosphor you happened to have in your CRT.
Today, everything has changed for the better. SSTV is now all computer based, and all you need is a small amount of hardware and a decent amount of software. The hardware is an interface module to connect the audio output of your transceiver to the serial (RS-232) port of your computer. I built a clipper circuit using a common 741C op amp, which cleans up and squares the received pulses. Everything else is software!
The software I now use was written by Ben Vester, K3BC. I had been collecting his entire series of articles in QST, knowing that some day I would use them, starting with January 1994. There were more updates in June and December 1994, and July 1996. Ben is a nice guy, and has provided a lot of assistance and additional software and pictures to me by email. His next-to-latest version, rev. l, (that's lower case L, not One) is available on the Internet. You can download it from the University of Oakland, which mirrors the ARRL BBS, at http://oak.oakland.edu:8080/pub/hamradio/arrl/BBS/programs/. (There's
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lots of other great stuff there too.) His program is called Vester_1.zip, and is actually written in good old GWBASIC. Why not QBASIC? Quoting the author, "QBASIC is larded up with pull-down menus, etc, and uses 195kB of low memory, where GWBASIC occupies only 80 kB. New computers with higher speeds and large amounts of RAM change the tradeoffs somewhat, but there are still a lot of older machines around being used by hams." I like it because I find it easy to make changes and experiment with the program using GWBASIC. I'm not a software person, and it's the only programming language I ever learned. I'm rusty, but it's coming back.
K3BC has mailed me rev. m, which adds the capability of superimposing your call letters or anything else you wish to write on any picture. Both revs also have the capability to convert TGA, JPEG, PCX, dnd GIF, and TIF files.
Now the fun begins! Just tune to 14.230 MHZ, where all the slow-scanners hang out. (The hams aren't slow, only their scan rates!) Most of the time, you'll hear a musical, chirping signal. A voice will break in and say, "I'll send the next picture in Scottie 1" (or Martin 1, the two most popular modes, or occasionally others.) Then you'll hear more chirping for a minute or two. Other hams in the round table will comment on the received picture, and then proceed to send their own. In the early evening, I often turn the beam toward Europe and receive lots of nice pictures, all in the Martin 1 mode. If you have a good VGA display, the pictures are in beautiful color with excellent resolution. They can be saved if you wish to look at them later or retransmit them.
What about going on the air with my own pictures? I haven't gotten around to that yet, but will do so this fall and winter. There's not much time for ham radio in the summer, when there are too many outdoor activities. I simply have to add a transformer to my circuit board, and all the rest is already in place.
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SSTV is a fascinating mode that can bring lots of enjoyment to ham radio. I'm having fun with it, and will be getting further into it in the future.
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