AMATEUR SLOW SCAN TELEVISION (SSTV)
by: Cletus Berkeley, 9Z4CLB
Thinking back on my early days as an electronics-engineering student and my fascination with communications. I recall participating in a science fair at the University of the West Indies under the banner of associate membership in TTARS. My project presentation was “Voice Transmission via a Modulated Light Beam” (lots of help at the time from my now deceased friend and mentor Dr. Winston Akong 9Y4WFA). I recall being completely blown-away by a Slow-Scan Television demonstration put on by Frank Brooker, 9Y4VU, with his home-brewed SSTV equipment.
In those days, slow scan receiving equipment was built around a surplus long persistence, radar cathode ray tube and a cocktail of tubes, transistors and the early 7400 series IC’s (not to mention the miles of hookup wire and occasional blue smoke that emitted from the selenium pile rectifiers). Indeed the display was monochromatic (with radar CRT’s, you had a choice of either green or orange phosphor screens) and the picture quality was pretty iffy at best, but it worked, we could transmit an image across the continents.
Today, all we need is a personal computer, some free (yes the best things in life are FREE) software and a few resistors and capacitors (add a relay and transistor if you really want to get fancy here) and you are all set. But wait, there’s a bonus (just when you thought it couldn’t get any better), today’s SSTV pictures are in full color!
So what is this SSTV thing? Well, the best way to explain slow scan TV is to have you imagine it as fax pictures but sent over the radio rather than the phone. The pictures are transmitted via tones (1200-2300 HZ) over the air. The frame rate is particularly slow (e.g. 110 seconds per frame in Scotty 1 mode) therefore only still pictures may be sent. You actually see the picture build on the screen line by line, top to bottom.
There are several simple ways to get setup for slow scan TV, the simplest of which is to use your computer and software with a hardware interface (I say again, the best software may be had for the outrageous price of FREE). There are interface circuits which work excellent and cost less than TT$100.00 to build a deluxe version if all parts are bought new or ZERO if you do like me build it from your junk box. Better yet, an interface can be built in less than an hour.
My experience with slow scan continues to be extreme fun. I've exchanged pictures with people in many different countries all over the world. The picture quality is dependent upon the computer, (monitor & graphics card), the software and band conditions of course. The software supports Hicolour which gives typical picture resolutions of 320 x 240 in 32,000 colors. These pictures are almost photographic quality and are extremely impressive.
Imagine being able to swap photos with other Amateurs all over the world and see who you're talking to.
Now if that’s not enough to get your adrenalin pumping, get in on the cutting edge technology with Digital High Definition SSTV. This mode is still very much in the experimental stages the available software is very raw and is actually being updated daily at the time of writing. The inner workings of Digital SSTV or HDSSTV is way beyond the scope of this article, however, basically, the picture is sent by tones, but there’s an encoding format that processes picture information into a WAV file that contains the audio signal corresponding to 8 DPSK modulated audio carriers and convolutional coding which is then transmitted as audio. At the receiving end, the WAV file is recorded followed by decoding using a very complex and CPU intensive algorithm and then converted back to the original file format (usually JPG). So what’s the big deal? The picture quality is astounding even under very high QRM and the worst of band conditions. Where a typical analog SSTV picture would be unrecognizable, the HDSSTV image is breathtaking. If that don’t increase your hart-rate, nothing will!
Note: You need a pretty fast CPU to efficiently decode HDSSTV (CPU’s approaching 2ghz is recommended).
For further reading on SSTV:
Just use your favorite search engine and search: sstv
There’s tons of information with all the downloadable freeware. I highly recommend MMSSTV Ver.: 1.09 software by Makoto Mori, JE3HHT http://www.qsl.net/mmhamsoft/
http://www.ultimatecharger.com/SSTV.html
Where to look on the bands for SSTV:
160m 80m 40m 20m 15m 10m 6m
1.916 3.845 7.170 14.230 21.340 28.680 50.680
3.857 7.172 14.233 28.690
14.236 28.700
For further reading on Digital SSTV:
Use your favorite search engine and search: digital sstv or hdsstv
These are "Must See" sites::
http://www.digisstv.net.tc/
http://www.svs.net/wyman/examples/hdsstv/
Where to look on the bands for Digital HDSSTV:
40m 20m 17m 15m 10m
7.228 14.233 18.1625 21.340 28.550 (FM)
TIME: 1730z 2100z 1330z
2100z
Don't be afraid to break-in on frequency and ask for info. The SSTV hams are very willing to help other interested parties or help you get started in slow scan. Hope to see you on the air.
About 10% of my HF DX work is voice, the rest is split between SSTV and PSK31 …am I hooked?
Cletus, 9Z4CLB