"Jim Shaffer, Jr." ) writes:
On Sun, 04 Apr 2004 18:13:33 GMT, "Maximus" wrote:

OK, so what the heck is SSTV s ?

Slow-scan TV. It's basically a color version of a fax transmission. There's a
good free decoder program at http://www.qsl.net/mmhamsoft/mmsstv/


I wouldn't call it that.

The scheme is 45 years old. It's tv with the vertical and horizontal
frequencies slowed down, so the resulting video signal can fit into
the bandwidth used by a voice signal, The tradeoff is that the
scan rate is so slow that it takes seconds to transmit a picture,
something like 8 seconds originally. When it came into being circa
1958 by Copthorne Macdonald (I can't remember if he was the outright
inventor or if he made it practical based on theory from elsewhere. He
wrote some articles in 1958 or 1959 for QST, and that would be the original
source. He wrote a column for Mother Earth News in the seventies, and was
the SSTV columnist for CQ magazine in the early seventies), there weren't
computers to do the work, so the only way to display them was with long
persistence cathode ray tubes, the specific type surplus out of radar
screens I think. Without that long persistence, by the time the full
picture had been "painted" on the CRT the earlier portion would have faded.
It's absolutely no good for motion, because it take so long to scan
the original image.

I do know when surplus fax machines came along in the seventies, this is
old style fax machines, they were compatible or closely compatible with
SSTV, and I don't know if that was a deliberate part of the SSTV specs.

IN the seventies, as digital logic and memory became common and cheaper,
there was a drive away from special displays. So scan converters came
into being which would assemble the picture from the SSTV signal, and
then read it out at the fast rate that a common tv set expected. And
people moved to common TV cameras, again with the use of converters,
so it was a whole lot easier to set up, and regular video equipment is
a lot more common than SSTV equipment.

Of course, when computers came along, they could be used to do
that work, and I suspect virtually nobody uses dedicated equipment
at this point. I think there were color experiments in the seventies,
but they really made a splash once computers were common. It made
it a whole lot easier. But color is not what makes SSTV.

The standards may have change, I haven't kept track in recent years.

Michael