Moderators: Dave Moll, Andrew Davie, Steve Anderson
Steve Anderson wrote:I'm astounded by the effort quite a number have put into this thread.
For years I have had mechanical SSTV on a back-burner, though only in my head. Harry (especially), well done. Other contributors also muchly thanked.
I had always intended to attack this from the 'camera' end first, but my thoughts and scribbles on paper have come to nothing more than just that.
With my impending return to the UK about a week away and my workshop effectively mothballed I'm not going to able to add much here.
Thanks again to those that can.
Interested? You bet!
Steve A.
Steve Anderson wrote:I must admit I had not considered a mechanical SSTV monitor because as you say, the P7 phosphor was the answer...pre computer age.
At that time I had no way of generating a 'real' picture, either done live or some form of scanning a photo...much like a fax machine. Simple patterns like bars or grille was just a few power-guzzling (7400 original type) logic chips. But that's not a picture to me.
I did a lot of sketches and spent hours/days on this and nothing ever came of it. I was still a student, therefore impoverished* and still had the chore of studying to do.
Late last year I started working on something akin to this but it wouldn't have generated a true waveform, either SSTV or NBTV on it's own. It still required some number-chunching by a small micro, though not a PC.
It was based on a standard 33RPM turntable...yes I still have one! This 33RPM provided the horizontal scan. Only about 30-40 degrees would have been used as the field of view. So most of the time during rotation would have been wasted.
The vertical scan was to be provided by some means of 'nodding' the sensor assembly slowly in one direction then a quicker retrace. This was to be for conventional top-left, bottom right scanning...as per SSTV.
Where I fell foul was the sensor assembly. I wanted to do this without any glassware of the optical vatiety, that is a pin-hole arrangement.
This is where I spent most of my effort and quite simply although I got a video waveform it was far too noisey. A PMT could well have done the job but as this whole thing was designed to sit on a rotating turntable with no slip-rings for signal or power that idea was shelved. Semis and on-board batteries it had to be. Signal out was by a vertical on-axis IR LED/photodetector link.
I'm sure it can be done but when I reached the point above the evil of work in the UK loomed, so that was that! I may go back to it later in the year but conceed that a lens IS required. At least in the manner I was trying to do it.
Steve A.
*I'm still impoverished, nothing changes!
kareno wrote:Here's an idea I had many years ago for an SSTV camera. It's a rotating lead screw that causes the end of a fibre to scan a raster of sorts (the drawing doesn't mention that the fibre can slide freely through the threaded component on the threaded bar).
I had hoped this could run at NBTV rates but I suspect there would be such violent movements that it would break itself off its mounts! So I never tried it.
Oh yes - I've left out all the microswitches/opto sensors that would be present to stop the thing falling off the end of the thread!
Klaas Robers wrote:Harry, don't expect too much from the afterglow. I remember from the time that I was experimenting with SSTV (about 1973) using a 7BP7 tube, that the afterglow is so short that you are looking at the last 1 or 2 cm of the picture, while the earlier part was already gone more or less. Only in the dark it was possible to get a kind of overview of the total picture of 8 seconds, however there was a huge difference in contrast between the lowest part, just written, and the upper part, written 7 to 8 seconds ago.
But if the afterglow had been much longer the previous picture would have been still visible through the new one. This happened already, when it was dark enough.
So you should have a still standing screen, where the lines are written while you can watch the coming of the picture. There is no use to scan (move) the screen and hope that you can watch the picture after it has been written when you halt the screen.
On top of that, if you would do so, the next picturelines are already coming in while you hope that you are observing the picture. So you will miss half the amount of pictures.
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