Moderators: Dave Moll, Andrew Davie, Steve Anderson
Andrew Davie wrote:I have some interesting (for me, anyway ) ideas about alternate mechanical TV configurations (such as using a loop of 35mm film instead of a Nipkow disk, with the ‘holes’ being clear areas in the film, and passing this at fairly high speed over a LED array/monitor. This would of course allow a compact monitor – but more interesting to me, the use of film allows the overlapping of the ‘holes’ and the opaqueness of the overlapped edges to be set (say, 50% when overlapped) such that the lines between scanlines become “invisible”. Using 35mm film would presumably allow very precise positioning of the ‘holes’ (or transparent windows, really), because the film could be printed from computer generated output. This would also make it much cheaper, I assume, making a TV monitor. I’m aware that the film would need to be moved at pretty high speed.
Klaas Robers wrote:The idea of the film-loop monitor is very old. Drawings were published long ago. But there is a not so simple to see problem that prevents us from using it.
First the film has to pass the sprocket wheels so frequently that it will wear out very fast. Film and sprocket wheels are not designed to run so fast, it will be running 400 frames per second. Then the speeds and the forces are too large to be handled by the rather fragile plasic of the film.
However this can be overcome by glueing the loop in a circle onto a disc and producing a drum monitor.
But there is another problem that I will try to point out for you.
1. Suppose we have a system with 32 lines and 32 dots per line. In total this is 1000 dots per frame.
2. One frame has a transparent dot with a surface of 1/1000 of the total frame.
3. So 99,9% of the frame is black.
4. So we throw away 99,9% of the light en keep only 0,1%.
This explains why we need so much light and still remain a rather dim picture.
I use 32 high brightness LED's at 50 mA peak current. You can't look into that.
5. Each dot of the picture is illuminated by this light 0,1% of the time through the "hole" of the disc.
6. If you use film this is attenuated by the transparency of the hole, say 0,9.
7. So this gives an illumination of 0,09% of the brightness of the LED-array. Still about 0,1%.
8. But each dot of the picture is also illuminated in the remaining 99,9% of the time
9. This is attenuated by the darkness of the film.
10. Suppose the attenuation of the film is 500 x. This is already very good for silver halide film.
11. Then each dot is illuminated as well with 0,2% of the light of the array during the other dots.
12. Say that the light is on for 50% of the dots,
13. Then this stray-illumination is 0,1%, the same as the illumination through the "hole".
14. So white dots get an illumination of 0,2%,
15. and black dots an illumination of 0,1%.
16. The so called contrast ratio is 1 : 2, which is extremely poor.
I know that a contrast of 1:500 can be reached by Black and White (silver halide) film. There is also special graphical material that can do only total black and total white, no gray shades. I don't know its contrast ratio, may be it is somewhat better. But if we want to come to 1:100 we need an improvement of a factor of 50 x. I doubt this. Besides this material will only be available as sheets, not as 35mm film. There is no need for graphical material in the movie industry.......
Klaas Robers wrote:You also could consider two film strips that run a different path and only come on top of each other in the viewing window. Then the worse bendability is overcome.
Klaas Robers wrote:I have a drawing of the film loop scanner from the thirties, but I don't know how to share this with you on the Forum...
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