[GreenKeys] Paper tape ---> Photos

[Harold Hallikainen]

On Thu, August 28, 2025 3:19 am, Ken LoCasale wrote:
> An interesting post on Hackaday...
>
>
> https://hackaday.com/2025/08/26/picture-by-paper-tape-wire-photos/

Thanks for posting! I had seen a description of that system before. It's
an amazingly complex system to get a 6 level gray scale. I think we do
better with Baudot art. In fact, on the receive side, it would be
interesting to have a teletype where instead of letters, we had 31 dots
(plus one with no dot) of increasing size. This would directly print
halftone gray scale ready for print.

Doing the encoding using photography seems more complex. Printers (the
people, not the machines) use a half tone screen with "dots" that are
clear in the center and get darker as you go out radially. This is put
over the high contrast film in the copy camera. Darker areas of the photo
expose the center of the screen dots enough to get over the photographic
threshold. Brighter areas get the film over the threshold over a bigger
area, making a larger dot
on the film. I may have this backwards since it has been more than 50
years since I operated a printshop copy camera. In any case, high contrast
film results in varying dot size.

If they had the availabiliy of a photocell, they could have done a
mechanical successive approximation analog to digital converter using 5
neutral density filters. Shine a light on a pixel area of the original.
Put a 50% filter in front of the photocell. Is the remaining light above
or below the threshold? If below, the most significant bit is 0. If above,
it is 1. If the light was above the threshold, leave the filter in place
and put a 25% filter in also. Are we above or below the threshold? Any
time we are below the threshold, the filter that put us below the
threshold is removed for the next test.

I can imagine a system with the original photo on a drum, solenoids moving
filters in and out of the light path, and tape being punched.

Another method might be to vary the brightness of the light source. Use
resistors and relays to drive a light bulb with an R2R digital to analog
converter (with resistor values adjusted to deal with the nonlinearity of
an incandescent lamp). Each bit, in sequence most significant to least
significant, would be tested to see if the photocell illumination was
above a threshold. After the five relays click, the 5 bit code
representing the pixel brightness would be ready for tape punching.

MANY years ago, I visited a shortwave listner who had a "wirephoto"
machine. He would put photographic film on the rotating drum and print
very high quality photos received over the air. And then I worked on Alden
weather fax machines at an FAA FSS.

Interesting technology!

Harold

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