Robert Rolf <Robert.Rolf <at> ualberta.ca> writes:


> OK, I looked. I didn't see any URLs for this 'scanner' based software.
> Am I missing a post?

I haven't posted it yet. I saw it somewhere years ago (1990s). I'll find it and
post it later.

> A lot of the paper tape in my junk pile is BLACK, so black on black
> gives lousy contrast. (PDP 11 & PDP 8 diagnostics tapes).
> It also would be difficult to correctly light aluminum (metal) tape 
> (used for bootloaders and other high repeated use stuff).

So one uses a white background for black tapes.
 
> Backlighting works well if you set the camera gain manually.
> With correct lighting intensity you get nearly 100% modulation
> of the video signal, making it easy to discriminate.

There ar no controls on the cheap kind of webcam I have in mind for this.
 
> And you wouldn't need a PC to do it.
> Just pull the tape horizontally, and have the PIC A/D sample a few
> microseconds into the scan line for whatever scan lines have
> the holes. Use the sprocket hole as your clock for 'data valid'.
 
> Or pull the tape vertically and dissect a single scan line into
> the bits. Could even be done with hardware shift register and
> comparator + PLL. Just need to have a stable optical setup.

You know the golden rule: cheap/affordable, good/works, now, pick any one.
 
> How about a 'Rube Goldberg' style virtual design contest to come
> up with the most unusual way to read a paper tape?

Doh. Xylophone driven by holes and software running on soundcard (hey, one could
use the little springs from musi boxes).

Anyway Rube Goldberg and crc-less records of any length (longer than a few
lines) are a no-no. Just for laughs compute the required error rate to read just
1 kilobyte of data from a paper tape. That's 1024*3.5 holes to get right. Or the
error rate must be < 2^-13 or so to read it right 50% of the time, and it gets
worse as the length increases. Does not look good for 'Rube Goldberg' systems.

Peter P.


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