Digitizing video frame for printing

Peter Corlett abuse at cabal.org.uk
Mon Sep 28 09:25:54 CDT 2020

On Mon, Sep 28, 2020 at 03:12:50PM +0200, Lawrence Wilkinson via cctalk wrote:
> Sorry I accidentally deleted this message from Dag Spicer, so here it is
> for cctalk. Reply to him or the list, not me!

[I'm not going to attempt to clean-up the top-quoted mess; check your archive
if you can't remember what it said.]

I don't know anything about the unit described, but we can make an educated
guess based on the known facts.

A quick search tells me that the printer has 132 columns and can do 330 cps.
Assuming square characters, 99 rows are needed to maintain the 4:3 aspect ratio
of the video image. 99 is convenient for neither computers nor NTSC, so some
nearby round figure is indicated. I'll arbitrarily pick 120, i.e. every other
scanline, since this is a reasonable upper bound.

Taking 132 samples of a video line requires a sample rate of roughly 2MHz. I'm
not sure of the state of the art in 1976 but that feels achievable. 26
greyscale levels only needs a 5-bit ADC, which also sounds doable. 132x120 is
15,840, which is close to 16,384, and given the 4116 was launched in 1975, five
of those would be perfect. This has a certain elegance and given the "instantly
freeze" claim, my money's on this design.

None of the dimensions are convenient powers of two, nor small integer
multiples thereof, so the actual page size and greyscale depth would be tweaked
to make the digital logic simpler. 128x80 (every third scanline) or 120x96
(every second scanline plus a bit of cropping) feel most likely, and perhaps a
depth reduction to 4 bits since who is going to check if it's really 26 levels
rather than merely assume so because it's made out of letters? If I'm
overestimating the abilities of mid-70s digital electronics, halve the
horizonal figures: digitise at 1MHz and print 64 columns (in 80 column mode).
It'll still impress the great unwashed. One may as well make it 64 rows as well
so it fits in a cheaper 1K DRAM.

A tape loop could be sampled a row at a time at the convenience of the digital
hardware. It still has to sample with 500ns precision, but not every 500ns.
Again, my calculations suggest six samples per line is sufficient to feed the
printer at 330cps. However, while this saves on high-speed digital components,
it adds a complex and unreliable analogue device which might not take kindly to
the hostile environment it's placed in, so I doubt it.

Another alternative is the wheeze done with cheap video digitisers on the 8-bit
micros: one sample per scanline, slow-scanning horizontally, and the subject is
told to sit still for the required duration. The one I saw was PAL and output
to a BBC Micro with its 160x256 mode, so it'd need to sample over 160 fields,
or 3.2 seconds. That's not exactly "instantly" but might be close enough to
fool enough people, especially when compared to a traditional photo booth.

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