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.
