Comment by perching_aix

13 days ago

Regardless of whether we go with 512x324, 512x342, or 512x384, the claim of 72 PPI (exact) and 9" of diagonal size (exact) are not simultaneously possible.

Extremely nitpicky thing I know, but this kinda stuff really bugs me, could somebody please clarify what was the real size (and/or PPI) here?

For reference:

512x324 @ 72 PPI = 8.42" (or 214 mm) (rounded)

512x342 @ 72 PPI = 8.55" (or 217 mm) (rounded)

512x384 @ 72 PPI = 8.89" (or 226 mm) (rounded)

The first two don't even yield an integer result for the number of diagonal pixels, let alone yield an integer multiple of 72. Or would there be bars around the screen, or how would this work?

7 comments

perching_aix

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pavlov  13 days ago

For CRTs, the diagonal measurement was of the physical tube. The actual viewable area was smaller. Part of the tube’s edges were covered by plastic, and there was always also some margin that wasn’t used for picture so it was just black.

It was a 9” tube with 3:2 aspect ratio. Your calculation of a 8.5” image at 72 dpi sounds right.

  • Suppafly  13 days ago

    >For CRTs, the diagonal measurement was of the physical tube. The actual viewable area was smaller.

    That's also why TVs and monitors of that era always seemed smaller than advertised. I remember having to explain that to a lot of people.

msephton  13 days ago

You're quite right, the screen would be centred with margin/border around it.

Whilst the CRT is 9", according to period repair guides the screen should be adjusted so that the visible image was 7.11" x 4.75", pretty much exactly 1.5:1. This meant 72dpi, which was to match PostScript point size for print output and WYSIWYG.

So it's your 8.55" diagonal.

Some classic Macintosh users today are unaware of this screen size reasoning, or don't agree with it, and stretch the screen to fill the whole CRT. Yikes!

BTW, I posted pretty much the same info earlier today at https://news.ycombinator.com/item?id=44105531 — what synchronicity!

johnklos  12 days ago

A square that's one thousand units by one thousand units doesn't give a rational number, much less an integer one, for the diagonal.

A 9" CRT would never be precisely 9", because beam trace width and height are analog, plus there's overscan, so a 9" screen would simply give something pretty close to 9".