Comment by adrianmonk

Have you ever seen high speed footage of a CRT in operation? The phosphors on most late-80s/90s TVs and color graphic computer displays decayed instantaneously. A pixel illuminated at the beginning of a scanline would be gone well before the beam reached the end of the scanline. You see a rectangular image, rather than a scanning dot, entirely due to persistence of vision.

Slow-decay phosphors were much more common on old "green/amber screen" terminals and monochrome computer displays like those built into the Commodore PET and certain makes of TRS-80. In fact there's a demo/cyberpunk short story that uses the decay of the PET display's phosphor to display images with shading the PET was nominally not capable of (due to being 1-bit monochrome character-cell pseudographics): https://m.youtube.com/watch?v=n87d7j0hfOE

There are no pixels in CRT. The guns go left to right, ¥r¥n, left to right, while True for line in range(line_number).

The RGB stripes or dots are just stripes or dots, they're not tied to pixels. There would be RGB guns that are physically offset to each others, coupled with a strategically designed mesh plates, in such ways that e- from each guns sort of moire into only hitting the right stripes or dots. Apparently fractions of inches of offsets were all it took.

The three guns, really more like fast acting lightbulbs, received brightness signals for each respective RGB channels. Incidentally that means they could go between brightness zero to max couple times over 60[Hz] * 640[px] * 480[px] or so.

Interlacing means the guns draw every other lines but not necessarily pixels, because CRTs has beam spot sizes at least.

No, you don't sacrifice refresh rate! The refresh rate is the same. 50 Hz interlaced and 50 Hz non-interlaced are both ~50 Hz, approx 270 visible scanlines, and the display is refreshed at ~50 Hz in both cases. The difference is that in the 50 Hz interlaced case, alternate frames are offset by 0.5 scanlines, the producing device arranging the timing to make this work on the basis that it's producing even rows on one frame and odd rows on the other. And the offset means the odd rows are displayed slightly lower than the even ones.

This is a valid assumption for 25 Hz double-height TV or film content. It's generally noisy and grainy, typically with no features that occupy less than 1/~270 of the picture vertically for long enough to be noticeable. Combined with persistence of vision, the whole thing just about hangs together.

This sucks for 50 Hz computer output. (For example, Acorn Electron or BBC Micro.) It's perfect every time, and largely the same every time, and so the interlace just introduces a repeated 25 Hz 0.5 scanline jitter. Best turned off, if the hardware can do that. (Even if it didn't annoy you, you'll not be more annoyed if it's eliminated.)

This also sucks for 25 Hz double-height computer output. (For example, Amiga 640x512 row mode.) It's perfect every time, and largely the same every time, and so if there are any features that occupy less than 1/~270 of the picture vertically, those fucking things will stick around repeatedly, and produce an annoying 25 Hz flicker, and it'll be extra annoying because the computer output is perfect and sharp. (And if there are no such features - then this is the 50 Hz case, and you're better off without the interlace.)

I decided to stick to the 50 Hz case, as I know the scanline counts - but my recollection is that going past 50 Hz still sucks. I had a PC years ago that would do 85 Hz interlaced. Still terrible.

You assume that non interlaced computer screens in the mid 90s were 60Hz. I wish they were. I was using Apple displays and those were definitely 30Hz.