Comment by mjg59
2 days ago
The limiting factor is the horizontal refresh frequency. TVs and older monitors were around 15.75kHz, so the maximum number of horizontal lines you could draw per second is around 15750. Divide that by 60 and you get 262.5, which is therefore the maximum vertical resolution (real world is lower for various reasons). CGA ran at 200 lines, so was safely possible with a 60Hz refresh rate.
If you wanted more vertical resolution then you needed either a monitor with a higher horizontal refresh rate or you needed to reduce the effective vertical refresh rate. The former involved more expensive monitors, the latter was typically implemented by still having the CRT refresh at 60Hz but drawing alternate lines each refresh. This meant that the effective refresh rate was 30Hz, which is what you're alluding to.
But the reason you're being downvoted is that at no point was the CRT running with a low refresh rate, and best practice was to use a mode that your monitor could display without interlace anyway. Even in the 80s, using interlace was rare.
Interlace was common on platforms like the Amiga, whose video hardware was tied very closely to television refresh frequencies for a variety of technical reasons which also made the Amiga unbeatable as a video production platform. An Amiga could do 400 lines interlaced NTSC, slightly more for PAL Amigas—but any more vertical resolution and you needed later AmigaOS versions and retargetable graphics (RTG) with custom video hardware expansions that could output to higher-freq CRTs like the SVGA monitors that were becoming commonplace...
Amigas supported interlace, but I would strongly disagree that it was common to use it.
CGA ran pretty near 262 or 263 lines, as did many 8-bit computers. 200 addressable lines, yes, but the background color accounted for about another 40 or so lines, and blanking took up the rest.
Everything runs at 262.5 lines at 60Hz on a 15.75KHz display - that's how the numbers work out.
The irony is that most of those who downvote didn't spend hours in front of those screens as I did. And I do remember these things were tiring, particularly in the dark. And the worst of all were computer CRT screens, that weren't interlaced (in the mid 90s, before higher refresh frequency started showing up).
I spent literally thousands of hours staring at those screens. You have it backwards. Interlacing was worse in terms of refresh, not better.
Interlacing is a trick that lets you sacrifice refresh rates to gain greater vertical resolution. The electron beam scans across the screen the same number of times per second either way. With interlacing, it alternates between even and odd rows.
With NTSC, the beam scans across the screen 60 times per second. With NTSC non-interlaced, every pixel will be refreshed 60 times per second. With NTSC interlaced, every pixel will be refreshed 30 times per second since it only gets hit every other time.
And of course the phosphors on the screen glow for a while after the electron beam hits them. It's the same phosphor, so in interlaced mode, because it's getting hit half as often, it will have more time to fade before it's hit again.
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
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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.
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If they weren't interlaced then they were updating at 60Hz, even in the 80s. You're being very confidently wrong here.