[mjg59]

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.

[bitwize]

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...

[mjg59]

Amigas supported interlace, but I would strongly disagree that it was common to use it.

[LocalH]

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.

[mjg59]

Everything runs at 262.5 lines at 60Hz on a 15.75KHz display - that's how the numbers work out.

[cm2187]

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).

[adrianmonk]

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.

[mjg59]

If they weren't interlaced then they were updating at 60Hz, even in the 80s. You're being very confidently wrong here.