Michael J. Mahon wrote:
No, the Apple II video is 60 fps, non-interlaced.
I do not agree with this. NTSC is 30 frames per second, in two fields. 60 fields per second where each field is every other scan line. If the Apple did not agree with this format you could not use a TV as a monitor, with or without a modulator. I used it with a TV and modulator for many years, so I am certain of this. Perhaps you have some sort of video card which is different.
The vertical scan rate does not determine the bandwidth required-- it determines the number of horizontal lines.
If you assume that the number of scan lines are the same, at 60 frames per second you have half the time for each scan line. To have the same horizontal resolution you have to double the bandwidth. Or are you saying that the number of lines is cut in half at 60 fps? You have a 262 scanline monitor? It must be made by HP.
It might be worth pointing out that NTSC is 30 frames per second. When you change the frame rate you are no longer NTSC compatible, strictly speaking. So if the frame rate changes I wouldn't be surprised if the number of lines was also changed, but these are arbitrary decisions made by the equipment manufactuer.
OK, I looked up NTSC. A scan line 63.5 usec. The line rate is 15750 scan lines per second. A screen is 525 lines. So 15750 / 525 = 30 frames.
The horizontal blank is 0.16 of the line duration, so the picture has (0.84)(63.5 usec) = 53.4 usec for the line. If you have 7 pixels per character, and 80 characters per line, then you need a resolution of 560. 53.4 usec / (560/2) = 0.1907 usec, or about 5.25 MHz. This would be an absolute bare minimum.
If everything else is the same (525 lines) but you double the frame rate the time per pixel would be cut in half. You would need 10.5 MHz. Which agrees with your previous range of 10-14 MHz. (You need to give some extra bandwidth or you may get some interference from adjacent pixels, plus transition bands for the filters)
Don't worry--all 80-column cards produce frame rates which are well within the lock range of a nominal 60Hz monitor. And all that I know of use non-interlaced video, since the display of characters with single-line features creates annoying flicker if an interlaced display is used without an unusually long-persistence phosphor.
I'm not worried at all. It is possible that the flicker is a problem. Interesting that it was never an issue at 40 columns on an interlaced display. But I think TVs have longer persitance than the typical monitor. I should look up the actual video circuit. The Apple had something line 192 rasters so you should get more than 2 scan lines per pixel. Maybe that had something to do with it. Big fat blurry pixels. It amazes me how much these guys did with not much more than TTL.