Path: news.uiowa.edu!iagnet.net!math.ohio-state.edu!magnus.acs.ohio-state.edu!freenet.columbus.oh.us!not-for-mail From: dalloff@freenet.columbus.oh.us (Dave Althoff) Newsgroups: comp.sys.apple2 Subject: Re: VGA to Monitor GS (Was Re: Apple IIGS on a VGA monitor...the hard way?) Date: 2 Oct 1997 23:10:00 -0400 Organization: The Greater Columbus FreeNet Lines: 62 Message-ID: <611nm8$506@login.freenet.columbus.oh.us> References: <607pd5$1dg_006@news.blkbox.com> <60bh1i$ois@uni.library.ucla.edu> NNTP-Posting-Host: login.freenet.columbus.oh.us X-Newsreader: TIN [version 1.2 PL2] Xref: news.uiowa.edu comp.sys.apple2:127529 A word about video displays... NTSC video is 262.5 lines per field, two fields per frame, thirty frames per second. Since the fields are interlaced (a little like Apple's 80-column screen memory map, turned on its side...odd lines, then even lines on the next field), that means a screen resolution of 525 lines by however many pixels will fit. Remember, NTSC video is analog, so the horizontal resolution is not rigidly defined. Okay, so that means that for each frame, the screen has to trace over 525 lines. Multiply that by 30 frames/second, and you get 15,750 line traces per second. That is the familiar 15.75 kHz horizontal oscillator frequency. So the vertical retrace rate is 60 Hz (one retrace for each field), and the horizontal retrace is 15.75 kHz. Now, this limits resolution to 525 vertical and...about 700 horizontal if you are using square pixels. That would be enough for 640x480 VGA, except that many computers prefer not to interlace the fields, which then cuts the resolution down to 700 x 262.5. With that in mind, you can see why the IIgs graphic modes go up to 640x200. Now, if we want to increase vertical resolution, we need to figure out a way to scan more horizontal lines during the screen refresh...and this is what XGA monitors do. Say we want an 800 x 600 display, and we want that non-interlaced. That means we need at least 1,200 scan lines per frame, and at 30 fps, we need to draw 36,000 lines per second, which means bumping the horizontal oscillator up to 36 kHz. Which is why the GS monitor won't display XGA resolutions on a Second Sight board. Of course, one of the problems is that computers don't do the timing necessary for interlacing very well. Interlaced displays tend to flicker because the signals don't line up exactly, so most computers use non-interlaced displays...both fields are identical. I don't know if XGA specifies progressive scanning (scan all lines in one pass instead of every other line) or not, but that would theoretically result in more display flicker because more time would be required to redraw the whole screen. Which is why most progressive-scanning schemes considered for high-definition video specify a frame rate of at least 60 frames/second...so that there will be no more flicker than for 30 fps interlaced. I know that some of the nice XGA monitors out there also switch to a higher vertical scan rate...say, 74 Hz rather than 60 Hz. Of course, for our 800x600 display, that means bumping the horizontal scan rate up to 44.4 kHz. I suspect that the high scanning frequencies may also be part of the reason that modern computer displays cannot generate the brilliant colors that our GS displays can. The dot pitch is so small that the amount of screen devoted to each of the three color pixels is curtailed...less area per color pixel (and I am talking about screen pixels, all of which are red, green, or blue, not to computer pixels) would tend to yield a more muted color...but the trade off is much higher resolution. I hope that makes some sense...Corrections are encouraged.. --Dave Althoff, ][. -- /-\ _ _ *** Just a few more weeks!!! *** /XXX\ /X\ /X\_ _ /XX\_ _ _ _____ /XXXXX\ /XXX\ _/XXXX\_ /X\ /XXXXX\ /X\ /X\ /XXXXX _/XXXXXXX\__/XXXXX\/XXXXXXXX\_/XXX\_/XXXXXXX\__/XXX\_/XXX\_/\_/XXXXXX