On Tue, 13 Mar 2001, Pablo Manalastas wrote:
..
> I noticed that even if you have 2MB, 4MB, or more on
> you video card, you are effectively using only
> 1MB for a maximum of 1024x768 resolution at 8bpp depth,
> if you monitor has the usual frequencies 30-50 H and
> 50-100 V. The monitor seems to limit what you card
> can do. Is this correct? Can someone give me the
> correct relationship?
Well, the total framebuffer memory is H x V x BPP/8 so for example,
1024x768x32bit would require 3.14MB of framebuffer. If you go for a
refresh rate of 85Hz (optimal for 1024x768) you need a total RAMDAC
bandwidth of 267MB/second. Generally, you can't get that much from an
older video card but that's no sweat for the new AGP video cards.
ESR wrote a long, boring treatise about the relationship between monitor
modelines and resolutions you can get, but briefly:
a typical 17" monitor has a maximum horizontal frequency of 70kHz. A
15" usually goes up to 64kHz. of course a good monitor (Sony, or high-end
ViewSonic) can do better.
Simple rule: dot clock = horiz freq x vert freq ( == refresh) x horiz dots
x vert dots
Horiz dots = horiz # of visible pixels plus "front door" and "back
door" (the "doors" are the black portions at left and right of the image
that are non-visible)
Same for vert dots, except we're talking top and bottom non-visible areas.
So given the maximum HF and VF of a monitor, this limits the maximum
resolution. Also, if your video card can't pump out a high dot clock
(pretty rare these days) your resolution will also be limited.
As a simple rule, the horiz dots is about 110% of the horiz freq (5% for
front door and back door) same for vert.
N.B. XF4 can query the monitor via VESA DDC (if it's a plug-n-pray
monitor, most built in the past 3 years are) and automagically select the
optimal modeline.
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