At 22:16 9/24/98 -0600, Drew Dunn wrote:
>The problem also may be related to the RAMDAC speed. The RAMDAC is
>a clock on the video card that controls the video timings regarding
>refresh rates and resolutions. If it's not fast enough, then you
>won't be able to display >certain resolution/color combinations.
>
>Jose, your situation with the Stealth 3D 3000 is an example of that
>problem. I have a 3D 2000 on the Linux machine at
>adsl12.bois.uswest.net that will display 1200x1024x16 bits, but will
>not display 800x600x32 bits because the RAMDAC (or dot clock, as
>Linux calls it) is not fast enough. The data path is probably an
issue, too, because if a larger "chunk" of data can be transferred,
>that would reduce the physical number of data transfers that have
>to be made to provide a given resolution/color display.


There could also be a problem with the bandwidth of your monitor.
Better monitors will handle more data per second.

Just as your RAMDAC is rated in MHz (it's speed), the bandwidth of
monitors is also measured in MHz. Look at the specs of your monitor
and see if the bandwidth (in MHz) is specified. I am not aware of
a real simple way to combine the resolution, color depth, and
refresh rate into an equation that you can compare to RAMDAC speed
and/or monitor bandwidth. For example, the RAMDAC speed of both my
Matrox Millennium II and my Matrox Millennium G200 video cards
is 250 MHz. But the Millennium II allows a refresh rate at
1600x1200x32 of 85 Hz whereas 1600x1200x32 with the G200 will
only give 60 Hz. (Note that my monitor has a bandwidth of 250 MHz
I don't know if having the same number for the RAMDAC and monitor
bandwidth is a coincidence or a good match of video components.)

Summary:

What resolution and color depth (e.g. 1200x1024x16) you can get
is determined by the RAM on the video card. (Multiply the numbers,
divide by 8 and you get the amount of RAM in Bytes that you need.)

The refresh rate for a certain resolution and color depth,
assuming your video card has enough memory, is determined by the
bandwidth of your monitor and the speed of your video card (an
indication of which is the RAMDAC speed) assuming, as Drew
mentioned, that the data path between is not a bottleneck.

Also, RAMDAC speed alone is not enough to compare the speeds
of two different models of video cards.

Regards,
Bill

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