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Re: Is monitor flicker a function of video card or monitor?

On Thu, 31 Aug 2000, Krzys Majewski wrote:

> Can I take advantage of a new video card to reduce the flicker I 
> see in X Windows, or is this strictly a function of the monitor? 

Both.  In your case, it's probably the monitor.  Your video card is kind
of low-powered too.

The thing that determines flicker is the refresh rate.  This is not able
to exceed the VertRefresh line in XF86Config and it too is limited by the
monitor itself.  Unfortunately, a monitor is also limited by the maximum
dot-clock it can support (this is also called bandwidth) and the
horizontal sync rate.  If the dot-clock is higher than the monitor can
support, the image becomes blurry and it becomes impossible to distinguish
individual dots.  If the horizontal sync is too high, the monitor can't
display a stable picture (and it's no good for the monitor, either).

My monitor (an NEC XV17) is 5 years old and has, even for the time, a
pretty low H-sync of 65.5 KHz, and lowish bandwidth of 85MHz which I run
at 100MHz.  This is good enough for me to get 1152x864 at about 70 Hz
refresh rate (or 1280x1024 at about 62 Hz - most unpleasant flicker and
the wrong aspect ratio too).

As resolution increases, both horizontal and vertical frequency increase.
This is why refresh rates are lower at higher resolutions.  Horizontal
sync rates tend to place the limit on maximum resolution.

The dot-clock is equal to the H-sync times the refresh rate times the
vertical frequency, and refresh rate cannot exceed vertical frequency.  
So you might find that your monitor has plenty of vertical frequency
available to display whatever refresh rate you want, but cannot use it at
high resolutions because you will exceed your bandwidth.  You can push the
limits with your rated bandwidth (in an emergency, you can sometimes
double it) to buy refresh rate, but your picture quality will decrease,
and it will increase the wear on the monitor.  You should try to avoid
exceeding your vertical sync rate and never ever exceed the horizontal
sync rate.

Although dot-clock cannot exceed the bandwidth of the monitor, it cannot
exceed the capabilities of your video card.  This is usually determined by
the video card's RAMDAC.  My video card (A #9 771 with an S3-968 chip) has
a pretty good 170MHz maximum dot clock; I think the average mach64 is
stumbling around at 100MHz or so.  Of course you don't lose anything if
this is higher than your monitor bandwidth.  XFree86 should tell you when
it starts up, anyway.

> I copied the modelines from my old machine to my new machine and they 
> work fine, but I'm wondering if I can do better. 

Typically modelines are monitor-limited.  The only case where they would
not be is if your video card is limited by the dot clock.

> Any pointers to rtfm on the meaning of things like "hsync" and "vsync"
> and "dot clock" also appreciated.

Look for the XFree86-Video-Timings or Modelines HOWTO.  If you can't find
it, I'll send it to you.

>    VendorName      "Magnavox" # I typed this in
>    ModelName       "CM2015D1" # I typed this in

See if you can't dig the specs up somewhere.

>    HorizSync       31.5 - 53
>    VertRefresh     50-90
>    Modeline "1024x768c" 65.0    1024 1036 1180 1304  768 771 777 802 -hsync -vsync

You can almost certainly do better than that - if your monitor is 17" or
larger.  If it's only 14" or 15" you might have fairly accurate readings

Any 17" or better monitor will have a better HorizSync and bandwidth than
that and, by extension, better refresh rates.  That is only 65MHz of
bandwidth, not very much.

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