VGA output scan rates

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Does anyone know if it is possible (practical) to adjust
the horizontal and vertical scan rates on the VGA output
on a VIA EPIA motherboard?

I am toying with the idea of driving 3 monochrome monitors
with different signals from the RGB output but would need
the HSync to be 15 to 17Khz and VSync at 50 or 60 Hz.

The system would be running Windows XP Embedded.

My programming skills are HLLs such as Delphi and VB
with a bit of PIC and AVR assembly, so writing a driver
would probably be too much effort for this project.


Re: VGA output scan rates
On Tue, 10 Jan 2006 15:47:20 +1000, "Mike Warren"

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Getting the relevant information on the chipsets to do this sort of
thing is next to impossible. You might look at Linux driver source for
this chipset to get the needed info.  If you are not up to writing a
device driver, then even if you had all the info, you will not get
very far.

  Anton Erasmus

Re: VGA output scan rates
Hi Mike,

Have a look at Powerstrip ( ), I use
it to drive my scart (TV) from a VGA port.


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Re: VGA output scan rates
Mike Warren schrieb:
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That probably won't help you much - but Xorg (and XFree86) allow to set
(nearly) arbitrary frequencies.

See for XFree86 information, for a DOS TSR, for general
information; maybe
helps, too.

Maybe there's something in it for you.



Re: VGA output scan rates

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I did this for a company that was selling refurbished fixed-frequency
monitors many years ago, so yes it is possible. (Actually I did it on
an S3 chipset, which is an ancestor of the Apollo chipset used in the
Epia boards).

If you are OK with _only_ getting good video output in Windows, it is
not impossibly difficult to write a little program that pokes the CRTC
values and simply put that program in your startup folder.

You _MAY_ be able to get by with only poking the standard VGA CRTC
locations, but it's unlikely. What I suggest you do is tune XFree86 to
the syncrates you want, then dump out the chipset parameters.

Re: VGA output scan rates
On Tue, 10 Jan 2006 15:47:20 +1000, "Mike Warren"

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Please note that these are interlaced modes requiring the right kind
of synch pulses. Some graphical cards are reportedly capable of
generating it, some other might not. The low HSync frequency can also
be a problem.

Unless you are using monitors with the green extremely slow phosphor,
you should limit your vertical resolution to about 240 or 288 pixels
to avoid the annoying 30/25 Hz interlace flicker on small vertical
details. Practically the same image should be sent to both odd and
even frames.


Re: VGA output scan rates - Thanks
Thanks for all the answers.

Powerstrip looks promising. Since this is a one-off,
the $30 cost is not an issue. I have downloaded it
and will try it tomorrow.


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