
See:

  http://www.tamasi.org/irix/O2/vfc/1600/

Copy of text:

Subject: Video Formats + experiences with vfc on SGI IRIX (with O2)

Author: Rob Fielding <rf(a)tamasi.org>
Date: 20101015

Hi there!

http://forums.nekochan.net/viewtopic.php?f=10&t=15616&start=0&hilit=o2+vfc

Great thread. So to keep up the good work I'd like to share my experiences with
O2 VGA-DVI on HP LP2475W (native 1920x1200).  This all started because my O2
and my screen weren't "autoadjusting" to the clock/phase very well at anything
over 1024x768, leading to a headache. The screen has and Image Control menu
item for adjusting the clock/phase timings but there wasn't enough play in the
setting to get it to stabilise the test image, explained here:

	http://www.techmind.org/lcd/phasexplan.html

I found that pulling the modelines from xorg log (debian, lenny) I could tune a
vfc for the O2. This still wasn't perfect - auto adjust still wouldn't get it
right - but it did get near enough so that the clock/phase adjustment was
possible in the Image Control menu on the screen. I found I could get
1600x1000_60 and not feel sick. I expanded this little experiment after reading
about CVT, the follow on from GTF VESA standard, particularly the section
concerning Reduced Blanking (RB) lines.

	http://www.uruk.org/~erich/projects/cvt/

As explained on the cvt project page the benefit of CVT + RB is that you can
run the same resolution at a lower clock, which theoretically means high
resolutions are possible on flat pannel displays - even with analog
connections.

So for the SGI O2,

* 1920x1200_60 32x32
	Possible but you end up with the visual in the bottom right reflecting the
	top left. But it works!
* 1920x1200_60 16x16
	Possible, but a different effect - like horziontal noise tearing the visual.  
* 1600x1200_60 32x32 
	Good, but my screen greys out Custom Scaling option and auto stretches the
	display which I don't want. I also get a small part of the top left in the 
    bottom right.

My attention was therefore focused on 1600x1000, non-stretched as this is
acceptable for my use of my O2 (blender and terminals): 

* 1600x1000_60 32x32 132.79MHz clock
	My working resolution based on the monitor modeline
* 1600x1000_60 32x32 108.87MHz clock
	My new working resolution based on CVT Reduced Blanking modeline !!!

I have the same visual at a lower clock. Blender works great and I'm hoping the 
lower clock will mean the hardware is taxed less.

Modeline "1600x1000"x59.9  132.25  1600 1696 1864 2128  1000 1003 1009 1038 -hsync +vsync (62.1 kHz
Modeline "1600x1000_60.00_rb"  108.50  1600 1648 1680 1760  1000 1003 1009 1029  +HSync -Vsync

I wrote a perl script to parse the modelines, compute the back/front porch and 
then generate a vfs from a template.

  ./modeline template.tmpl < xorg.log

Use the lowest number version of the vfo file, ie

tamasi_1600x1000_60___1031.vfo <--- use this one
tamasi_1600x1000_60___1032.vfo
tamasi_1600x1000_60___1040.vfo
tamasi_1600x1000_60___1041.vfo
 
YMMV

See also tools/ for the cvt source and a compiled mips binary.
