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Old 09-18-2008, 03:04 PM   #1
Registered: Feb 2004
Location: USA
Distribution: Debian
Posts: 174

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Force xorg to use specific monitor resolution / disable DDC?


Is there a way to force xorg to always use the resolution specified in the xorg.conf file and not use DDC to try to detect the monitor? I have the xorg.conf working as shown below, as long as the monitor is directly connected to the PC (Kubuntu 8.04). I need to add in a 100 foot VGA extension cable, which works if I add in while the xorg is running, but if the X Server starts/restarts with the extension cable, it apparently tries to re-detect the monitor and comes up with a lower resolution.

Note I've tried the settings in the Device section I have commented out without any luck - perhaps I'm missing something?


Section "InputDevice"
        Identifier      "Generic Keyboard"
        Driver          "kbd"
        Option          "XkbRules"      "xorg"
        Option          "XkbModel"      "pc105"
        Option          "XkbLayout"     "us"

Section "InputDevice"
        Identifier      "Configured Mouse"
        Driver          "mouse"
        Option          "CorePointer"

Section "Device"
        Identifier      "Configured Video Device"
        Boardname       "ATI Radeon (fglrx)"
        Busid           "PCI:3:0:0"
        Driver          "radeon"
        Screen  0
        Vendorname      "ATI"
#       Option          "MergedFB"      "off"
#       Option          "IgnoreEDID" "true"
#       Option          "DDCMode" "false"
#       Option          "NoDDC"
#       Option          "PanelSize" "1920x1200@60"

Section "Monitor"
        Identifier      "Configured Monitor"
        Vendorname      "Generic LCD Display"
        Modelname       "LCD Panel 1920x1200"
        Horizsync       31.5-74.5
        Vertrefresh     56.0 - 65.0
        Modeline  "1920x1200@60" 193.16 1920 2048 2256 2592 1200 1201 1204 1242 -hsync +vsync
        Gamma   1.0

Section "Screen"
        Identifier      "Default Screen"
        Monitor         "Configured Monitor"
        Device          "Configured Video Device"
        Defaultdepth    24
        SubSection "Display"
                Depth   24
                Modes   "1920x1200@60"

Section "ServerLayout"
        Identifier      "Default Layout"
        Screen 0 "Default Screen" 0 0

Section "Module"
        Load            "glx"
        Load            "GLcore"
        Load            "dri"
        Load            "v4l"

Section "ServerFlags"
Old 09-18-2008, 03:44 PM   #2
Registered: Sep 2005
Posts: 199

Rep: Reputation: 19
No idea could startx with verbose option 6 and see why it dumps the valid mode. The resolution would be what I did with my telly and get the EDID information of your monitor on a separate file - save that on harddrive and call it from xorg.conf this way you will bypass detection procedure, but... be careful

How to accomplish this would be here...
Old 09-18-2008, 04:12 PM   #3
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Registered: Nov 2005
Location: Annapolis, MD
Distribution: Arch/XFCE
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I would have thought the modeline would do it.....

Things to try:

1. Set the vert refresh to 59.9 - 60.1

2. Uncomment the "noDDC" option in the device section.
Old 09-19-2008, 09:47 AM   #4
Registered: Feb 2004
Location: USA
Distribution: Debian
Posts: 174

Original Poster
Rep: Reputation: 16
Well, thanks for info. It seems I actually need the DDC on or else the "radeon" driver or card sticks with some failsafe resolutions - I snipped pin 12 off a VGA cable which should remove the hardware's ability to probe DDC and it would not do the 1920x1200.

Instead of fiddling with this anymore, I shrunk the size of the video image my setup ultimately displays to look okay at the lower resolution. The PC basically just runs Firefox to display some web accessible network video cameras.



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