The Undeclared Variable

At one time I had issues with one of my older computers displaying 1440x900 resolution correctly. This has not been an issue in a couple years but I feel the information here is still worth keeping around. The solution that worked for me was really quite simple by adding a "Modeline" to xorg.conf with the correct settings. There are endless forum threads concerning this topic and people stating how they got it to work through trial-and-error or using various sites with modeline generators. In my experience these generators caused additional headache and where less the helpful.

What is "Modeline"? 
Modeline is a configuration line that can be added to your xorg.conf file to drive a specific display resolution. An individual Modeline has ten parameters, beginning with a label for the resolution being specified. The second parameter specifies the rate of the pixel clock in megahertz. Then follow four numbers that together specify the x-resolution (width) and related parameters, and four numbers that specify the y-resolution (height). ** Example:

Modeline "1440x900_60.00"  106.47  1440 1520 1672 1904  900 901 904 932  -HSync +Vsync

There are numerous links to generators that will generate a Modeline for you based on input values of resolution, refresh rates, and additional info (they are all a little different). However Linux does this for you! Using terminal a user can generate a Modeline by entering only minimal information. The "gtf" command The solution is simple, use the gtf command and enter the desired resolution. In the example below I am querying for a Modeline based on 1440x900 resolution with a refresh rate of 60hz.

$ gtf 1440 900 60

The terminal returns:

$ # 1440x900 @ 60.00 Hz (GTF) hsync: 55.92 kHz; pclk: 106.47 MHz  Modeline "1440x900_60.00"  106.47  1440 1520 1672 1904  900 901 904 932  -HSync +Vsync

Now to add it to the xorg.conf, if you haven't edited this file before continue reading because this file requires you to be logged in as root.

$ su -

Next the gedit command will be used to edit xorg.conf, as a note you can view xorg.conf through GUI but the edit permissions are restricted to root.

$ gedit /etc/X11/xorg.conf

Next locate the "Monitor" section and add the Modeline, below is a sample from my xorg.conf file.

Section "Monitor"
	Identifier   "Monitor0"
	ModelName    "LCD Panel 1440x900"
	HorizSync    31.5 - 56.0
	VertRefresh  56.0 - 65.0
	Modeline     "1440x900_60.00"  106.47  1440 1520 1672 1904  900 901 904 932  -HSync +Vsync
	Option	     "dpms"

In some cases users have had to add the display type manually. Locate the "Screen" section, and underneath that locate the "Display" sub-section and add a "Modes" line like the example below.

Section "Screen"
	Identifier "Screen0"
	Device     "Videocard0"
	Monitor    "Monitor0"
	DefaultDepth     24
	SubSection "Display"
		Viewport   0 0
		Depth      24
		Modes	   "1440x900_60.00"
	EndSubSection

Save the file and restart the xserver by pressing CTRL+ALT+BACKSPACE, this will take you back into the login window. This is what worked for me, I hope that this information was useful. Remember I am not a Linux/Fedora expert so use at your own risk!

 ## Important note on Fedora 10 ##

  I recently upgraded to Fedora 10 and to my surprise the display manager was not installed by default. This is the dialog opened through the following command:

$ system-config-display

If this is the case with you use the command below to install the display manager.

$ yum install system-config-display

The reason I wanted this installed was that the xorg.conf file was blank by default. Opening the display manager and just saving the current settings will fill in the information needed making it a lot easier only having to add the Modeline.

- Ed

Sources: ** Modeline @ Wikipedia