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Using xrandr and gtf to add a new mode to your X configuration at runtime

You know that your chipset and monitor are capable of supporting a particular mode (say 1440x900 at 59.9Hz) but that is currently not listed as being supported in the xrandr output. The following steps show how xrandr and gtf can be used to add a new modeline at runtime to your current X configuration. Please note that this is non-persistent. For making it persistent, one option is to add these lines to your /etc/profile file.
The following has been tested on a Sony Vaio VGN FJ290P Laptop running Ubuntu 8.10 and xrandr version 1.2. The laptop is connected to an external monitor (eMachines E19T6W with max 1440x900 resolution) via VGA. The display hardware is as follows:
00:02.0 VGA compatible controller: Intel Corporation Mobile 915GM/GMS/910GML Express Graphics Controller (rev 03)
00:02.1 Display controller: Intel Corporation Mobile 915GM/GMS/910GML ExpressGraphics Controller (rev 03)
  1. Use xrandr to make sure that the new mode can fit within the maximum framebuffer size
    neoblitz@n30:~$ xrandr | grep maximum
    Screen 0: minimum 320 x 200, current 1440 x 900, maximum 1680 x 1680

    The maximum virtual framebuffer size of 1680 x 1680 is sufficient to support our 1440 x 900 mode. Please note that this cannot be increased but can be decreased.

  2. Use gtf to create a mode line
    neoblitz@n30:~$ gtf 1440 900 59.9

      # 1440x900 @ 59.90 Hz (GTF) hsync: 55.83 kHz; pclk: 106.29 MHz
      Modeline "1440x900_59.90"  106.29  1440 1520 1672 1904  900 901 904 932  -HSync +Vsync

    gtf is a program to calculate the VESA GTF (Generalized Timing Formula) mode lines. Given the desired horizontal and vertical resolutions and refresh rate (in Hz), the parameters for a matching VESA GTF mode are printed out.

  3. Add new mode using xrandr
    neoblitz@n30:~$ xrandr --newmode "1440x900_59.90"  106.29  1440 1520 1672 1904  900 901 904 932  -HSync +Vsync

    Copy the text after Modeline from the above output as is and paste it after the --newmode option in xrandr.

  4. Add this newly added mode to the desired output (VGA/LVDS etc)
    neoblitz@n30:~$  xrandr --addmode VGA 1440x900_59.90

    As my monitor is connected via VGA, i will add this new mode to VGA

  5. Choose the new mode
    neoblitz@n30:~$  xrandr --output VGA --mode 1440x900_59.90

    At this point if everything above succeeds without errors then your display would switch to the new mode.