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Adding undetected resolutions
Due to buggy hardware or drivers, your monitor's correct resolutions may not always be detected. For example, the EDID data block queried from your monitor may be incorrect.
If the mode already exists, but just isn't associated for the particular output, you can add it like this:
xrandr --addmode S-video 800x600
If the mode doesn't yet exist, you'll need to create it first by specifying a modeline:
xrandr --newmode <Mode``Line>
You may create a modeline using the gtf or cvt utility. For example, if you want to add a mode with resolution 800x600 at 60 Hz, you can enter the following command: (The output is shown following.)
cvt 800 600 60
# 800x600 59.86 Hz (CVT 0.48M3) hsync: 37.35 kHz; pclk: 38.25 MHz
Modeline "800x600_60.00" 38.25 800 832 912 1024 600 603 607 624 -hsync +vsync
Then copy the information after the word "Modeline" into the xrandr command:
xrandr --newmode "800x600_60.00" 38.25 800 832 912 1024 600 603 607 624 -hsync +vsync
After the mode is entered, it needs to be added to the output using the --addmode command as explained above.