We’ve had a steady stream of requests for supporting HDMI modes that aren’t from the official CEA/DMT list of modes.

I’ve implemented CVT:
http://en.wikipedia.org/wiki/Coordinated_Video_Timings
You specify the new mode in config.txt with the following config string:

CODE: SELECT ALL

hdmi_cvt=<width> <height> <framerate> <aspect> <margins> <interlace> <rb>
width        width in pixels
height       height in pixels
framerate    framerate in Hz
aspect       aspect ratio 1=4:3, 2=14:9, 3=16:9, 4=5:4, 5=16:10, 6=15:9
margins      0=margins disabled, 1=margins enabled
interlace    0=progressive, 1=interlaced
rb           0=normal, 1=reduced blanking

The first three arguments are required. The remaining ones are optional. aspect defaults to 16:9 if not specified.

It should appear if you do

CODE: SELECT ALL

tvservice -m DMT

An example:

CODE: SELECT ALL

hdmi_cvt=1680 1050 60 5 0 0 1

CODE: SELECT ALL

tvservice -m DMT
...
mode 87: 1680x1050 @ 60Hz 16:10, clock:119MHz progressive


(The custom mode will be 87). You might want to test it:

CODE: SELECT ALL

tvservice -e "DMT 87"
fbset -depth 8 && fbset -depth 16


(The fbset gets the console back after closing the display).

You can also do the usual:

CODE: SELECT ALL

hdmi_group=2
hdmi_mode=87

to make the custom mode the default.

Please note. There is no guarantee your display will support the resolution/framerate produced by this.
But if you are currently running a monitor at a non native resolution, then this may be worth a try.

The original post is from

https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=24679