I have installed a Mini HDMI Cat5e/6 Extender (Digitech AC-1726) onto my Ubuntu 22.04 LTS system to allow a second monitor to be operated in the next room. Everything is working except that there is now a very pronounced colour cast (magenta) on the second monitor. This colour cast is not present when a HDMI cable is used instead of the extender ethernet cable. I get the same problem if I use another Ubuntu computer or if I swap the monitors around or if I use other ethernet cables. Using just one monitor with the extender also causes the problem.
To troubleshoot this problem further I installed Windows 11 on the computer and the colour cast disappears and extender seems to work perfectly. I then installed Pop!_OS (an Ubuntu derivative) on to the computer and again, the colour cast disappears and extender seems to work perfectly.
I even tried a third instance of Ubuntu residing on a USB thumb drive and again I get the colour cast when I use the extender. What could there be about Ubuntu that causes the colour cast when an extender is used, but when Windows 11 or Pop!_OS is used there is no problem?
FEEDBACK FOR VIDARLO
Thanks for the heads up on xrandr @vidarlo. I wasn’t aware of that command and I can see I’ll be making use of that in other areas. I have included 4 outputs to cover the scope of two cabling scenarios (DisplayPort and Extender/ethernet) and two operating systems (Ubuntu and Pop_OS). I can’t see a clue in comparing outputs but maybe a keener eye can. I did observe one interesting outcome, though I doubt it is a clue as to what’s going wrong. With Ubuntu, the first attempt to make a connection to the monitor using the extender cable resulted in resolution being set to 1920x1080. However with Pop_OS the first attempt to make a connection to the monitor using the extender cable resulted in resolution being set to 1280x720. I then tried both these resolutions on both operating systems. The upshot is that when using the extender cabling Ubuntu always suffers with the colour cast while Pop_OS always displays without any hint of a colour cast. When using Diplayport or HDMI cabling there is never a colour cast regardless of operating system.
1. DP-Cable & Ubuntu:
zig@ubuntu:~$ xrandr
Screen 0: minimum 8 x 8, current 3840 x 2160, maximum 32767 x 32767
HDMI-0 disconnected (normal left inverted right x axis y axis)
DP-0 connected primary 3840x2160+0+0 (normal left inverted right x axis y axis) 597mm x 336mm
3840x2160 60.00*+ 29.98
2560x1440 59.95
2048x1280 60.20
2048x1152 60.00
2048x1080 24.00
1920x1200 59.88
1920x1080 60.00 59.94 50.00 23.98
1680x1050 59.95
1600x1200 60.00
1280x1024 75.02 60.02
1280x800 59.81
1280x720 59.94 50.00
1152x864 75.00
1024x768 75.03 60.00
800x600 75.00 60.32
720x576 50.00
720x480 59.94
640x480 75.00 59.94 59.93
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)
USB-C-0 disconnected (normal left inverted right x axis y axis)
zig@ubuntu:~$
2. Extender Cable & Ubuntu:
zig@ubuntu:~$ xrandr
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 32767 x 32767
HDMI-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 890mm x 500mm
1280x720 60.00 + 59.94 50.00
1920x1080 59.94* 50.00 49.95 29.97 23.98
1280x1024 60.02
1024x768 60.00
800x600 60.32
720x576 50.00
720x480 59.94
640x480 59.94 59.93
DP-0 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)
USB-C-0 disconnected (normal left inverted right x axis y axis)
zig@ubuntu:~$
3. DP-Cable & Pop!_OS
zig@pop-os:~$ xrandr
Screen 0: minimum 8 x 8, current 3840 x 2160, maximum 32767 x 32767
HDMI-0 disconnected (normal left inverted right x axis y axis)
DP-0 connected primary 3840x2160+0+0 (normal left inverted right x axis y axis) 597mm x 336mm
3840x2160 60.00*+ 29.98
2560x1440 59.95
2048x1280 60.20
2048x1152 60.00
2048x1080 24.00
1920x1200 59.88
1920x1080 60.00 59.94 50.00 23.98
1680x1050 59.95
1600x1200 60.00
1280x1024 75.02 60.02
1280x800 59.81
1280x720 59.94 50.00
1152x864 75.00
1024x768 75.03 60.00
800x600 75.00 60.32
720x576 50.00
720x480 59.94
640x480 75.00 59.94 59.93
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)
USB-C-0 disconnected (normal left inverted right x axis y axis)
zig@pop-os:~$
4. Extender-Cable & Pop!OS – Change res to 1920x1080 then reboot:
zig@pop-os:~$ xrandr
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 32767 x 32767
HDMI-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 890mm x 500mm
1280x720 60.00 + 59.94 50.00
1920x1080 59.94* 50.00 49.95 29.97 23.98
1280x1024 60.02
1024x768 60.00
800x600 60.32
720x576 50.00
720x480 59.94
640x480 59.94 59.93
DP-0 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)
USB-C-0 disconnected (normal left inverted right x axis y axis)
zig@pop-os:~$
If I adjust gamma and brightness in an attempt to reverse the colour cast affect that Ubuntu combined with the HDMI Extender introduces, using the command line:
xrandr --output HDMI-0 --gamma 0.2:0.8:1.5 --brightness 1.5
I get much closer to a normal colour rendering, but still not acceptable.
I mention this, not because I think it might be a solution to my problem, but rather to illustrate how severe the colour cast problem is. Normally gamma would be set to gamma R:G:B =
--gamma 1.0:1.0:1.0
