In the early days computer monitors had an analog video signal as input similar as black and white TV's. To support color, they did not modulate the color information to the black and white signal (as PAL and NTSC). PAL and NTSC work well for video sequences but for a color output the picture was not sharp enough. The color PC's used different color signals and separated the sync signals from it. Many different connector variations appeared, but finally the VGA 15pin connector became the standard.

As PC evolved a digital interface between PC and monitor was requested, to increase the performance and since digital not loose any information. The DVI (Digital Visual Interface) got introduced. The 3 channel color makes use of DC free encoding, differential signals and are clocked between PC and Monitor supporting 24bit data per pixel. The dual link feature doubles the number of channels and expands the limits of the single link implementation. Optionally the DVI connector can have additional pins that make the analog signal available to connect VGA monitors. DVI makes use of DDC to communicate with the monitor.

Nowadays the gap between PC and multimedia devices gets smaller and smaller. Multimedia devices require not just a video signal but also audio signals. The High Definition Multimedia Interface (HDMI) is therefore quite common. HDMI considers DVI for the Video signal and adds other missing feature to the connector (Audio and HDMI Ethernet Channel HEC, bidirectional audio using AOC Audio Return Channel). Since the physical size gets quite important for multimedia devices, there are many different connectors available: type A (single link), B (dual link), C (Mini-HDMI single link), D (Micro HDMI), E (Automotive). The smaller connectors make not use of the dual link feature of DVI. There are no audio pins on the HDMI connector, since audio is transmitted the same way as DVI uses for video. This might cause compatibility problems and therefore many devices still use separate audio cables. HDMI makes also use of to communicate between the devices. There is also Content protection (HDCP) to encrypt data between two devices for copy protection reasons (but there are also HDCP strippers to remove this).


Since HDMI is digital and bidirectional it might happen than a small fault makes the signal disappear. One source of errors are connectors and sockets that do not fit well together. Connectors might have too much rubber that prevents plug them in completely.

The i2c interface of the graphic card needs to be setup. Check the i2c section of this book about how to do this. Finally a good command to see if the i2c adapter of your graphic card is available is: i2cdetect -l (after emerge i2c-tools). i2c-tools comes with ddcmon and decode-edid that use the (not in the kernel included) kernel module eeprom to read the edid from the monitors eeprom. So first modprobe eeprom and to confirm lsmod will confirm that the eeprom kernel module is loaded.

There are also dedicated programs for that:


To read from (and write to) the monitor emerge ddccontrol and read

If all is ok, gddccontrol pops up a gui and detects the monitor.


To read the edid data emerge read-edid from Extended display identification data (EDID). The two programs are get-edid and parse-edid. Per default get-edid reads the data and sends it directly to parse-edid so human readable data appears on the screen among other bla bla bla output text that get-edid produces. Optionally get-edid > <edid-file> sends the binary edid data in a file. The file could then be used by manually tweaking Xorg or can later parsed using parse-edid < <edid-file> to see the edid data. The edid data can be copied and pasted to a /etc/X11/xorg.conf.d/10-Monitor.conf file.


The modeline lines in the /etc/X11/xorg.conf.d/10-Monitor.conf file might cause syntax errors due to the missing first three mandatory numbers. Just comment the lines with # that having a syntax error.

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