Direct HDMI Profiling
There is a lot of confusion regarding the direct use of a PC's HDMI output as a patch generator, mainly related to historic issues with old graphics cards and chip-sets.
The reality is that modern Nvidia graphics cards and Intel HD Graphics chip-sets have very accurate output signals, and can be used for extremely accurate patch generation and hence display profiling and calibration, if the VCGT is correctly nullified using SpaceMatch DCM.
Note: the standard Windows API/reset call for the VCGT doesn't generate a true Null setting, as explained within the SpaceMatch DCM page.
Additionally, AMD/ATI graphics cards have proven to not be transparent, and are therefore not recommended for direct patch generation and profiling.
A direct HDMI connection from the LightSpace CMS PC can therefore be the one of the best options for accurate display profiling, with the VCGT issue managed correctly, and the correct graphics card/graphics chip set used.
When using direct HDMI (or DVI/DisplayPort) output for patch generation there are a number of primary considerations that must be kept in mind.
A major potential issue with HDMI output is the use of ICC profile within the PC for internal calibration. Such ICC profiles will distort the HDMI output, making any attempt to use the output signal for profiling and calibration totally inaccurate.
Graphics Card Settings
As with ICC profiles, any inaccurate set-up of the graphics card settings will also distort the output HDMI signal.
EDID / DDC-CI / MCCS
Extended display identification data (EDID) is a data structure provided by displays to describe their capabilities to a graphics card. It is this that enables the graphics card to know the correct signal information to send to the display.
DDC-CI stands for Display Data Channel Command Interface, and is a collection of protocols for communication between a display and a graphics card that enable the PC to adjust display parameters, such as brightness and contrast.
Monitor Command Control Set, or MCCS, defines the protocol for controlling the properties of a display from a PC, or set-top box, etc., often using DDC-CI for communication.
PC Desktop Set-up
To use a PC (normally a Laptop) for display profiling via its HDMI output, first connect the display to calibrated to the HDMI output, set the desktop to 'Extended' mode (not 'Clone' mode), and re-start the PC, so the display is correctly identified.
The above shows the standard Intel HD Graphic configuration for Extended Desktop, with the Second Display as the display to be profiled.
With any graphics card ensure you select the correct display to configure - usually the external HDMI connected display.
With the PC now connected to the display to be profiled set the PC desktop background to 'black', so when displaying the profiling patches the background is not intrusive, or potentially changing the profiling results, as with plasmas ABL.
Extended Desktop, with the left hand side being the LightSpace Laptop, and the right hand side the display being profiled.
ICC Profile and VCGT Management
ICC Profiles can be managed manually, if you know what you are doing. If you are unsure SpaceMatch DCM can be used to verify the status of any active ICC profile and the graphics card's VCGT, and can re-set any found active to Null, as is needed for accurate HDMI output.
Using a true Null LUT to replace the VCGT data is critical, as outlined above.
The above shows an active ICC profile as reported via SpaceMatch DCM. This needs to be re-set to a Null profile before the HDMI output can be used for display profiling, which SpaceMatch DCM can do using a true Null LUT generated via LightSpace CMS.
Graphics Card Setup
The PC graphics card, or graphics chip-set, needs to be set to standard default settings, as any setting changes will again distort the HDMI output.
The above shows the Null settings for an Nvidia graphics card, with all settings set to their default values, and the below the same for the Intel HD Graphics chip-set.
(Note: the VCGT must be set to a true Null status via the user of SpaceMatch DCM and a true Null 3D LUT.(
The above Graphics Card controls can use DDC-IC / MCCS commands and protocols to directly adjust the display where applicable, or directly adjust the graphics card output. For accurate calibration it is therefore necessary to 'Null' all such settings, to enable the LightSpace CMS calibration to operate as required.
EDID is important for HDMI use for patch generation, as the protocol can be difficult to manage if it operates incorrectly, due to incorrect display and graphics card communication. Luckily, such problems are few and far between with modern displays and graphics cards.
DDC-CI and MCCS can effectively be ignored, so long as all graphics card settings are set to Null, as above.
EDID on the other hand attempts to ensure the image signal supplied by the graphics card is compatible with the display's expectations. That is all well and good when the communication is accurate, but can be a pain to overcome if the communication is inaccurate.
Inaccuracies are usually incorrect black and white levels, and are usually obvious to see, as defined in the Data vs. TV Legal Levels user guide.
With some graphics cards, such as Nvidia shown below, you can use the 'video' mode to compare the main graphics card output to the video output.
These settings change the black/white point for video output (only visible when the video is playing!), enabling a comparison to be made with the desktop output.
The above shows a live video stream set to 16-235 TV legal levels, with the desktop background set to 0-255 data levels.
When EDID goes wrong there are a number of options, depending on the graphics card or chip set being used.
ATI graphics cards have a manual option to force the card's output to data or TV legal ranges (0-255, 16-235).
Nvidia graphics cards do not have the same option, but can be forced via a utility file - such as NV_RGBFullRangeToggle.exe - if needed (although this is not always successful, depending on your graphics card drivers).
An alternative approach to force data range (0-255) is to set a 'custom resolution' (using the actual resolution figures), with a Hz rate that is slightly different to the expected standard - say 59Hz, rather than the standard 60Hz.
One device we have found rather useful with EDID issue is the small Dr HDMI box.
This allows independent control of most aspects of the HDMI signal, and we have used it will great success to overcome all issues we have not been able to manage with other approaches.
Additional Technical & Support Info.
German language version of this page: displaycalibration.de