Direct HDMI

Using the in-built ColourSpace patch generator is one of the easiest way to perform accurate display calibration, and negates the need for separate signal generator hardware.

  • ColourSpace Integration
  • Closed Loop Profiling
  • Direct TPG Operation
  • Accurate Patch Generation

Overview

There is a lot of confusion regarding the direct use of a Windows PC's direct HDMI output as a patch generator, mainly related to historic issues with old graphics cards and chip-sets.

The reality is that modern graphics cards and Intel HD Graphics chip-sets have very accurate output signals, and can be used for accurate patch generation, and hence display profiling and calibration, if the VCGT is correctly nullified, and any active ICC profiles disabled.

The standard Windows API/reset call for the VCGT doesn't generate a true Null setting, and should be reset by the use of a Null 1D LUT upload via ColourSpace, or SpaceMatch.
Additionally, Apple operating systems have major colour issues, especially with regard to direct HDMI output, and have proven to not be transparent, and are therefore not recommended for direct patch generation and profiling.

A direct HDMI connection from a Windows PC running ColourSpace can therefore be the one of the simplest, and best, options for accurate display profiling, with VCGT/ICC issues managed correctly.

Windows PC Configuration

When using direct HDMI (or DVI/DisplayPort/USB-C) output for patch generation there are a number of primary considerations that must be kept in mind.

ICC Profiles

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. Specifically, Windows OS uses a VGCT (Video Card Gamma Table) that is often set via the applied ICC profile, and needs to be disabled/removed for the HDMI output to be accurate.

SpaceMatch DCM can be used for this purpose.

Graphics Card Settings

Inaccurate set-up of the graphics card/chip set settings will also cause inaccurate HDMI signal output, specifically with signal range settings not matching the display range - Full or Limited.

The Control Panel software provided for specific graphics cards/chip sets can often be used to set the required output signal range, depending on the graphic card/chip set in use, as well as the output colour format.

Nvidia/AMD graphics cards usually have more options/capabilities than Intel chip sets.

Nvidia Signal Range
Nvidia Control Panel - Range Settings

Note: the Output Dynamic Range options do not rescale the signal sent to the display. When set to Limited any image data below 16 (8 bit) is clipped, as is any image data above 235 (8 bit). When set to Full, all image data from 0 to 255 (8 bit) is sent to the display. This is not the same as Video Colour Setting, which stretches, or compresses, the source video as required for the display, and only affects video sources.

Nvidia Video Range
Nvidia Control Panel - Video Range

Bit depth can be set to 8 bit, 10 bit, or 12 bit, depending on the Output colour format selected. Usually, RGB allows for 8 bit or 12 bit, while YCbCr 422 allows for 8 bit, 10 bit, or 12 bit, and YCbCr 444 allows for only 8 bit, although different graphics card models can have different options.
(The in-built ColourSpace TPG is presently 8 bit, as unless using an external TPG for manual calibration of 10 bit displays with 10 bit control points, greater than 8 bit is not required. The madTPG, part of madVR, can be used for 10 bit direct HDMI profiling.)

Nvidia RGB Bit Depth
Nvidia Control Panel - RGB Bit Depth
Nvidia YCbCr Bit Depth
Nvidia Control Panel - YCbCr 422 Bit Depth

When profiling, the graphics card/chip set Range settings should be set to match the display's expected range, with the required Patch Scale set within ColourSpace to match. For most home TVs, this means setting the graphics card to Limited, and Legal set for Patch Scale within ColourSpace.
(As there is no valid broadcast or video disc source that has such over-range signals, Legal is the recommended Patch Scale setting, and not Extended.)

There is however yet another setting with Nvidia that will alter the signal as interpreted by the display as being Legal or Full Range.

Nvidia Content Type
Nvidia Control Panel - Content Type

In most situations, setting to Desktop programs will instruct the display the signal is Full Range, while setting to Full-screen videos will instruct the display the signal is Legal Range.

Regardless of the above settings, how the graphics card/chip set interprets the attached display, via EDID, has to be understood.

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. It is the various data blocks within the EDID data that are often interpreted differently by different graphics cards/chip sets, such as CEA data being used to define TVs vs. PC display.

The above Graphic Card settings, in part, define the EDID communication.

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.

The problem is such control information can potentially override manual settings within the graphics card/chip set, and cause unexpected range scaling...

EDID/Graphics Card/Chip Sets Issues

A potential issue with graphics cards/chip sets is the way they interpret the connected display as being Full range, or Limited. ColourSpace will need to be configured to match the graphics card/chip set assessment, or a levels miss-match will occur.

It is entirely possible for two different graphics cards/chip sets to interpret the same display, with the exact same setting, differently.
One graphics card/chip set can see the display as being Limited range, while another can see it as being Full... The ColourSpace Patch Scale setting will need to be set to match.

Testing for graphic card/chip set patch scale range errors, based on EDID assessment, can be performed with the in-built Test Patterns, and measurement of a Zero Black patch.

The first test is to use the Brightness Test Pattern, and assess for either black clipping or lift.

The Test Patterns follow the Patch Scale values set within Settings, and will need to be set to Full to show the above issues as defined. See the Patch Scale page for more info.

Windows PC Desktop Set-up

To use a PC (normally a laptop) for display profiling via its HDMI output, first connect the display to be calibrated to the HDMI output, set the desktop to Extended mode (not Clone mode), and if necessary, re-start the PC so the display is correctly identified.

Extended Desktop
Extended Desktop
(ColourSpace on Laptop screen & Patch Window on TV)

Using the graphics card/chip set control program, select the correct display and configure as outlined above.

Open the pop-up colour patch window within ColourSpace, and place on the extended desktop, on the display to be profiled.

Profiling can then be performed as normal.

Overcoming EDID Issues

When EDID goes wrong there are a couple of options available to try to attain the required range settings.

It can be possible to force data range (0-255 - 8 bit) by setting a custom resolution, using the actual resolution values required, but with a Hz rate that is slightly different to the expected standard - say 59Hz, rather than the standard 60Hz.

Alternatively, the various HDMI management devices from HDFury can be used to manually alter the display's EDID data.

Patch Scale & Resolution

Patch Scale & Resolution Guide