Signal Processing issues are where the video processing distorts the image signal in an unexpected, and inaccurate way.
Such issues can be exasperated if the image path is more complex, and uses a number of different hardware components.
Signal Processing
Signal Processing issues can cause serious issue with displays with Internal TPGs, as if there are any signal path distortions prior to the location of the TPG, the internal TPG patches will not compensate for the signal path, making any generated LUT inaccurate.
The issue with signal path processing errors is they can be difficult to spot, as an internal TPG positioned after the signal processing will not reveal any problems. An external TPG, that send the patches through the full signal path, is required to spot calibration issues.
Signal Path Image Processing Errors - Internal TPG
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Patch Generation
Patch colour defined by ColourSpace, and sent to display as RGB Triplet values via Network/USB connection
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Data Connection
USB/Network data connection to the display TPG
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Signal Path Processing
Video electronics distorts normal image signal, but is bypassed by the TPG signal
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Patch Generator
Display Internal Test Patch Generator (TPG) located after the signal processing
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Uploaded LUT
LUT uploaded into the display modifies the patch colour as defined by the RGB Triplet colour data
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Display Measurement
LUT corrected patch colour displayed and measured on monitor
In the above signal path the video processing electronics adds a distortion to any real image signal, before it reaches the screen.
(The video processing can be within the display, or any external video processing system within the signal path.)
As a workflow example, a skin tone colour with triplet value (145, 107, 113) is sent to the display. The video processing electronics distorts the triplet values to 146, 105, 114, and it is that value that is then displayed on the screen.
However, when ColourSpace sends a patch triplet value to the displays TPG of the same original value (145, 107, 113), that is the colour the TPG shows on the display screen, as it is not distorted by the video processing electronics.
Therefore, the Calibration LUT generated will be inaccurate, as it will not compensate for the distortion introduced by the video processing electronics.
The potential for such signal path issues is an extremely valid reason for an external TPG to always be used as the input to the whole signal path chain.
Signal Path Image Processing Errors - External TPG
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Patch Generation
Patch colour defined by ColourSpace, and sent to external TPG as RGB Triplet values via Network/USB connection
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Data Connection
USB/Network data connection to the display TPG
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3rd Party External Patch Generator
External Test Patch Generator (TPG) located before the signal processing
(Could also be Direct HDMI TPG from ColourSpace PC) -
Video Connection
Video connection from the TPG to the display
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Signal Path Processing
Video electronics distorts the TPG signal, as well as normal image signals
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Uploaded LUT
LUT uploaded into the display modifies the patch colour as defined by the RGB Triplet colour data
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Display Measurement
LUT corrected patch colour displayed and measured on monitor
In the above signal path the video processing electronics adds a distortion to both the patch colour, as well as any real image signal, before it reaches the screen.
(The video processing can be within the display, or any external video processing system within the signal path.)
As a workflow example, a skin tone patch with triplet value (145, 107, 113) is defined within ColourSpace, and sent to the display. The video processing electronics distorts the triplet values to 146, 105, 114, and it is that triplet value that is then displayed on the screen and is the resulting colour measured by the probe.
When ColourSpace generates a calibration LUT it will generate a correction within the LUT that corrects for the colour the display showed, which includes a correction for the distortion introduced by the video processing electronics.
However, the LUT correction will have a different result depending on where in the video path the LUT is then located...
If the LUT is positioned before the video processing electronics it will apply the correction to the signal in advance of the error being introduced. So, the 145, 107, 113 value will be come 144, 109, 112, and when this colour reaches the video processing electronics, the error introduced will most likely not be the same relative error as before, as the colour triplet value is NOT the same! So the LUT correction will now be wrong, as the introduced error will be different!
The LUT must be positioned AFTER the video processing electronics to correct the introduced errors.
See LUT Processing for information on LUT processing issues.
