For any calibration work it is imperative the workflow to be used for real work is well understood and the calibration path set-up accordingly.
With most grading systems this means understanding how to re-scale the image workflow as required - for example, mapping 0-1023 Data levels to 64-940 Legal levels.
The need to understand this cannot be stressed enough, and this page attempts to address the various problems that can be encountered.
Also see the Signal Range page.
Initial Understanding
There are two fundamental basics to image levels - creative/grading systems that will output either Data range images (0-255 or 0-1023), or TV Legal levels (16-235 or 64-940), and displays that expect the input signal to be either Data range images, or TV Legal levels, and will display accordingly.
(The Legal values quoted are for RGB and luma (Y). In YCbCr signals the colour-difference channels (Cb/Cr) use 16-240, or 64-960 at 10-bit.)
What is important to understand up-front is that all displays show black as black and white as white, when correctly configured for the input signal level. This means that regardless of the input signal level, if the display is correctly matched to the signal input level, black and white will look identical.
When incorrectly configured, black and white levels will be displayed clipped/crushed, or lifted/compressed.
It is also important to understand that whatever range is used on the connection, the display's panel itself is always driven at full (Data) range, with fixed black and white. When a display is set to input Legal levels, its internal processing expands the incoming 16-235 (64-940) signal to the panel's full range. When set to input Data levels, 0-255 (0-1023) passes straight through. Correct display therefore depends on the display being set to, or with HDMI being told, the correct input range.
Not all displays can perform this rescaling. Many computer monitors only accept Data levels, so a Legal signal will always appear compressed on them unless it is rescaled to Data levels within the source.
Data vs TV Legal
The effect configuring incorrect Data vs Legal signal path levels has on an image can be summarised as follows, with the correct, Nominal image in the centre. The images either side show issues when an incorrect Data vs Legal level configuration is used, with the darker, Clipped, or brighter, Compressed image error depending on where the incorrect configuration exists - in the process chain, or within the display - as defined in the rest of this page.
(Click any image to cycle through the variations.)
Continue Initial Understanding
The requirement for accurate calibration (and accurate general image display) is to match the output from the source system to the expected input to the display, and/or vice versa.
The following examples all use the BarsandBlack Cal Image, which is part of the downloadable Calibration Images, and which is supplied as a Data level image. When loaded into a grading system it may immediately be rescaled to Legal, or rescaled only on output (keeping the image Data level on the timeline), or may have no scaling applied at all.
SDI vs HDMI Connections
The signal paths on this page show an SDI connection, where the display's input range is set manually, and nothing within the signal changes it.
HDMI is different. An HDMI source sends AVI InfoFrame metadata with every frame, flagging the quantisation range of the signal as Full (Data) or Limited (Legal). A display with its range setting on Auto (often called HDMI Black Level, RGB Range, or similar) will follow that flag, regardless of the range the source system is actually outputting. If the actual output range and the signalled range disagree, or the display ignores the InfoFrame, the clipping and compression errors shown below will occur, even though every setting appears correct.
Additionally, HDMI YCbCr signals are almost always Limited range, while the default range for RGB depends on the video format (TV timings such as 1080p or 2160p default to Limited, PC timings to Full), and on whether the display's EDID declares that the range can be selected. Within a PC environment, the graphics card driver makes these choices automatically based on the display's EDID, making an understanding of EDID rather important in any HDMI signal path.
See the HDMI InfoFrames page and the Direct Profiling User Guide, as well as the EDID Viewer in Graph Options, for further info.
Valid Signal and Display Level Settings
Valid Signal and Display Level Settings
The following signal paths assume an SDI connection, with the display's input range set manually. With HDMI, the display's input range may instead be set by the source's AVI InfoFrame - see the SDI vs HDMI note above.
Data to Data Levels
The following signal path shows a simple configuration, where the original image is Data levels, and is maintained at Data levels before being sent to the display. The display is set to input Data levels, and maps black and white correctly to the display's screen.
This is nominally the default signal path for most post-production workflows, as compression to TV Legal range is performed after post-production creative work has been completed.
The relative impact on the image in each of the three stages can be seen below. In this instance there is no change at all.
Legal to Data Levels
The following signal path is basically the same as above, but with the original image in Legal levels, which the source system rescales to Data levels before being sent to the display. The display is set to input Data levels, and maps black and white correctly to the display's screen.
This is a standard post-production workflow, when source footage is already compressed into Legal levels.
The relative impact on the image in each of the three stages can be seen below. In this instance the change occurs immediately within the source system.
(Click any image to cycle through the variations.)
Legal to Legal Levels
The following diagram shows an alternative valid signal path configuration, where the original image range is Legal, and is maintained Legal through to the display input. The display is set to input Legal levels, and maps black and white correctly to the screen.
This is another default signal path for post-production workflows, where the entire signal path is for direct TV deliverables. It is also the default for home TV viewing, as broadcast, streaming and disc based media are all delivered in Legal levels. However, home TVs use HDMI, not SDI, so the TV's input range is normally set automatically from the source's AVI InfoFrame (with the TV's range setting on Auto), rather than manually - see the SDI vs HDMI note above.
In this configuration the Legal levels are used throughout, and the display is expecting Legal levels as its input, and internally rescales the lifted black back to Black, and pushes the pulled-down white back to White.
The relative impact on the image in each of the three stages can be seen below. In this instance, the Connection between source and the display is Legal levels, and as the display is set to input Legal levels the screen image is displayed correctly.
Data to Legal Levels
The following diagram shows an alternative signal path to the above, this time with the original image range as Data, which is compressed to Legal levels before being sent to the display. The display is set to input Legal levels, and maps black and white correctly to the screen.
In this configuration the source system outputs Legal levels, rescaled from Data levels, and the display is expecting Legal levels as its input, and internally rescales the lifted black back to Black, and pushes the pulled-down white back to White.
The relative impact on the image in each of the three stages can be seen below. In this instance, the Connection between source and the display is Legal levels, and as the display is set to input Legal levels the screen image is displayed correctly.
Invalid Signal and Display Level Settings
Invalid Signal and Display Level Settings
As above, these signal paths assume an SDI connection. With HDMI, the same errors are often caused by a mismatch between the range actually being output and the range signalled in the AVI InfoFrame.
Data Source Display Clipping
The following signal path shows the most common invalid configuration, where the original image is Data levels, and is maintained at Data levels when sent to the display, but with the display set to input Legal levels, which means the image is stretched when displayed.
The display therefore incorrectly maps black and white, clipping the image on the display's screen.
In this configuration the source system outputs Data levels, but the display is expecting Legal levels as its input, and therefore clips blacks below 16/64, and clips white above 235/940, as it expands 16/64 to black and 235/940 to white.
The relative impact on the image in each of the three stages can be seen below. The clipping on the display screen is obvious.
Legal Source Display Clipping
The following signal path is basically the same as above, but with the original image in Legal levels, which the source system rescales to Data levels before being sent to the display. The display is set to input Legal levels, which means the image is stretched when displayed.
The display therefore incorrectly maps black and white, clipping the image on the display's screen.
In this configuration the source system outputs Data levels, but the display is expecting Legal levels as its input, and therefore clips blacks below 16/64, and clips white above 235/940, as it expands 16/64 to black and 235/940 to white.
The relative impact on the image in each of the three stages can be seen below. The clipping on the display screen is obvious.
Data Source Clipping
The following diagram shows a configuration problem with incorrect matching of the output of the source system signal level, where input Data images are clipped to Legal levels on output, going into a display expecting Legal levels.
The display therefore shows the clipped image.
In this configuration the colour system is outputting TV Legal levels, but the internal source image is full Data range, and is being clipped on output. The display is expecting TV Legal levels as its input, and is therefore displaying the clipped image as it is output from the colour system.
The relative impact on the image in each of the three stages can be seen below. Clipping on the signal connection and the display screen is obvious.
Data Source Display Compressed
The following diagram shows a configuration problem with incorrect matching of the display to the source system output, where the display is expecting Data levels, but the source system is outputting a rescaled Legal level signal.
The display therefore shows a compressed image, with black lifted, and white reduced.
In this configuration the source system outputs Legal levels, rescaled from a Data level image. The display expects Data levels as its input, and is therefore displaying the compressed Legal image, with black lifted, and white reduced.
The relative impact on the image in each of the three stages can be seen below. The lifted black, and reduced white on the display screen is obvious.
Legal Source Display Compressed
The following signal path is basically the same as above, but with the original image in Legal levels, with the display expecting Data levels.
The display therefore shows a compressed image, with black lifted, and white reduced.
In this configuration the source system is operating with Legal levels throughout. The display is expecting Data levels as its input, and is therefore displaying the compressed Legal image, with black lifted, and white reduced.
The relative impact on the image in each of the three stages can be seen below. The lifted black, and reduced white on the display screen is obvious.
