Data vs. TV Levels

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.


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.

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.

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.)

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.

If used within a PC environment, with a graphics card HDMI connection to the display, the associated EDID will alter the image scaling automatically, making understanding of EDID rather important in any signal path. See the HDMI User Guide for further info.


Valid Signal and Display Level Settings

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 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.

Data to Data Levels
Source Data-Data Levels
Connection
Connection Data levels
Data to Data Levels
Display Data Levels

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 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.

Legal to Data Levels
Source Legal-Data Levels
Connection
Connection Data levels
Data to Data Levels
Display Data 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 deliverable. This is also the default signal path for all home TV configurations, as all broadcast and disc based media is delivered in Legal levels.

Source Legal-Legal Levels
Source Legal-Legal Levels
Connection Legal Levels
Connection Legal Levels
Display Legal Levels
Display Legal levels

In this configuration the Legal levels are used throughout, and the display is expecting Legal levels as its input, and rescales the lifted black back to Black and the pulled down white is pushed back to White, internally within the display.

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 displays 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, and 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.

Source Data-Legal Levels
Source Data-Legal Levels
Connection Legal Levels
Connection Legal Levels
Display Legal Levels
Display Legal levels

In this configuration the source system outputs Legal levels, rescaled from Data levels, and the display is expecting Legal levels as its input, and rescales the lifted black back to Black and the pulled down white is pushed back to White, internally within the display.

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 displays 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

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 display's screen.

Data to Data Levels
Source Data-Data Levels
Connection
Connection Data levels
Data to Data Levels
Display Data Levels

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 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 display's screen.

Legal to Data Levels
Source Legal-Data Levels
Connection
Connection Data levels
Data to Data Levels
Display Data Levels

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.

Data to Data Levels
Source Data-Data Levels
Connection
Connection Data levels
Data to Data Levels
Display Data Levels

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.

Source Data-Legal Levels
Source Data-Legal Levels
Connection Legal Levels
Connection Legal Levels
Data to Data Levels
Display Data Levels

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.

Source Legal-Legal Levels
Source Legal-Legal Levels
Connection Legal Levels
Connection Legal Levels
Data to Data Levels
Display Data Levels

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.