Data vs. Legal TV video 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 DI systems this means understanding how to re-scale the image workflow as required - for example, mapping 0-1023 data levels to 64-940 TV legal levels.

The need to understand this cannot be stressed enough, and this page attempts to address the various problems that can be encountered.

There are two fundamental basics to image levels - Colour systems that will output either full 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 full 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. This means that regardless of the input signal, if it is correctly matched to the display's expected input, black and white will look the same. So, if a display has the option of setting the expected input to 'data' levels, or 'TV legal' levels, when correctly match to the input signal, black and white will look identical.

This is hard to get your head around, but understanding this is critical!

So, the requirement for accurate calibration (and accurate general image display) is to match the output from the colour system to the expected input to the display.

The following examples all use the BarsandBlack Cal Image, which is part of the downloadable Calibration Images.


Signal and Display Level Options

The following diagram shows the most simple of the various configurations.

Data Levels to Data Levels

In this configuration the colour system is outputting full data range levels, and the display is expecting the same as its input.

Data Levels Data Levels Data Levels

The above three images show that no scaling is applied to the image in this configuration, and all is therefore good for calibration.

The following diagram shows the another of the most simple configurations.

TV Legal to TV Legal

In this configuration the colour system is outputting TV Legal levels, rescaled from full data range levels, and the display is expecting TV Legal levels as its input, and is then rescaling the lifted black back to 'black' and the pulled down white is being pushed back to 'white' internally within the display.

Data Levels Rescaled to TV Legal Levels Display scales back to full black and white

The above three images show that no scaling is applied to the image in this configuration, and all is therefore good for calibration.

The following diagram shows a configuration problem with incorrect matching of full range Data output from the colour system to a display that expects TV legal level input.

Data Levels to TV Legal

In this configuration the colour system is outputting full range Data levels, but the display is expecting TV Legal levels as its input, and is therefore clipping the black backs below 16/64, and clipping white above 235/940 as it expands 16/64 to black and 235/940 to white.

Data Levels Data Levels Display clips black and white

The above three images show that as a full range data signal is being exported from the colour system, white the display expects TV legal levels, the image data below 16/64 and above 235/940 is clipped. This is an incorrect configuration for calibration.

The following diagram shows a configuration problem with incorrect matching of the output of the colour system signal level, where internal full range Data images are clipped to TV legal levels on output, going into a display expecting TV legal levels.

Data Levels clipped to TV Legal

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.

Data Levels Colour system clips black and white Display clips black and white

The following diagram shows a configuration problem with incorrect matching of the display to the colour system output, where the display is expecting Data levels, but the colour system is outputting a rescaled TV level signal.

Data Levels rescaled to TV Legal on a Data level display

In this configuration the colour system is outputting rescaled TV legal levels. The display is expecting full range Data levels as its input, and is therefore displaying the image with lifted blacks, and reduced whites.

Data Levels Colour system rescales black and white Display lifts black and reduces white

This is one of the hardest incorrect configurations to spot, as there is no obvious clipping in the image, just lifted blacks, and lowered whites.

The only way to really check for the above error is to measure black with a probe:

With the display turned on, but with no input signal measure the black level. Then send zero black to the display and re-measure. There will likely be 'some' small variation, but not much, if all is correct. If there is a mismatch the variation will be a lot greater.

Hopefully, from these examples you can understand the potential problems, and extrapolate the other possible incorrect configurations.

As has been stressed repeatedly, it is critical to calibration to fully understand the potential problems associated with incorrect Data vs. Legal TV Level setups.