COLOR SPACE ISSUES ON VIDEO DISPLAYS

Incorrect Playback Issues

All video displays are RBG devices. If an RGB signal is delivered to a display, the display’s processing electronics have very little work to do. Aside from a computer source, or in Hollywood, where production workflows may be RGB after being converted from RAW camera files, content delivered to consumers from popular video delivery media such as streaming, cable, or disc devices is invariably in Y’CbCr component form with chroma downsampled to 4:2:0.  

When a component video signal is delivered to a contemporary video display, the display ultimately performs color space conversion into RGB. Suppose the incoming signal consists of chroma subsampling at 4:2:0. In that case, the conversion process dictates the video be converted from 4:2:0 to 4:2:2, then to 4:4:4, and finally, RGB, where it is then sent to the display’s image process controller for formatting (depending on the display technology). 

If chroma upsampling is selectable on the source device, it may be possible to output 4:2:2, where the display will only be required to scale Cb and Cr to create a 4:4:4 image for conversion to RGB. If chroma upsampling is output selectable at 4:4:4, no scaling is necessary; only the conversion to RGB is required. If the display is sent an RGB signal, all conversion steps are bypassed, and the signal is sent directly to the display’s image processing controller. 

Where conversion takes place should be determined by what device performs color conversion best. The design of flagship flat panels or projectors likely dictates these products incorporate the best signal processing (and most accurate conversion process) a manufacturer can offer. Therefore, consideration must be given to whether a $99 streaming device that provides chroma upsampling adjustment performs this better than the $4,999 premium flat panel. 

A side issue discussed elsewhere is why send the most complex bandwidth-intensive signal down the pipeline to the display, only to have the display do little to no work converting it to RGB (unless the source is outputting RGB)?

This article mentions the above for reference purposes. Its purpose is to recognize conversion errors when they occur. While all displays are designed to automatically identify the color space of the incoming signal, menu adjustments that force conversion rather than allow the display to decide may result in the display incorrectly displaying the incoming signal. 

If the incoming signal is from a game console and is in RGB, but a menu adjustment on the display forces all signals to be displayed in Y’CbCr, the image will be pinkish/greenish (see Fig.1).

Fig. 1

Similarly, image discoloration will result if a source device menu adjustment forces output at Y’CbCr but the display is adjusted for only RGB signals (see Fig. 2). The overall Hue of the image results in a purplish cast, with darker colors and black appearing as greenish.

Fig. 2

While a natural inclination is to suspect device, infrastructure, or display failure, check all menu adjustments first. If the problem persists, best practice is to default all devices and the display to their original factory settings.

While rare when dealing with a newly installed display that shows no outward sign of damage, image discoloration could be from an internal failure, such as the TCON (timing control) board. This is a common issue on displays that have seen years of use. Occasionally, a failed HDMI cable may prevent a color channel from passing. 

Another best practice is to have test gear capable of troubleshooting the issue. A Murideo Fox & Hound kit will help determine the problem in minutes.

              

Technical History and Additional Background Information

Content from sources to today’s video displays is delivered using one of two color models (mathematical representations of a defined range of colors). At the onset of the development of HD television in the late 1980s, the  Consultative Committee on International Radio (CCIR) made the formal technical recommendation for a stated goal of a worldwide HDTV standard designed to overcome the challenges of various formats in different global regions. Their proposal, REC. 709 was superseded by the International Telecommunications Union, the international governing body of the United Nations for electronic signal transmission and broadcast, in 1992, releasing ITU-R BT.709-1. 

In August 2012, the ITU announced the next generation of broadcast television specifications, ITU-R BT.2020, also in use today.

 

BT.709 and BT.2020 use the same illuminant D65 white point (x = 0.3127, y = 0.3290). BT.709 produces 35.9% of all colors the human eye can perceive and is mastered at 100 nits peak luminance. BT.2020 more than doubles this to 75.8% and allows for ten times the luminance, up to 1,000 nits (the threshold target for mastering…some displays are capable of light output nearing 4,000 nits). 

You may read about a Hollywood production color space called DCI-Protocol 3 or DCI-P3. This is only presented to the consumer theatrically and is not used in any other media. It originally had a different, green-leaning white point (x = 0.134, y = 0.351) to accommodate theatrical film projectors using xenon-arc lamps. In early 2024, Digital Cinema Initiatives approved a consumer gamut workflow white point to the D65 x and y points, called DCI-P3-D65 (and Apple’s Display P3). However, it is rarely referred to as that, with the D65 often omitted. However, the color primaries remain at the DCI-P3 theatrical points, with Y at 300 nits, to align with digital production standards. If this color space is chosen to view content rendered in SDR BT.709, colors will be exaggerated and more saturated than the content creator intended due to the color space mismatch. 

DCI-P3 produces 53.6% of human-perceived color.

 

Using the same white point primaries as BT.2020, the Society of Motion Picture and Television Engineers (SMPTE) in 2014 adopted a Dolby development known as the perceptual quantizer (PQ) and standardized it as a transfer function replacing the fixed gamma curve used in standard dynamic range (SDR) as SMPTE ST 2084. The ITU adopted BT.2100 in 2016, representing a high dynamic range via PQ for streaming and movies and HLG for broadcast TV. BT.2100 uses the same white point as BT.2020 but is future-forward, accommodating luminance levels up to 10,000 nits. 

SDR content is delivered by cable, streaming, and DVD/Blu-ray/UHD Disc as component video (YCbCr) 4:2:0 (though most DVD players chroma upsampled to 4:2:2 to overcome limitations in early player color MPEG conversion). This compression is virtually imperceptible and was chosen as the most efficient means of delivering content with no apparent color fidelity loss. Though technically superior, using 4:2:2 or 4:4:4 would only consume bandwidth and not provide a substantial improvement tradeoff in color depth, saturation, or accuracy. 

So, to say it again, SDR content is delivered in 4:2:0, with some devices, such as Apple TV, able to perform chroma upsampling onboard. This burdens system infrastructure by the device doing the “heavy lifting” the display would generally do, needlessly forcing a higher bandwidth signal down the pipeline and typically not performing the chroma upsampling with the same degree of accuracy.