Wide color gamut TVs are capable of producing colors that are outside of the Rec. 709 standard. In the TV market, this is generally meant to apply to TVs that have a color gamut capability that includes the 709 standard as well as additional colors outside of this standard.

The Status of Wide Color Gamut UHDTVs

Chris Chinnock | Insight Media

Introduction

UHD TVs with a resolution of 3840x2160 are now common in the market place and will soon surpass sales of 1080p TVs.  But the roll out of UHD TVs and UHD content will not be uniform.  There are several aspects to improved image quality besides more pixels.  These enhancements, producing what are sometimes called “better pixels”, include:

  • Wider Color Gamut (WCG)
  • High Dynamic Range (HDR)
  • Higher Frame Rate (HFR)

TVs and content will be offered that feature various combinations of the above features.  And, these features have variations within them that offer different levels of performance.  It is a complex matrix of content offering and display capabilities that is sure to be very confusing to the end user.

In this white paper, we will focus on the improvements that wide color gamut will bring to new content and to TVs that have this capability.  Since WCG will also be coupled to HDR in many cases, we need to discuss this to some degree as well.  HFR will not be discussed.

The report will discuss what wide color gamut is, the challenges to creating and delivering wide color gamut content to TVs, what platforms are now ready to deliver this content and what TVs are ready to receive and display WCG content.  In addition, we will dive into the various approaches to creating WCG TVs as there are several methods with various trade-offs.

Market research suggests that WCG displays are desired by consumers.  According to a recent IHS Technology forecast, WCG flat-panel displays – from smartphones to TVs - will grow from 2.8% of the display market in 2015 to 25% by 2020 in terms of display area shipped.  In 2015, TV displays will constitute 52% of the WCG market, rising to 86% in 2020.

 

Figure 1: Wide Color Gamut Flat-Panel Forecast for 2015 by Technology (Source: IHS) Quantum Dot technology will be adopted strongly by TV makers.  According to IHS, the market share of the QD solution will grow from less than 1% in 2015 to more than 9% in 2020, driving the growth of the wide color gamut display market.

IHS also published a forecast on High Dynamic Range (HDR) TVs.  As we will see, wide color gamut and high dynamic range will be closely coupled in content and display.  This forecast offers unit shipments of around 2.9 M sets in 2016 growing to about 32.6M sets.  Adoption really starts to take off in 2017 with China taking the lead on unit sales and Japan running up a high percentage of sales over the forecast period.

 

Figure 2: IHS HDR Forecast by Region

Color and Color Gamuts

What is Wide Color Gamut?

Color science is a complicated topic that we will attempt to simplify here (with apologies to the color scientists).  Color science must describe:

  • The naturally-occurring colors in the world
  • The total volume of colors (natural and artificially generated) - What our eyes can see

In addition, there are aspects of color engineering that were developed to help capture color images, prepare them for distribution to end users and to present them on electronic displays.  These aspects are captured in standards for:

  • Color encoding
  • Color sub-sampling 
  • Color gamut

The presentation of color images on display devices is done with a number of technologies.  The choices of these technologies have an impact on how the human eye and brain perceives these images.

Color Volume

Color is best described as a three-dimensional volume.  But most of the time, color is discussed as a region of a two-dimensional plane.  2D charts describe the color performance of the display at a single luminance (brightness) level, but not all of the luminance levels the display can create, and the color performance may differ with luminance; it can be hard to maintain a very saturated color at low luminance, for example.  Thus luminance is the third dimension in the color volume representation of color.

Figure 3 shows a common 2D method for showing the color capabilities of the display - the 1976 CIE u’v’ chromaticity diagram.  The horseshoe shaped area of the diagram represents all the colors that the human eye can see.  Interestingly, nature does not create colors to fill this horseshoe.  The irregularly shaped circle (Pointer’s Gamut) represents the surface of natural colors in the world.  Artificial light sources like neon, LED lights, and laser sources have colors that extend beyond the surface colors boundary.  In addition, color can be computer-generated as cartoons and graphics that can be anywhere in the horseshoe – even extending to its edges (the spectral locus).

 

Figure 3: Surface Colors (Pointer’s Gamut) within the 1976 CIE Chromaticity Diagram

 

Figure 4: The u'v' Chromaticity Diagram

The 2D version of color representation using u’v’ coordinates (Figure 3 & Figure 4) was developed so that the perceived distance between two color points was more linear and consistent versus the 1931 version that used x,y coordinates. 

But the best way to describe all possible perceived colors is a three-dimensional coordinate system.  Many different coordinate system exists, one of which is CIELUV.  Figure 5 shows the color volume of the Rec. 709 color standard, which is the one used for HDTV.  The horizontal axes are x and y chromaticity coordinates while the vertical axis is luminance or brightness (Y). The left graphic shows Rec. 709 as a 2D representation, while the other two show two different views of the 3D representation.

 

 

Figure 5: Color Volume of the Rec. 709 Standard (Source: Dolby)

 

 

Figure 6: Comparison of the Display Color Volume (wire grid) vs. the rec 709 Color Gamut

Color volumes are often used to compare the volume of a color standard to what a particular device can display.  Figure 6 shows typical comparison with the monitor color volume represented by the wire grid and the ITU-BT 709 (Rec 709) standard represented by the solid volume.  Note that the monitor’s color space completely envelops the 709 standard meaning it can display all of those colors in the 709 standard.  (If you rotated this color volume around so that you could see “the back of it”, you would find the wire grid continued to always enclose the solid volume.)  The color volume of this monitor is 176% of the 709 volume.  However, note that this measurement does not say how accurately the monitor displays the incoming colors – only that the monitor has the capability of displaying them accurately.  That is why calibration is needed.

Also note that at low luminance levels (the bottom of the shapes) and high luminance levels (top), the color volume is smaller than it is at the middle.  That is why the 2D xy or u’v’ representations are not as accurate as color volume.  Most likely, any marketing materials’ specification of a device’s color gamut using a 2D metric will choose a luminance level that yields the biggest horizontal cross section of the volume – which will only be accurate for that single luminance level!

To illustrate this point consider Figure 7.  The orange color of the volcano lava is shown in a 2D and color volume representation.  The 2D representation suggests that the color is within the Rec. 709 gamut.  The 3D representation shows that this point is actually outside of the Rec. 709 gamut.  The reason is that the luminance value is not considered in the 2D representation.  As noted earlier, the 2D values are the best case which occurs at lower luminance values.  For the fairly bright lava content, the luminance level now places it outside of the Rec. 709 color volume.  How this is rendered depends completely on the capabilities of the display and color processing.

 

 

Figure 7: Misleading Use of 2D Color Gamut Representation (Source: Dolby)

Color Standards

There are many color standards that have been developed.  These standards are needed to provide an agreed-upon set of colors to master content to so that it can be faithfully recreated on a display.

 

 

Figure 8: Various 2D Color Standards using u’v’ Coordinates

The sRGB color space is the primary standard used in the majority of input devices, such as cameras and scanners, as well as display devices such as computer monitors, printers, and smartphones.  The sRGB color space uses the RGB primaries (not shown in Figure 8), with a specific gamma curve to determine the steps between gray levels. Most Internet content is rendered using the sRGB color space. 

The HDTV specification for color is represented by ITU-R BT.709, sometime called simply 709 or Rec. 709 (Figure 8).  This is typically referenced in televisions and home theater projectors, and uses the same RGB primaries as sRGB but a different gamma curve.  In other words, a comparison of series of red blocks with increasing levels of brightness between sRGB and Rec. 709 will show many identical levels, with only some slight differences in the darker regions.

The DCI (P3) color space is used in digital cinema, while the ITU-R BT.2020 standard is the new color space for Ultra-High Definition TVs.  Rec 2020 is the largest color space being used for video content today.  It does the best job of showing all of the colors available in nature, plus many additional colors (neon lights, LED lights, computer generated colors, etc.).  The largest triangle in Figure 8 represents the coverage of BT.2020, showing Pointer’s gamut for reference as well.

The white point of the display is that combination of red, green and blue light that creates white light.  But white light has a “color temperature”.  Movie and TV content is typically graded and should be displayed using a D65 white point.  This is supposed to have a relationship to the white light from a blackbody radiator at 6500 degrees Kelvin (although it is more complicated than that).  

Almost all TVs ship with a default mode white point color temperature of 9,000 to 11,000 degrees Kelvin.  These displays will appear distinctly bluish in their white point.   This setting is mostly about selling the TVs.  In a showroom environment for example, the whites look brighter and crisper than a set tuned to movie/TV mastering standard of 6500 Kelvin (D65).  But for home TV, a D65 white point is needed for accurate color rendition as the content is mastered on a display with a D65 white point and specific color standard (709 in the case of HDTV).  That is why most TVs also have a “cinema” or “movie” mode that tunes the white point to D65 and should offer the closest color gamut to Rec. 709 it can.

Wide color gamut TVs are capable of producing colors that are outside of the Rec. 709 standard.   In the TV market, this is generally meant to apply to TVs that have a color gamut capability that includes the 709 standard as well as additional colors outside of this standard. 

 

 

Here is the table of contents showing whats in the rest of this paper.
It's a free download from Insight Media

 

Table of Contents

Introduction......................................... 5 

Color and Color Gamuts...................... 6 

What is Wide Color Gamut?........................ 6

Color Volume..................................................... 7

Color Standards................................................ 10

Benefits of Wide Color Gamut.................. 11

Additional and More Saturated Colors............... 11

Coupling with High Dynamic Range (HDR)...... 13

An Ideal HDR/WCG Display?........................... 16

Challenges with Wide Color Gamut.......... 17

Bit Depth.......................................................... 17

Tolerance on 2020 Primaries............................. 19

Five HDR/WCG Techniques to Consider........... 20

HDR/WCG Content........................... 22

HDR/WCG Content Creation.................... 22

Issues with HDR/WCG Production.................... 22

Getting Studios to Master to 2020...................... 23

Accessing HDR/WCG Content.................. 24

Ultra HD Blu-ray.............................................. 24

Over-the-Top.................................................... 27

Over the Air...................................................... 28

Cable................................................................ 29

Satellite............................................................ 29

IPTV................................................................ 30

Game Consoles................................................. 31

Playback of HDR/WCG Content............... 31

Playback on HDR/WCG TVs.................... 32

Wide Color Gamut Display Technologies..................................... 35

LCD Display System Considerations......... 36

Phosphor Film............................................ 37

Quantum Dots............................................ 38

Quantum Dot Technology................................. 38

Quantum Dot Film............................................ 39

Quantum Dot Edge Optic.................................. 41

Adjusting Color Filtering........................... 43

LEDs.......................................................... 43

Direct View RGB LEDs.................................... 43

RGB LEDs in Projectors................................... 44

Phosphor-Enhanced LEDs................................. 45

QD-Phosphor Hybrid........................................ 47

OLED......................................................... 48

Emerging Technologies............................. 50

QD-LEDs......................................................... 50

Hybrid and Multi-Primary................................. 51

Summary of Current Wide-Color-Capable TVs...................................... 54

 

Interested in the rest of the paper?
It's a free download from Insight Media


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