In several of my past posts, I’ve referred to Wide Color Gamut (WCG) and indicated that you can’t talk about HDR without also talking about WCG. It’s time for us to discuss that in more detail. As always, this is not intended to be an in-depth discussion – just enough to give you the ammunition you need to understand WCG as it relates to TV, or to give you a launching point for your own research.

DOCUMENTING HUMAN PERCEPTION OF COLOR

Academics have been studying human vision for quite a while, and several systems have been developed to attempt to describe it in precise terms – such as the Munsell color system, which was first published in 1905 and is still in use, in an updated form, today – largely for color matching. Like many other systems, the original “Munsell Atlas Of Color” dealt with relatively few hues, which limited its widespread adoption. The most well-known of these systems today is the CIE 1931 color space, which is the incumbent color space for the film and television industries.

CIE 1931 – THE STANDARD COLOR SPACE

In 1931, the International Commission on Illumination (abbreviated to CIE from the French version of the name), created a pair of color space definitions based on a series of experiments to establish links between perceived colors and their wavelengths in the electromagnetic spectrum – specifically the responsiveness of cone cells (for a more detailed description of cone cells, see my previous post “how we see: the human visual system”). The results were then weighted and tabulated into a “tristimulus graph” of an average person – known as “the CIE Standard Observer” –and is shown in Fig. 1.

Don’t let the term “tristimulus” disturb you – there are many ways of describing light in tristimulus values: R,G,B and even Y,U,V are examples that we use every day. In the case of CIE color space, the terms used are X, Y & Z, which are derived from each of their respective curves in Fig. 1. Without going into huge detail, Y in this system is generated from the y(ƛ) color matching function and is the luminance component. X, not surprisingly, is generated from the x(ƛ) color matching function, and Z is generated from the z(ƛ) color matching function. The amplitudes of all three of these functions are always non-negative and are proportional to intensity.

You should also note that these curves are not definitive – different people will have slightly different distributions than those shown in the Standard Observer chart. In fact, the original research that ultimately produced the CIE color space only used 17 test subjects!

In order to create a graph of the CIE color space, we can now create 2 chromaticity values from these tristimulus parameters as follows:

We now have our X and Y axes for the graph, which is now the well-known “CIE horseshoe” shown in Fig. 2. Two things to note here: (1) the locus (the outside path shown as a black line) indicates the c and g coordinates for a monochromatic source as its wavelength is tuned from 400nm to 700nm, and (2) there is no line at the bottom of the graph. Anything outside of the horseshoe-shaped region is not a visible color.

STANDARD COLOR GAMUT FOR TV

We now have a model that shows the full color spectrum as perceived by the Standard Observer. Practical limitations in technology mean that we can only process and deliver a subset of that color space. In normal HDTV, the transmittable color space was defined by an International Telecommunications Union (ITU) recommendation ITU-R BT.709, more commonly known as “Rec 709”. 

The term for the legal color space is “gamut;” any color that is outside of this defined color space is termed “out of gamut.” What a system or display will do with an out of gamut signal is not defined, though in many cases, it will just result in clamping to the legal limit. Really bad things can happen with seriously out of gamut signals, though, so a great deal of effort is put into keeping signals within the legal values. Fig. 3 shows how much of the CIE space is actually legal in Rec 709.

The dot in the center of the diagram labelled D65 simply details the illuminant declared to be flat white. As you can see, there are large areas of the CIE color space that are outside the range of Rec. 709. This means that there are a significant set of colors that simply can’t be reproduced on an HDTV system. We really need to do better!

WIDE COLOR GAMUT AS IMPLEMENTED IN HDR

Now let’s take a look at the color space that is available through an HDR path. The specification here is BT-R.2020, commonly referred to as “Rec 2020” or simply as “BT.2020”. Fig. 4 shows how that relates to the CIE space.

Clearly, there are many more colors that can be reproduced. In particular the range of red and green colors is substantially increased.

This much wider range of reproduceable colors is a significant factor in the impact that HDR television brings to the presentation of video. Not only are colors more vivid, but they are also more nuanced.

Paul Turner is founder ofTurner Media Consultingand can be reached atpault@turnerconsulting.tv

Other articles in this series:

The Practical Side Of HDR

Candelas And Lumens And Nits – Oh My!!

How We See: The Human Visual System

HDR: What Is It And Why Do We Need It?

LAS VEGAS: There are few set standards for 4K, UHD, high dynamic range, high frame rate or wide color gamut. Each has a particular path from capture to the home through an HDMI cable into a 4KTV set from one of several manufacturers offering a variety of technologies. So how will adoption be achieved?

That’s what four top broadcast engineers take on during the NAB Show in a Tuesday, April 19 at 2:30 p.m. Super Session entitled, “4K, UHD, HDR and More—The Future of Video.” TV Technology’s Deborah D. McAdams will grill video expert Mark Schubin, SMPTE President and CBS Vice President of Engineering and Advanced Technology Robert Seidel, broadcast veteran Jim DeFilippis and Ericsson’s Senior Vice President of Technology for TV & Media Matthew Goldman with crowd-sourced questions from their peers.

Here, Mark Schubin provides a preview.

CROWD MEMBER:What provides the best bang for the buck: 2160p resolution, high dynamic, wide color gamut, or high frame rate (120 fps)?
SCHUBIN: In my opinion, HDR and HFR offer the most bang for the buck. HDR might be easier to implement, but it can increase motion artifacts, bringing us back to HFR.

C.M.:The Canadians are now doing baseball, basketball and hockey in 4K, and distributing it via cable. What is holding up the U.S. broadcasters?
SCHUBIN: They’re more sensible.

C.M.:What are their immediate challenges?
SCHUBIN: Viewers.

C.M.:What about standards? What’s needed?
SCHUBIN: Agreement (as usual in the case of standards).

C.M.:Beyond sports, what other genre will benefit and have a business ROI?
SCHUBIN: HFR is primarily for sports. HDR works on everything; 4K and wider color gamut are tough sells to viewers for anything.

C.M.:Will broadcasters be “forced” into 4K/UHDTV adoption similar to the evolution of HDTV?
SCHUBIN: I don’t think so. There is a much smaller perceptual improvement from HD to 4K than there was from SD to HD.

C.M.:What are the key dominos in the chain that need to be knocked over for the consumers to feel this is real and start spending? Is it availability of “great titles,” consumer devices, branded premium channels, marketing or something else not yet concocted?
SCHUBIN: 4K, again, has very limited perceptual grab. Anyone who doubts that should visit Bob’s demo at CBS with side-by-side giant screens. Wider color gamut sells only in a side-by-side demo; consumers would have to see it in stores that way, and store shopping is on its way out. HDR and HFR are both perceptually significant.
Some fear HFR is perceptually significant in the wrong direction for scripted programming. HDR might be easier to implement, but it depends a lot on the home viewing environment. HFR needs agreement on a bit-rate reduction system (compression).

C.M.:The young adults of today are notorious for consuming social content on small, portable devices. They are the future money for this. Why do they need it?
SCHUBIN: HFR is significant on any size screen at any viewing distance but primarily for fast-moving content. HDR is going to be tough on a handheld screen outdoors. 4K is largely irrelevant on a handheld screen, and WCG probably won’t be noticed. But even young adults are using larger screens for longer programming.

C.M.:How do we as an industry start our advancement to UHD and HDR?
SCHUBIN: Standards.

C.M.:What is the immediate opportunity for a TV group with regard to these technologies?
SCHUBIN: PR.

C.M.:Are we now headed toward 1080p60 with HDR and wide color as a first step, with 4K used in production and on-set display etc.?
SCHUBIN: From your words to the powers that be! Except maybe it’ll be just HDR on regular HD, and forget the 4K.

C.M.:What are your thoughts on HDR HD facilities versus native 4K HDR?
SCHUBIN: HDR works just fine on HD.

C.M.:Is the notion of HDR for HD now officially dead?
SCHUBIN: No one told me!

C.M.:Given the file sizes, do you think we should be significantly compressing in acquisition and post, just as we did in the early stages of HD?
SCHUBIN: For 4K, it’s probably necessary. For the other improvements, no.

C.M.:HDCam, while not perfect, was a pragmatic way of getting to HD and getting material out to audiences. What’s the equivalent for UHD?
SCHUBIN: HD.

C.M.:4K, 8K, UHD, and HDR require a very large network topology to support file transport…
SCHUBIN: I don’t understand that statement. What does “topology” have to do with anything? And the increased data rate for HDR borders on the insignificant; for the Philips version, it’s just 35 bytes per scene (not per frame or per second).

C.M.:…how do we justify that investment when the delivery to the home will not support the transport without heavy compression for years to come?
SCHUBIN: As you can tell from my other answers, I’m not a big fan of a move to 4K, but, if I were, I would point out that, for entropy coding, the sharper the pixels and the less change between pixels, the easier the compression. So raw 4K might be 8x the data rate of HD (1080i or 720p), but the compressed-data-rate increase would be a small fraction of that. Gary Demos showed some compression at the HPA Tech Retreat in February with extraordinary spatial resolution and dynamic range and not a high bitrate.

C.M.:Will 4K become a standard transmission? 
SCHUBIN: Not on my account.

C.M.:What are your views on affordability of transmission methods for 4K, that is, broadcast, broadband and/or satellite?
SCHUBIN: See my comments on “heavy compression” above.

C.M.:Do you think there is benefit to using HDR and/or 4K to author a better HD product that can be delivered to consumers without significant changes to current infrastructure, instead of racing to provide 4K?
SCHUBIN: In the case of HDR, absolutely. Grass Valley likes to show an image of a combined HDR/SDR sports production in which the SDR video controllers are constantly making adjustments (as usual) and the HDR video controllers were leaning back with their hands behind their heads. In general, the more you can offer post, the better. As for 4K, there have been mixed opinions about extracting an HD image from a portion of the frame. Potentially, increased sensor resolution would improve sharpness even for HD.

C.M.:Shooting UHD side-by-side with HD is too costly. How can we reduce the cost of producing sports in UHD and HDR?
SCHUBIN: For 4K, relatively simple downconversion is probably fine. For HDR, many people have been working on some form of algorithmic downconversion; at the moment, human intervention might be necessary (no worse than the job of a video controller).

C.M.:How can we do live production of HDR sports without needing separate “shaders” for an HDR/wide color gamut output and an SDR/normal color gamut output for legacy TVs?
SCHUBIN: See answer above. At worst, it would be one “shader” converting the HDR to SDR.

C.M.:Do you think that high frame rate is suitable for all genres?
SCHUBIN: Tough question! I think it’s great for anything intended to look “live” (sports, news, talk, game shows, concerts, operas, ballets, etc.). For scripted programming, everyone alive today has been watching it largely at 24 or 25 fps, and they are accustomed to that look. Is it just that they are accustomed to it, or is there some psychophysical reason that 24 or 25 fps helps suspend disbelief? I don’t yet know.

C.M.:Given the propensity of advertisers for brighter brights and whiter whites, how do we avoid the CALM Act for brightness?
SCHUBIN: Those helpful algorithms, again! But there is a real issue associated with long-term exposure to very high light levels: retinal “bleaching.” If you’re watching skiing for half an hour at high brightness and change channels to a night scene in a noir movie, it might be like driving into a tunnel at noon on a sunny day.

C.M.:How do we integrate interstitials into programming without brightness and color wars?
SCHUBIN: Those helpful algorithms, again!

C.M.:How well can we produce for both HD and HDR without serious compromises, particularly in graphics or saturated colors?
SCHUBIN: Keep the graphics in the SDR subset of HDR, and don’t sweat the saturated colors. As George Joblove pointed out at the HPA Tech Retreat, we should be dealing with gray as a reference, not black or white.

C.M.:Given the constraints of the television, cable and satellite system, how comparable will the broadcast experience be to the ultra Blu-ray?
SCHUBIN: Could be the same; could even be better.

C.M.:Given the wide disparity of displays and capabilities on the market, how well can we author once and use everywhere, and how much will the user experience vary?
SCHUBIN: Never mind the equipment! If the viewer turns lights on or off or opens or closes drapes, the experience changes. But that’s always been the case. Viewer experiences today vary greatly; that’s why The New Yorker was able to run a cartoon by Joe Dator on Sept. 8 showing a doctor telling a patient that he’d like to see about fixing his aspect ratio. Plenty of viewers are watching stretched people today.

C.M.:As HDR displays get better and do a nice job of presenting SDR-graded content better than it would look on a legacy UHD display, does this challenge the value of specific HDR grading? What percentage of average consumers will appreciate the difference?”
SCHUBIN: I haven’t yet seen HDR upconversion; I have no idea what it might look like.

C.M.:Whenwill lower-end 4K cameras offer servo lenses?
SCHUBIN: Define “lower end.” AJA’s “RovoCam” 4K camera has a built-in servo lens and costs $2,495 including the lens; is that “lower-end” enough?

Also see…
April 16, 2016
“Q&A: Matthew Goldman on 4K, UHD, HDR and More”
“Is it worth the investment to build a native 2160p infrastructure for the gain in user experience? This is what the broadcaster needs to grapple. This question is very different between a Hollywood studio production and live TV production or distribution.”

April 13, 2016
“Q&A: Jim DeFilippis on 4K, UHD, HDR and More”
“We have many standards, but what we are missing is clarity of the problem they are supposed to solve.”