By this point, you probably already know that 4K/Ultra HD is the future of HDTV and that it offers four times the resolution of 1080p. Given how many Ultra HD TVs are expected to hit the market this year, you could argue that 4K is actually the present of HDTV, but I'm not willing to make that jump until I see a bit more 4K content to go along with the displays - perhaps by the end of this year, if companies like Netflix follow through on plans to introduce 4K streaming to a wide audience.
You've likely also heard the debate over whether 4K has any real value at smaller screen sizes, where it is very difficult to discern the step up in resolution from 1080p at the viewing distances from which most people watch their TVs. During such debates, you may have heard a statement along the lines of, "When we get the better color that goes along with that higher resolution, then 4K will really appeal to the masses, at any screen size." We've certainly said as much on our site over the past few months, but what is the "better color" we're talking about? Upon what are we basing this statement?
The answer is the ITU-R Recommendation BT.2020, or Rec 2020 for short. ITU stands for the International Telecommunication Union, and it's one of several organizations that sets the standards upon which the production, broadcast, and display industries rely to ensure that what's being done on the content-creation end is being accurately transmitted and displayed on the TV/projector end. As we've discussed previously, when we measure our displays, we evaluate their accuracy based on standards developed in part by the ITU. When your HDTV is accurately calibrated to ITU color standards, then you can be confident that the colors you're seeing are the same colors that the director approved during the post-production process. (Read our article "How We Evaluate and Measure HDTVs" for more info on this topic.)
In the slideshow below, we've included shots of the CIE 1931 diagram that shows the full color spectrum visible to the human eye, as well as images of different color-space triangles within that spectrum. The Rec 709 triangle shows the color palette that the ITU defined for current high-definition sources; each point of the triangle represents the target for primary red, green, and blue, and all of the colors that lie within the triangle can be displayed within the current standard. Rec 709 was an improvement in color saturation over the previous NTSC standard, a difference you can clearly see when you switch from a standard-def TV source to a high-def one. Now check out the Rec 2020 triangle, and you can see just how much farther the red, blue, and especially green color points extend beyond Rec 709, which means more colors and more lifelike color saturation. The ITU believes that these are colors that the cameras can capture, the system can transmit, and the Ultra HD displays can reproduce.
As a side note, you may notice in your current 1080p HDTV or projector a video option called Color Space or Color Profile that allows you to set different color spaces. Options might include Normal (which should be close to Rec 709), Auto (which should adjust the color space according to the input signal), Adobe RGB, and Wide or Native, among others. A mode like Adobe RGB can be valuable if you are using your display to view content that was specifically shot in the Adobe color space. The Wide or Native mode on newer displays will likely reproduce colors far outside the Rec 709 triangle. A wider color gamut is good, right? That's the goal, right? We should all set our TVs to the Wide mode and enjoy more saturated color, right? Wrong. If you care at all about accuracy, you must understand that the current HD source you're watching was created within the confines of the Rec 709 standard, and exaggerating the color space may make the colors pop and appear more vibrant, but it is not an accurate representation of what the content's creators envisioned. If you don't care about that accuracy and just want your colors dialed to 11, well ... it's your TV.
Rec 2020 also addresses color bit depth, or the number of possible shades of each color. Our current standard is eight-bit, which equals 256 shades per color. That's 256 red x 256 green x 256 blue, for a total of 16.78 million colors. The Rec 2020 standard mandates 10- or 12-bit color: 10-bit offers 1,024 shades per color for a total of 1,073,741,824 colors, while 12-bit offers 4,096 shades per color for a total of 68,719,476,736. Even if your eyes can't see the step up in resolution between 1080p and 4K, almost everyone will be able to see this combination of higher bit depth and wider color gamut.
Beyond issues of color, Rec 2020 defines two specific resolutions: 3,840 x 2,160 (4K) and 7,680 x 4,320 (8K), at frame rates from 24 up to 120 frames per second. The recommendation does away with interlaced content (think 480i DVD or 1080i HDTV) and focuses exclusively on the progressive format. It does not specify a minimum screen size for Ultra HD TVs, but does offer an optimal viewing distance, which is listed at 1.5x the screen height for 4K and 0.75x for 8K. For reference, a 70-inch 16:9 TV has a height of approximately 34 inches, which would equate to a recommended viewing distance of 51 inches (4.25 feet) for 4K and just 25.5 inches (2.1 feet) for 8K.
As its name suggests, Rec 2020 is currently just a recommended standard, and it will likely evolve over time to reflect the realities of the industry, just as Rec 709 did. For us to enjoy the benefits of the standard, it must be adopted all through the chain, from camera to post-production to display. It's certainly not going to happen overnight, but at least it sets a target at which all sides can aim (there is a three-phase roadmap that has the standard fully implemented by the year 2020). Also, it gives you, the shopper, some valuable insight as you consider whether or not to invest in an Ultra HD TV at this early stage. While we encourage you to avoid the aforementioned Native or Wide color space in your 1080p TV right now because it doesn't accurately reflect the current standard, that same mode may be valuable in the new Ultra HD displays to handle Rec 2020's wider gamut when the time comes. This means it is valuable to know just how wide the TV's color gamut can be, which is something we will address in future reviews of Ultra HD displays. Another question that the press has asked of TV manufacturers regards the ability of their newly announced Ultra HD TVs to support at least 10-bit color in order to accommodate the standard, and we've heard a lot of "we're not sure yet" or "we'll get back to you" - this is another subject we will try to address in our individual reviews. If a current display has these abilities, combined with the inclusion of HDMI 2.0 or DisplayPort inputs that support 4K at higher frame rates, it helps to ensure that it is a bit more "future-proof," which should put you more at ease about making the investment at this stage in the game.