One of the most exciting developments in home entertainment is the rapid adoption of 4K HDR by TV makers and viewers. HDR’s enhanced contrast and rich colors deliver an obvious improvement to the home viewing experience. However, not all TVs have what it takes to properly reproduce today’s HDR content. Color volume capability is a crucial consideration.
HDR stands for high dynamic range, and TVs that support it must be able to reproduce deep blacks as well as bright highlights to convey the extended contrast delivered by the format. But HDR is about more than just contrast; richer color is part of the package.
Content mastered in HDR (whether it’s on Ultra HD Blu-ray, Netflix, Amazon, Vudu or YouTube) typically uses a color palette called DCI/P3—it comes from commercial cinema. Unlike the color standard used for SDR (standard dynamic range) video, DCI/P3 properly handles intense colors. Think of the red of a Coca Cola can, or the blue of a perfectly clear sky.
To present HDR content, displays such as quantum dot-based QLED and LED-phosphor LCD TVs use a technique called local dimming to improve black levels and overall contrast. And while the deep blacks of transmissive TVs are not “infinite,” they can get much brighter than emissive counterparts (OLEDs and plasmas), and can do so over a significantly larger part of the screen. Importantly, they do so using RGB sub-pixels.
In consumer OLEDs, on the other hand, color volume suffers, because that type of display relies on a white sub-pixel to achieve peak brightness. Despite exceptional native contrast ratios, the technology has limits in terms of how bright it can get.
The result is that with bright scenes, colors lose their intensity. Consequently, these displays do not deliver the viewing experience the director intended. Plus, even 2017 OLEDs have a hard time displaying the 1000-nit highlights that most HDR is mastered to, and can only achieve it in a tiny area and for a limited time.
During numerous demos over the past year, I’ve seen QLED and LED-phosphor HDR TVs from Samsung, Sony, and Vizio in side-by-side comparisons with OLEDs. On every occasion, I noted that the better-looking image depended on what was showing. When dealing with HDR scenes full of vibrant colors, the benefits of a display with higher brightness and superior color volume becomes readily apparent.
So, what is “color volume”? Technically, it’s a 3D model of the maximum color-reproduction abilities of a TV at all brightness levels. This can be represented as the 3-dimensional shape shown below.
The TV represented in the graph on the left can reproduce DCI P3, even at high luminance levels. The TV on the right cannot.
In contrast, the more familiar term “color gamut” refers the range of color that a TV can reproduce at one single predetermined brightness level, typically 75 or 80% of the peak. This is commonly represented by the CIE diagram showing a triangle defined by the red, green and blue points at that brightness.
On the left is the DCI/P3 gamut expressed in a 3D color volume diagram. On the right is a 2D color gamut chart showing BT.709, DCI/P3, and BT.2020 gamuts.
Going forward, it’s likely manufacturers will place a greater focus on the HDR capabilities of TVs—and in particular, on color volume.
The days when emissive displays like plasma and OLED were unquestionably superior to transmissive displays like QLED and LED-phosphor models are gone. Today, finding the best TV for the job requires weighing various factors, including cost, image size and viewing habits.
For example, if you love watching movies in total darkness, OLEDs remain appealing. But, if you are not ultra-rich and still desire something larger than a 65″ picture, an HDR LCD TV suddenly looks a lot more attractive.
Another key point is most consumers watch TV with some amount of ambient light in the room, whether it’s daylight from windows, or various forms of indoor lighting. The greater color volume, brightness capabilities, low screen reflectance of HDR LCDs give the picture that extra oomph needed to make imagery look punchy and “pop.” The result is a more dynamic viewing experience.
Ultimately, a choosing a TV that offers great color volume performance matters because it will deliver the experience the artist or director intended.