At the SMPTE UHD Symposium in October (which I wrote about here), several presenters, including Dolby, talked about the need for displays with greater dynamic range than current technology provides, especially in terms of higher light output. Last week, Mark Henninger highlighted a story from Engadget that offered a sneak peek at a new high dynamic-range (HDR) display technology from Dolby. Yesterday, I got to see it for myself, and I all I can say is—wow!
Let There Be Light
The presentation began with some information about light levels in the real world as well as from TVs and cinema projectors. In this discussion, the unit of measurement was nits, aka candelas per square meter, rather than foot-lamberts. Why? Because the nit is a metric unit of measurement used around the world, while the foot-lambert is an old-fashioned unit that's falling out of wide usage. Both measure the same thing—the amount of light emitted (or reflected) per unit area—with a simple conversion: 1 fL = 3.42 nits, 1 nit = 0.29 fL. According to Dolby, people involved with emissive, direct-view displays have switched to using nits, while commercial-cinema folks continue to use foot-lamberts.
In the real world, the sun emits over 1 billion nits, while light from the daytime sky is around 1 million nits, and starlight is 0.000001 nit. So the dynamic range from direct sunlight to starlight is about 15 orders of magnitude. On the other hand, a calibrated TV in a dark room has a peak light output of around 100 nits and a black level of perhaps 0.1 nit—not counting the ability to completely shut off the illumination—which yields a total contrast ratio of about 1000:1, or three orders of magnitude. (I've seen plenty of TVs that can reach lower than 0.1 nit, so if we say it's 0.01 nit, that's 10,000:1, or four orders of magnitude.) Commercial-cinema images exhibit about half the dynamic range of calibrated TVs.
What about the human visual system? In total, a person with normal vision is sensitive to a dynamic range of about 12 orders of magnitude—but not all at once. At any given instant, our visual dynamic range is about six orders of magnitude, and that range shifts up and down in response to the overall amount of light in the environment. That's why everything looks so black when you first walk into a dark room from a bright, sunny day, and why your eyes might actually hurt when you emerge from a dark theater into daylight. It takes your eyes some time to adjust, or accommodate, to the new ambient light level.
In an effort to bring display technology into the 21st century, Dolby decided to design for the limits of the human eye, not the limits of a particular technology, citing CRT and film as the limiting technologies on which current display capabilities are based. Neither technology is relevant any more, so why not design a display system that uses the entire range of human visual ability?
To study this issue, the company took a digital-cinema projector and focused it down to a 21-inch screen, then asked a large group of people what they preferred. The results were astonishing—the vast majority of viewers preferred 200 times more brightness than current TVs can deliver (20,000 nits) and 4000 times more contrast (4,000,000:1 with a black level of 0.005 nit, over six orders of magnitude).
Practical display technology can't reach 20,000 or even 10,000 nits, but Dolby has built a demonstration monitor based on its PRM4200 professional reference monitor that can output 4000 nits with a black level of 0.005 nit for a dynamic range of 800,000:1, almost six orders of magnitude. In the actual demo, we got to see the demonstration monitor side by side with a PRM4200 (both 1080p panels) in a completely blacked-out room displaying the same content, which had been graded separately for each display and sent as RGB 4:4:4 with 12 bits per color. The content included two segments shot by Dolby on ARRI Alexa digital cameras and some footage from Samsara, which was shot on 70mm film.
The PRM4200 was calibrated to 100 nits peak white and 0.005 nits black, which is a more-than-respectable dynamic range of 20,000:1. The PRM4200's color gamut was set to Rec.709, while the demonstration monitor displayed a color gamut conforming to the digital-cinema standard called P3, which is based on film emulsion; this gamut is larger than Rec.709 but smaller than Rec.2020. Both displays use full-array backlighting with RGB LEDs, and each LED is independently dimmable. The PRM4200 has 4500 LEDs (1500 for each color), while the demonstration monitor has 18,000 (6000 per color).
It is impossible to accurately capture the difference Dolby's new imaging technology makes using regular photography; it needs to be seen live. But here's a taste of the difference between a display using the new technology (left) and standard HD video (right). (Photo: Thom Brekke; image provided by Dolby Laboratories)
First, we were shown the content on the PRM4200 by itself, and it looked great—until the demonstration monitor was activated, at which point the PRM4200 looked totally washed out and dull. Of course, the demonstration monitor was much brighter, but it was not painful on the eyes at all. Instead, the entire image looked much more realistic, including greater detail in the bright and dark areas (often within the same shot), brighter highlights such as an arc welder and reflections of the sun from a shiny aluminum airplane, and richer colors, due in part to the wider color gamut but also because that gamut remained wider throughout more of the brightness range. In current TVs, the gamut often shrinks as the brightness increases, causing colorists to have to desaturate the colors in bright scenes.
In order to preclude compromises like that—and many others that must be made to accommodate current display technology—Dolby is proposing a new standard to SMPTE (the Society of Motion Picture and Television Engineers). The proposal includes a dynamic range from 10,000 nits down to true 0 with 12 bits per color.
You might think that even 12 bits isn't enough to avoid banding with a range from 0 to 10,000 nits, and you'd be right with a gamma-based system. Gamma is an exponential function that defines how a display responds to different levels of brightness in the signal; it was derived from how CRTs work, and it's still being emulated in today's displays, even though they don't need it. Part of Dolby's proposal is a new perceptual-coding system based on how humans see rather than gamma, and it is said to achieve the same smoothness of gradations as a 14-bit gamma-based system.
Another hallmark of the Dolby proposal is a signal format that remains the same for all display technologies, which would automatically adapt the signal for their particular capabilities. This is accomplished using a layered-coding approach, which adds things like extended dynamic range to the existing standard, making it backward-compatible. As a result, the content would need to be prepared, or "graded," only once rather than many times for different media as it is now. The studios already archive their content in the highest possible quality, so it would be relatively easy to transition to the new system.
It should be noted that many current TVs can produce more than 100 nits, especially in their "torch" mode—Dolby measured one Sharp Quattron model at 1000 nits. But virtually all consumer content is color graded for 100 nits, so reproducing it brighter only stretches—and thus distorts—the picture information as it's delivered today.
HDR is part of Dolby's effort to deliver better pictures, a goal characterized by three pillars—more pixels (UHD resolution), faster pixels (higher frame rates), and better pixels (greater dynamic range, more colors). Of course, all of this requires more bandwidth; UHD has four times as much pixel data as 1080p, and according to Dolby, HDR using its perceptual coding needs 25% more bandwidth at any resolution. New high-efficiency codecs like HEVC can cut the current data rate in half, and we will certainly see even greater gains in efficiency in the future.
As we've seen with UHD, TV manufacturers are rushing headlong toward more pixels, but not faster and better pixels, though Dolby believes that these technologies are finally ready to come together. In fact, greater dynamic range has much more impact on picture quality than resolution—if you're too far from the display to see the increased resolution of UHD, you can still clearly see the improvement of HDR.
Also, Dolby wants to make it very clear that it will not be manufacturing consumer TVs. Instead, it intends to license the technology to other companies, and company reps tell me that several OEM partners are planning to showcase displays with the new imaging capabilities at CES next month. All I can say is—I can't wait!