High dynamic range (HDR) displays are designed to provide a more vivid picture quality compared to standard dynamic range (SDR) displays – with greater brightness and contrast levels as well as a wider range of colors. Think of “darker” black tones and “brighter” white tones.
Within the past several years, HDR displays have gained significant traction within the consumer television market. Yet even today when consumers buy an HDR TV, they have no idea what performance level they are getting.
There seems to be nearly as many HDR logos and standards as there are brands. No brand is sharing details of their HDR performance. There is no fully transparent testing methodology nor fully public performance metrics. Typically, what consumers see is just a marketing sticker on a device with no real meaning behind it.
While the PC industry lagged behind the TV industry in adopting HDR, it has experienced a surge in HDR display product introductions during the past twelve months.
To help avoid a similar fate of confusion around HDR, the Video Electronics Standards Association (VESA) – the organization behind DisplayPort, Display Stream Compression and other electronics display standards – established a new tiered HDR compliance test standard in December 2017, called DisplayHDR. It provides a very clear and easy to understand logo system representing different performance levels for both laptop and desktop PC displays.
DisplayHDR defines eight key characteristics surrounding HDR display performance, including: black and white levels (which combined gives you a contrast ratio), bit depth and color gamut, as well as the performance and speed of the device’s response moving from black to white. Features that are unrelated to HDR performance, such as resolution, audio and aspect ratio, were excluded from the DisplayHDR specification.
A true ecosystem effort
Over two dozen companies including major OEMs that make displays, graphic cards and CPUs, have been involved in the development of the DisplayHDR specification. To date, several leading manufacturers have developed PC display products to these standards – nearly two dozen officially certified thus far – with more products expected to roll out in time for CES 2019.
The standard is fully open and publicly available for download from the DisplayHDR website (www.DisplayHDR.org). In addition, the website provides information about how to test the standard, the purpose for each of the tests, even the mathematical models for how to build the test files to create test routines if one is so inclined.
Putting DisplayHDR to the test
Creating a clearly defined standard and logo system to represent HDR performance that is easy for consumers to understand is an important step forward in improving the consumer experience with HDR PC displays. However, some consumers want to have the ability to verify the performance of their HDR displays and not simply accept the promise of HDR performance provided by the display maker.
Recognizing those concerns, VESA collaborated with Microsoft to launch an automated test app earlier this year that can be used to help automate the testing procedure for validating DisplayHDR performance of a new display.
The DisplayHDR test tool is free and can be downloaded through the Microsoft app store or the DisplayHDR website (www.DisplayHDR.org). Running on a Windows 10 platform, the DisplayHDR Test app projects patterns defined in the published VESA DisplayHDR certification tests onto the screen. The user then points a consumer-grade colorimeter (which must be purchased separately by the user) at the test patterns and uses the colorimeter’s software to measure and record the display’s capabilities. These results can be compared to the required performance values for each of the DisplayHDR logo tiers.
Breaking down HDR performance tiers
DisplayHDR currently encompasses three performance levels: DisplayHDR 400, 600 and 1000.
While the numbers in each tier correspond to peak luminance levels (a measure of brightness also known as nit, measured as candelas per square meter, or cd/m2 ), they are defined and distinguished by many other characteristics, as shown in Figure 1.
For example, not only must the displays achieve their 400, 600 or 1000 luminance levels for a 10% patch on the screen for a long testing duration, but the screen must also be able to deliver a brief full-screen flash at this same luminance level. For a gamer, or for someone watching explosions or flashes of light in movies, this provides for an incredibly realistic output – something that most other displays simply are not designed to achieve.
Display HDR Performance Tiers (Source: VESA)
The baseline performance tier DisplayHDR 400 is an entry-level tier designed to provide a measurable improvement in performance compared to SDR displays without a significant increase in cost, to help facilitate mass adoption of HDR.
Displays that are DisplayHDR 400 compliant are typically around 50 percent brighter than most SDR displays on the market today. DisplayHDR 400 displays are also true 8-bit displays – many with 10-bit simulation – versus the 6-bit with 8-bit simulation, which is common on SDR displays. All DisplayHDR certified displays are required to support HDR10 input.
Besides the 50 percent increase in luminance level between the DisplayHDR 400 and DisplayHDR 600 performance tiers, the most significant difference is that the specification requirements dictate a contrast ratio that is 6x higher at the 600 level than at the 400 level.
To achieve this much higher contrast ratio, all displays that have been certified at the DisplayHDR 600 level have been built using local dimming. This means that the backlight on the LCD panel is split into small segments that can be independently controlled.
Often, the end result is a display that exceeds the contrast ratio of DisplayHDR 400 by much more than 6x. In addition to the luminance and contrast increases, the DisplayHDR 600 tier is the first tier that pushes the color gamut beyond the typical SDR color gamut of sRGB/Rec.709 and into the DCI-P3 color gamut, requiring a minimum of 90 percent DCI-P3 gamut coverage.
The DisplayHDR 1000 performance tier represents the current state of the art in PC displays. Beyond the significant and immediately visible luminance difference of 1000 versus 600, the specification imposes higher full screen luminance requirements upon these displays.
Furthermore, the contrast ratio requirement at the 1000 level is double that of the 600 tier. Many display manufacturers have consequentially chosen to increase the number of local dimming zones to achieve this increased contrast ratio, since local dimming performance is improved as the number of zones is increased.
Based on the marketing message around HDR performance characteristics found in the TV market, some consumers may consider 1000 nits to be the minimum brightness level required for HDR PC displays and that anything less is not truly HDR. They might be surprised to learn that many HDR TVs would not even pass the DisplayHDR 600 spec.
Additionally, it is important to recognize that HDR PC and HDR TV displays are not viewed in the same way, and thus a comparison strictly based on brightness level is insufficient.
For example, the ideal viewing distance for display devices at 4K resolution is only 1.5X the diagonal screen size1 . For a monitor, this is in the sweet spot of where users actually sit. Unfortunately for TVs, users are almost always seated much more than 1.5X the diagonal screen size away from the screen and thus cannot fully benefit from the contrast and detail that high resolution screens provide.
Moving beyond LCD displays
When the DisplayHDR standard was being developed, more than 99.5 percent of PC displays on the market were based on LCD technology. As such, the first release of DisplayHDR was optimized for LCD panels.
Since then, newer display technologies have made further progress in development and, in some cases, are now being introduced into the PC market. Anticipating this development, VESA began work on a new DisplayHDR specification for OLED display technologies earlier this year. This new spec is currently planned for roll out before the end of Q1 2019.
Summary
HDR delivers significantly improved video quality, including more vivid colors and higher contrast and brightness, compared to standard displays for the PC market.
Yet, for all of the hype surrounding HDR, it is still not well understood by consumers. PC makers and consumers alike need to have a comparable standard to judge HDR picture performance between monitors.
With the DisplayHDR specification from VESA, PC makers finally have consistent, measurable HDR performance parameters with which to develop their products, while consumers have an HDR rating number and logo system that is meaningful and reflects actual performance.
More than two dozen companies representing market leaders from across the electronics supply chain have come together to establish this shared, open HDR standard for the PC industry that ensures all of the key display components involved in enabling HDR work together.
With several dozen PC display products having already achieved DisplayHDR certified, and new PC display products being certified every month, consumers will have many DisplayHDR certified products to choose from in time for the upcoming holiday season.
footnote: 1 S. Kindig. TV sizes and viewing distance: how to choose the right screen for your room. Crutchfield .
— Roland Wooster is a principal engineer and a display and platform technologist in the Client Architecture and Innovation Group at Intel. He is also chairman of the VESA Display Performance Metrics Group, responsible for leading VESA’s development of the DisplayHDR standard for HDR display-hardware performance.
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