High Dynamic Range and HEVC Update
This blog presents a overview of the “High Dynamic Range” capability being proposed in some quarters as one of the enhancements to be supported by future Ultra High Definition (UHD) displays and video sources/services. Also included in this blog is an update on the update to the High Efficiency Video Coding (HEVC) standard, which defines one of the critical UHD related technologies.
Update on HEVC Standard
This standard defines the High Efficiency Video Coding (i.e, HEVC codec) which is being used by several of the already announced 4K/UHD video services. The HEVC standard is being defined by a joint committee of ITU (International Telecommunications Union) and the ISO (International Standards Organization). These are two international standards bodies that frequently work together to produce joint standards. HEVC as defined by the ITU document that carries the nomenclature “Recommendation H.265″. HEVC will likely be specified as the codec for the upcoming Blu-ray UHD discs and perhaps also for future satellite and over-the-air broadcast UHD services. The H.265/HEVC standard, as approved about 1.5 years ago, defines an initial “profile” for HEVC that supports coding for 4K/UHD video. This existing profile supports the coding of movies in UHD resolution, with a bit depth of 8-bits, using the color (chroma) 4;2:0 subsampling scheme and at frame rates of only 24 and 30 frames per second. Thus, while the current version of HEVC defines the coding of the UHD resolution, this HEVC Main Profile only supports what are essentially the lowest color fidelity, lowest dynamic range and tradition movie frame rates. While these limitations may be considered perfectly acceptable for use with limited bit-rate UHD streaming video services, they are not considered by many to be desired for the higher fidelity video delivery formats, such as the upcoming Blu-ray UHD discs.
The joint ITU-ISO committee has been working on an amendment to the HEVC standard that will add a number of significant extensions for higher quality UHD video services. At the beginning of 2013 it was stated that the goal was to have this amendment approved by Jan. 2014. However, the amendment is still under development, and it now appears that perhaps the final technical and editorial details will be finalized at a meeting of the ITU/ISO “Joint Collaborative Team on Video Coding” (JCT-VC) that is convening for 10 days starting on June 30th.
High Dynamic Range
As the consumer electronic industry attempts to push forward with Ultra High Definition (UHD) TV, many within the broader entertainment industry have realized that other factors may offer potential benefits that may be just a obvious to the viewer as the increase in resolution and some may argue they are even more obvious, especially on typical flat panel TV sizes. Dolby Laboratories is one company pushing for certain of those ‘other’ enhancements. More specifically, Dolby has been conducting demos, and appears to be generating support within certain of the movie studios and consumer electronics manufacturers, for supporting video sources and displays with ‘High Dynamic Range’ (HDR). Dolby has formalized their specific approach under the trademarked name “Dolby Vision”. Dolby foresees future displays with increased dynamic range for both image brightness and color.
Brightness – HDR
It appears the primary focus of HDR is to provide an image that has high brightness on those picture elements that would appear very bright in nature, such as the reflection of sunlight off of a reflective surface while also providing very low black levels including providing excellent image details in the darkest areas of the image. Most of the discussions about HDR that I have seen on the web has been focused on image brightness, or luminance, expressed in the units of “nits”. The luminance numeric value is sometimes alternatively expressed in the units of cd/m2 which is equivalent to nits, so for example 100 nits the is same level of luminance as 100 cd/m2. Those of us working with projection systems typically use a measurement of lumens to express the light output from a projector and foot-lamberts (FL) as the luminance units for the image brightness from the projection screen. So we first need to understand how to relate a brightness figure expressed in nits to an image brightness in FL. A brightness of 1 nit = 0.29186 TL thus when it is said that a LCD/LED flat panel TV produces a 100 nit image that equates to having a 29 FL luminance level from a projection screen. In a fully light controlled home theater environment the most commonly recommended luminance level is around 16 FL, which is more aligned with that found in commercial cinemas than the higher luminance levels produced by most LCD/LED HDTVs.
A typical flat panel LCD/LED HDTV today will produce up to around 500 nits in vivid or dynamic mode, but perhaps no more than half that value once adjusted for a more accurate image. Professionally calibrated monitors used by the TV and movie studios typically produce a 100 nit image. Dolby would like to see the peak luminance increased to a several thousand nits for providing a more lifelike display of the brightest image elements. Dolby has used a display for their Dobly Vision HDR demos that has a extremely bright 4,000 nits, however for the next few years we will probably not see any consumer HD or UHD display with this extreme level of light output.
Another aspect of HDR calls for the display to handle black and near-black levels accurately without loss of image details. This relates to the display having a high contrast ratio (CR), which is the ratio of the reference/max white luminance level to the level of the reference black luminance level. From published information it appears that Dolby would ideally like to see a CR of 100,000:1, or higher. Commercial movie theaters have a rather poor CR as compared to what is available in most home theaters. The typical commercial theater only has a on/off CR of about 2000:1. Commercial theaters are limited by both the required lighting for exit signs, walkways and steps and also by the commercial cinema projector’s having a CR typically limited to 2000:1 to 3000:1 . Even with a higher contrast projector, the ultimate CR is limited by the inability to have a totally dark theater (due to the required safety lighting).
Compare this to best home theaters that may have an on/off CR of 50,000:1 or even greater. The LCD flat panels used in today’s better LCD/LED flat panels TVs typically have a native CR typically limited to perhaps 2000:1 while the much higher CRs values claimed for these TV are obtained by the dynamic dimming of the LED backlight. In a similar fashion, many home theater projectors employ a dynamic iris to achieve high on/of CR numbers. JVC, with their LCoS display technology, provides the highest native on/off CR values (i.e., without using a dynamic iris or lamp dimming), which in their best models provides over 50,000:1 with the manual iris fully open and over 100,000:1 with the iris stopped down (but with reduce light output from the projector in this latter mode). Sony’s best home theater projectors, also using LCoS display technology, have a native CR of perhaps 20,000:1 while the best DLP and LCD projectors have a native CR in the range of 5,000:1 up to perhaps as high 8,000:1. Note all of the above estimates are for on/off CR where a full maximum brightness white image is first displayed and its luminance value is measured followed by a luminance measurement when a full black image is displayed.
There is a second type of CR value frequently used, but seldom included in manufacturer’s spec. sheets, that represents the ratio of the brightest white level to the darkest black level that can be displayed within the same displayed image. One typical measurement for this is to display a black and white checkerboard test pattern and to then measure the ratio of the luminance value in the white squares as compared to the value black squares. This is the approach used for ‘ANSI’ CR measurements (ANSI is a standards organization that has published a specific test for this). Flat panel TVs have an advantage over projection systems for this performance parameter since light reflections within the projector itself and light dispersion within the projector’s optical elements will cause light from the white areas of the image to leak into black areas of the image. The higher-end home theater projectors will typically have an ANSI CR value of 300:1 to perhaps as high as 800:1. The very best performing plasma flat panel TVs achieve ANSI CR values of several thousand to 1, while the best LCD/LED flat panel TVs, i.e., those that use multi-zone local dimming, can also achieve high ANSI contrast ratios but are limited by the physical size of the local LED illuminated zones.
In order to support the increased dynamic range of image brightness, there will need to be an increase in bit-depth for the encoded video. Today’s consumer video sources use digitally encoded video with 8-bit depth capturing just 220 shades of grey (using digital levels 16 thru 235). Today’s better professional digital cinema cameras can capture a up to 14-bit depth and one recent model can capture a 16-bit depth. Providing consumers with video sources with 12-bit and greater bit depths will be needed to realize the full benefits of HDR.
Color – HDR
Going to 12-bit, 14-bit or 16-bit depth will not only provide for many more shades of grey, as mentioned above, but will also allow for many more shades to be reproduced for each family of colors. The Dolby proposal calls for an expanded color gamut. As I have discussed in a previous blog (HERE) the current HD standard (i.e., ITU rec. 709) has a rather limited color gamut, with limited saturation levels for the colors, while the standard proposed by ITU for UHD TV (ITU rec. 2020) has a much wider color gamut. Note that no current UHD TV nor projector supports the wide color gamut defined by ITU rec. 2020. The standard used by the digital cinema industry has a color gamut that falls between these two consumer standards. Thus the Dolby proposal would have displays capable of displaying greater bit depth images and with a greater color gamut that current HD or UHD consumer TVs or projectors are capable of providing.
Bottom Line for HDR
So what does all of this mean for the consumer?
In a recently published article (link HERE) written by Mike Rockwell, Executive Vice President, Advanced Technology Group, Dolby Laboratories, he stated:
“Our goals for Dolby Vision are ambitious. We’re working with television manufacturers to get Dolby Vision enabled TVs onto store shelves by the end of 2014. We’ve also developed the tools that content creators need to produce images that take advantage of all the power of Dolby Vision. And we’ve created technology that will help conventional broadcasters and streaming video providers get those images to your home with no loss in quality.”
HDR is being proposed by some for use with HD and/or 4K/UHD video but it appears that some industry members are specifically wanting to at least get some features of proposals for HDR incorporated into the upcoming UHD services and products. For flat panel HD and UHD TVs Dolby is said to be working with Sharp, TCL and Vizio on future products that will support at least some of the enhanced capabilities being discussed under the Dolby Vision banner.
For projection systems, either for home theaters or commercial cinemas, producing the peak brightness values along the high ANSI CR values envisioned by Dolby Vision would be a real technical challenge. Perhaps future projectors using laser light sources combined with improved optics will be able to offer some improvements in peak light output along with improved ANSI CR, but they are unlikely to reach performance levels envisioned in the current Dolby Vision proposal. However, projection technology does have the opportunity in the near term to provided the ability for displaying images with increased bit depth along with an increased color gamut. Note these two latter aspects of the Dolby Vision proposal are similar to those already being considered within some industry standards bodies that are working on UHD related technologies and services. Since it appears from the above quoted statement from Mike Rockwell, that Dolby Vision is an evolving concept and perhaps Dolby, or some other industry organization, will turn their attention to what can be accomplished with projection technology.