Ultra-HD Video Standards Move Forward
In several past blogs I have discussed international/industry standards that relate to the next generation of Ultra High Definition Television (UHDTV) devices (i.e., displays, video sources & video distribution). Specifically, in past blogs I have discussed (HERE and HERE) the next generation HDMI (i.e., version 2.0) and also the new High Efficiency Video Codec (HEVC) defined by the ITU-T Recommendation h.265 (discussed HERE and HERE). I have also previously discussed (HERE) the potential for a next generation Blu-ray Disc standard that would support 4K UHD resolution. For this new blog I am discussing another “recommendation” (i.e., standard) already published by the International Telecommunications Union (ITU) that defines the “Parameter values for ultra-high definition television systems for production and international programme exchange.”
The only consumer 4K video projector currently available is the excellent Sony VPL-VW1000ES. This projector, pictured below, was announced at the CEDIA Expo back in September 2011 and began shipping in early 2012. While that model was consistent with the video standards available at that time and it appears Sony attempted to anticipate some future developments, there has been significant progress made by the international standards community since Sony introduced that model in 2011 and future 4K projectors conforming to those more recent standards for Ultra-HD TV are expected to support certain video formats beyond those currently supported by the VPL-VW1000ES. However, Sony may be able to add support for certain additional capabilities via firmware updates, or possible even hardware upgrades.
This relatively new “Recommendation ITU-R BT.2020”, was released in August 2012, by the ITU “Radio Communications sector” (hence the ITU-R label) as a document in their “Broadcast Service (television)” series of recommendations. The ITU is a standards organization charted by the United Nations (i.e., government supported) and their standards/recommendations are frequently developed in coordination with the industry-supported International Standards Organization (ISO).
BT.2020 can be though of as setting the top-level technical framework for consumer UHDTV services. I use the term “consumer UHDTV” to distinguish this from the digital cinema 4K related standards. This ITU recommendation covers both the consumer version of 4K video as well as the even higher resolution 8K video. The ITU recommendation is intended to be applicable to both UHD program production and to delivery services, including possible future over-the-air UHD broadcast services.
The UHDTV service defined by BT.2020 specifies video with an aspect ratio of 16 x 9 (i.e., 1.78:1), which is the same as for today’s HDTV services. The 4K UHDTV service is defined to use 3840 x 2160 pixels which is exactly twice the horizontal and twice the vertical resolution (i.e., pixel count) of today’s 1080p HDTV. In the case of both HDTV and UHDTV formats, the pixels themselves are square in shape. BT.2020 also defines an 8K version of UHDTV that has four times the horizontal and four times the vertical resolution of 1080p HDTV with 7680 x 4320 pixels. By retaining the 16 x 9 aspect ratio of today’s HDTV and, to a lesser extent using exact multiples of 1080p’s resolution, the UHD format simplifies the accommodation of HDTV video sources/programs on UHDTV displays.
Note UHDTV, as defined by BT.2020, differs in both aspect ratio and resolution from what is used with commercial 4K digital cinema, where the slightly wider aspect ratio is 1.9:1 is used and the image is composed of 4096 x 2160 pixels.
While the most obvious difference between HDTV and UHDTV may be the increased resolution of UHDTV, there are a number of other technical improvements defined by BT.2020 that will result in higher quality images. The following paragraphs summarize each of these improvements and provides some brief background information on each.
Note: The codec to be used for the transmission or distribution of UHD programming is not defined by BT.2020. ITU has published Rec. h.265 which defines HEVC as one possible codec for coding of UHD video (plus audio). However, that standard currently only covers 2D video and lower frame rates for 4K UHD video. An update to h.265 is currently under development which is expected to add support for both 3D video and higher frame rates and this is expected to be issued as an amendment in early 2014.
The increase in image resolution enabled with UHDTV will only be of obvious benefit to the viewer if the screen size is large enough and/or the viewing distance is short enough. At the same time that BT.2020 was issued, ITU-R also released report BT.224601, titled “The present state of ultra high definition television.” Among other information included in this report are recommendations for the optimum viewing distance for video of various resolutions. These recommendations are based on persons with normal 20/20 vision (or visual acuity of 1.0). For a given screen size the viewer much be seated closer to the screen in order to achieve the full benefits of increased resolution. This also will result in a more immersive viewing experience since the image will now fill a larger portion of the viewer’s total visual field of view. The following chart compares the optimum viewing distance vs. screen size (diagonal size of a 16 x 9 aspect ratio screen) for both 1080p HDTV and for 4K (3840 x 2160) UHDTV based on the recommendations from ITU-R BT.2246-1. The recommended viewing distances shown in the chart below represent approximately how close to the screen you would need to be in order to be able to perceive all of the image details capable of being supported with that specific video resolution (either 1080P or 4K UHD). ITU has based their recommendations on a 32 degree horizontal viewing angle for 1080p video and a much wider 58 degree viewing angle for 4K UHD video. This assumes the video source, as well as the display, supports the full resolution possible with that specific video format. Note that a 1080p source video that has been upscaled for display on a 4K UHD display would not support the recommended shorter viewing distance shown on the chart for 4K UHD as that curve on the chart is for a native 4K UHD video source viewed on a native 4K UHD display. However, with excellent upscaling of 1080p source material for display at 4K UHD resolution, a seating distance of somewhat less than that shown for 1080p displays should be acceptable.
It should be pointed out just how close to the screen you would really need to be seated in order to get the full benefits of 4K UHD. For example, with a 120 inch 16 x 9 screen the optimal viewing distance (using the ITU recommendation) is approximately 15 ft. for a HD 1080p display but would be only about one half that distance for viewing a 4K UHD display. Of course that doesn’t mean you would have to sit this close to see some benefits in resolution from a 4K UHD display. For example, at a 10 ft. viewing distance from a 120 inch screen there would still be a perceived increase in resolution, as compared to 1080p, but with normal eyesight you would not be experiencing all of the benefits possible from the increased resolution of UHD. The bottom line is when you consider upgrading your home theater from a 1080p projection system you may want to consider moving up to a larger screen size and/or decreasing your viewing distance. However, trade-offs in the seating arrangements in your home theater will probably need to be made since for many years to come much of the available source material will be limited to 1080p resolution.
Note that some other industry sources have offered viewing distance recommendations for HD and/or UHD that differ a little from those summarized above from ITU.
HIGHER FRAME RATES
The current HDTV standards have a maximum frame rate of 60 Hz (i.e., 60 frames per second) for 720p video sources while 1080p video sources are generally limited to 24 Hz (1080p/24), or 30 Hz in a few cases. Note that while Blu-ray Disc players offer a 1080p/60 output mode, the source material recorded on the disc itself is 24 Hz. for virtually all movies. The recommendations of BT.2020 include support for frame rates up to 120 Hz for UHD video. The 4K UHD video sources and displays we are most likely to see come to the consumer marketplace within the next few years may be limited to maximum frame rates of 60 Hz for 4K UHD video. That being said remember that virtually all existing movies where created at a 24 Hz frame rate (the only exception is “The Hobbit” which was shot at 48 Hz.). Therefore, we can expect the see the vast majority of UHD video distributed with a 24 Hz. frame rate. Looking into the future it is possible that 4K UHD content delivered on optical discs or perhaps via over-the-air or satellite broadcast services may be able to support the higher frame rates for certain programming (e.g., sporting events or the occasional high frame rate movie).
INCREASED COLOR SPACE
The HD video standard (ITU Rec. 709) specifies a more limited color space that what has been specified for UHD by BT.2020. This difference in the colorimetry standard means that increased saturation of the displayed colors will be offered with UHD as compared to HD. The figure below shows (solid line) the color space specified for HD in ITU Rec. 709 vs. the expanded color space specified for UHD in BT.2020.
The technical details of how the video chroma (i.e. color) and luminance information is represented in digital form will impact the quality of the resulting displayed image.
The current HD standards, used for both broadcast and recorded media, limit the encoded video to 8-bit coding and with a digital range limited to 16 to 235 for encoding of the video levels for each of the three primary colors. This limits the maximum number of different colors and levels that can be represented to approximately eleven million values. While this may sound like a lot, it is really is a limitation of the current HD system and is less capable that the number of colors that can be encoded by the digital cinema standard or by many digital cameras. The UHD standard, defined by BT.2020, specifies that UHD will use either 10-bit or 12-bit encoding. 10-bit coding increases the total number of color and levels that can be represented to approx. one billion. This increased bit depth will reduce the occurrence of color banding sometimes seen on HD displays when displaying certain video scenes and may allow for UHD to provide improved shadow detail to be displayed with some source material.
Another important technical difference between the coding of HD vs. UHD that will impact the quality of the displayed video is how the chroma (i.e., color) information encoded. With HD source material (i.e., broadcast or Blu-ray Disc) chroma subsampling (Wiki article is HERE) is used where the color information contained in a block of 4 pixels (2 vertical by 2 horizontal) is combined into a single (e.g., averaged) value. This results in a 50% reduction both the vertical and horizontal chroma resolution. This is referred to as 4:2:0 chroma subsampling. While UHD, as defined by BT.2020, continues to allow 4:2:0 subsampling as an option, it also allows the use of subsampling using the higher fidelity 4:2:2 format (i.e., combines chroma information from 2 horizontal pixels) and also it allows the use of the highest fidelity 4:4:4 format (i.e., retains full chroma resolution).
THE BOTTOM LINE
UHD video sources and displays that are capable to supporting the technical features recommended by ITU will not only provide for a substantial improvement in image resolution but will also support improvements in a number of other areas effecting image quality.