ITU-R Recommendation BT.2020, more commonly known by the abbreviations Rec. 2020 or BT.2020, defines various aspects of ultra-high-definition television (UHDTV) with standard dynamic range (SDR) and wide color gamut (WCG), including picture resolutions, frame rates with progressive scan, bit depths, color primaries, RGB and luma-chroma color representations, chroma subsamplings, and an opto-electronic transfer function. The first version of Rec. 2020 was posted on the International Telecommunication Union (ITU) website on August 23, 2012, and two further editions have been published since then. It is expanded in several ways by Rec. 2100.
Rec. 2020 defines a bit depth of either 10-bits per sample or 12-bits per sample.
10-bits per sample Rec. 2020 uses video levels where the black level is defined as code 64 and the nominal peak is defined as code 940. Codes 0–3 and 1,020–1,023 are used for the timing reference. Codes 4 through 63 provide video data below the black level while codes 941 through 1,019 provide video data above the nominal peak.
12-bits per sample Rec. 2020 uses video levels where the black level is defined as code 256 and the nominal peak is defined as code 3760. Codes 0–15 and 4,080–4,095 are used for the timing reference. Codes 16 through 255 provide video data below the black level while codes 3,761 through 4,079 provide video data above the nominal peak.
|Color space||White point||Primary colors|
The Rec. 2020 (UHDTV/UHD-1/UHD-2) color space can reproduce colors that cannot be shown with the Rec. 709 (HDTV) color space. The RGB primaries used by Rec. 2020 are equivalent to monochromatic light sources on the CIE 1931 spectral locus. The wavelength of the Rec. 2020 primary colors is 630 nm for the red primary color, 532 nm for the green primary color, and 467 nm for the blue primary color. In coverage of the CIE 1931 color space the Rec. 2020 color space covers 75.8%, the DCI-P3 digital cinema color space covers 53.6%, the Adobe RGB color space covers 52.1%, and the Rec. 709 color space covers 35.9%.
During the development of the Rec. 2020 color space it was decided that it would use real colors, instead of imaginary colors, so that it would be possible to show the Rec. 2020 color space on a display without the need for conversion circuitry. Since a larger color space increases the difference between colors an increase of 1-bit per sample is needed for Rec. 2020 to equal or exceed the color precision of Rec. 709.
The NHK measured contrast sensitivity for the Rec. 2020 color space using Barten's equation which had previously been used to determine the bit depth for digital cinema. 11-bits per sample for the Rec. 2020 color space is below the visual modulation threshold, the ability to discern a one value difference in luminance, for the entire luminance range. The NHK is planning for their UHDTV system, Super Hi-Vision, to use 12-bits per sample RGB.
Rec. 2020 defines a nonlinear transfer function for gamma correction that is the same nonlinear transfer function that is used by Rec. 709, except that its parameters are given with higher precision:
- where E is the signal proportional to camera-input light intensity and E′ is the corresponding nonlinear signal
- where α ≈ 1.09929682680944 and β ≈ 0.018053968510807 (values chosen to achieve a continuous function with a continuous slope)
The standard says that for practical purposes, the following values of α and β can be used:
- α = 1.099 and β = 0.018 for 10-bits per sample system (the values given in Rec. 709)
- α = 1.0993 and β = 0.0181 for 12-bits per sample system
While the Rec. 2020 transfer function can be used for encoding, it is expected that most productions will use a reference monitor that has an appearance of using Gamma 2.4 transfer function as defined in Rec. ITU-R BT.1886 and that the reference monitor will be evaluated as defined in Rec. ITU-R BT.2035.
RGB and luma-chroma formats
Rec. 2020 allows for RGB and luma-chroma signal formats with 4:4:4 full-resolution sampling and luma-chroma signal formats with 4:2:2 and 4:2:0 chroma subsampling. It supports two types of luma-chroma signals, called YCbCr and YcCbcCrc.
YCbCr may be used when the top priority is compatibility with existing SDTV and HDTV operating practices. The luma (Y′) signal for YCbCr is calculated as the weighted average Y′ = KR⋅R′ + (1−KR−KB)⋅G′ + KB⋅B′, using the gamma-corrected RGB values (denoted R′G′B′) and the weighting coefficients KR = 0.2627, KG = 0.678 and KB = 0.0593. As in similar schemes, the chroma components in YCbCr are calculated as C′B = 2⋅(B′−Y′)/(1−KB) and C′R = 2⋅(R′−Y′)/(1−KR), and for digital representation the Y′, C′B, and C′R signals are scaled, offset by constants, and rounded to integers.
The YcCbcCrc scheme is a "constant luminance" luma-chroma representation. YcCbcCrc may be used when the top priority is the most accurate retention of luminance information. The luma component in YcCbcCrc is calculated using the same coefficient values as for YCbCr, but it is calculated from linear RGB and then gamma corrected, rather than being calculated from gamma-corrected R′G′B′. The chroma components in YcCbcCrc are calculated from the Y′, B′, and R′ signals with equations that depend on the range of values of B′−Y′ and R′−Y′.
HDMI 2.0 supports the Rec. 2020 color space. HDMI 2.0 can transmit 12-bit per sample RGB at a resolution of 2160p and a frame rate of 24/25/30 fps or it can transmit 12-bits per sample 4:2:2/4:2:0 YCbCr at a resolution of 2160p and a frame rate of 50/60 fps.
The Rec. 2020 color space is supported by H.264/MPEG-4 AVC and H.265/High Efficiency Video Coding (HEVC). The Main 10 profile in HEVC was added based on proposal JCTVC-K0109 which proposed that a 10-bit profile be added to HEVC for consumer applications. The proposal stated that this was to allow for improved video quality and to support the Rec. 2020 color space that will be used by UHDTV.
On May 22, 2014, Nanosys announced that using a quantum dot enhancement film (QDEF) a current LCD TV was modified so that it could cover 91% of the Rec. 2020 color space. Nanosys engineers believe that with improved LCD color filters it is possible to make a LCD that covers 97% of the Rec. 2020 color space.
On September 5, 2014, the Blu-ray Disc Association revealed that the future 4K Blu-ray Disc format will support 4K UHD (3840x2160 resolution) video at frame rates up to 60 frames per second. The standard will encode videos under the High Efficiency Video Coding standard. 4K Blu-ray Discs will support both a higher dynamic range by increasing the color depth to 10-bit per color, and a greater color gamut by using the Rec. 2020 color space. The 4K-Blu-ray specification allows for three disc sizes, each with their own data rate: 50 GB with 82 Mbit/s, 66 GB with 108 Mbit/s, and 100 GB with 128 Mbit/s. The first Ultra HD Blu-ray titles were officially released from four studios on March 1, 2016.
On November 7, 2014, DivX developers announced that DivX265 version 1.4.21 has added support for the Main 10 profile of HEVC and the Rec. 2020 color space.
On December 22, 2014, Avid Technology released an update for Media Composer that added support for 4K resolution, the Rec. 2020 color space, and a bit rate of up to 3,730 Mbit/s with the DNxHD codec.
On January 6, 2015, the MHL Consortium announced the release of the superMHL specification which will support 8K resolution at 120 fps, 48-bit video, the Rec. 2020 color space, high dynamic range support, a 32-pin reversible superMHL connector, and power charging of up to 40 watts.
On May 26, 2015, the NHK announced a 4K LCD with a laser diode backlight that covers 98% of the Rec. 2020 color space. The NHK stated that at the time it was announced this 4K LCD has the widest color gamut of any display in the world.
On June 17, 2015, Digital Projection International presented a 4K LED projector with support for the Rec. 2020 color space.
On January 4, 2016, the UHD Alliance announced their specifications for Ultra HD Premium which includes support for the Rec. 2020 color space.
Rec. 2100 is an ITU-R Recommendation released in July 2016 that defines high dynamic range (HDR) formats for both HDTV 1080p and 4K/8K UHDTV resolutions. These formats use the same color primaries as Rec. 2020, but with different transfer functions for HDR use. Rec. 2100 includes two such transfer function definitions that may be used for HDR:
- Perceptual Quantizer (PQ), which was previously standardized as SMPTE ST 2084, and
- Hybrid Log-Gamma (HLG), which was previously standardized as ARIB STD-B67.
The PQ scheme with 10 bits of color bit depth has also been called HDR10. Similarly, the HLG scheme with 10 bits of color bit depth has been called HLG10. The Ultra HD Forum guidelines for UHD Phase A include support for SDR formats with 10 bits of color bit depth based on both Rec. 709 and Rec. 2020 color gamuts and also both the HDR10 and HLG10 formats of Rec. 2100.
In addition to defining RGB and YCbCr color representations that are the same as in Rec. 2020 except for the transfer functions, Rec. 2100 also defines a constant luminance scheme known as ICtCp. Rec. 2100 does not support the YcCbcCrc scheme of Rec. 2020.
- UHDTV – Digital video formats with resolutions of 4K (3840 × 2160) and 8K (7680 × 4320)
- High Efficiency Video Coding (HEVC) – Video standard that supports 4K/8K UHDTV and resolutions up to 8192 × 4320
- Rec. 709 – ITU-R Recommendation for HDTV
- Rec. 601 – ITU-R Recommendation for SDTV
- Rec. 2100 – ITU-R Recommendation for HDR HDTV and UHD
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