ITU-R Recommendation BT.2020, more commonly known by the abbreviations Rec. 2020 or BT.2020, defines various aspects of UHDTV such as display resolution, frame rate, chroma subsampling, bit depth, and color space. It was posted on the International Telecommunication Union (ITU) website on August 23, 2012.
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 can not 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 digital cinema reference projector 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 allows for RGB and YCbCr signal formats with 4:4:4, 4:2:2, and 4:2:0 chroma subsampling. Rec. 2020 specifies that if a luma (Y') signal is made that it uses the R’G’B’ coefficients 0.2627 for red, 0.6780 for green, and 0.0593 for blue.
Rec. 2020 defines a nonlinear transfer function that can be used for gamma correction with RGB and YCbCr. RGB may be used when the best quality program production is needed. YCbCr may be used when the top priority is compatibility with SDTV/HDTV operating practices and allows for chroma subsampling. The luma and chroma components in YCbCr are calculated from gamma corrected RGB. Rec. 2020 also defines a linear encoded version of YCbCr called YcCbcCrc. YcCbcCrc may be used when the top priority is the most accurate retention of luminance information. The luma and chroma components in YcCbcCrc are calculated from linear RGB and are then gamma corrected.
- 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 a 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.
Rec. 2100 defines the high dynamic range (HDR) formats. The HDR formats are Hybrid Log-Gamma (HLG) which was standardized as ARIB STD-B67 and the Perceptual Quantizer (PQ) which was standardized as SMPTE ST 2084. HDR10 uses PQ, a bit depth of 10-bits, and the Rec. 2020 color space. UHD Phase A defines HLG10 as HLG, a bit depth of 10-bits, and the Rec. 2020 color space and defines PQ10 as PQ, a bit depth of 10-bits, and the Rec. 2020 color space.
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: 50GB with 82 Mbit/s, 66GB with 108 Mbit/s, and 100GB with 128Mbit/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 August 27, 2015, the Consumer Electronics Association announced their definition for HDR compatible displays which would need to be able to process HDR10 Media Profile video which uses the Rec. 2020 color space, SMPTE ST 2084, and a bit depth of 10-bits.
On January 4, 2016, the UHD Alliance announced their specifications for Ultra HD Premium which includes support for the Rec. 2020 color space.
- 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
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