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The UHD-1 Format – 4K at Home
Ultra high definition (UHD-1) is a broadcast video format that contains four times more pixels than a broadcast high definition (HD) frame (3840 x 2160 pixels vs. 1920 x 1080 pixels). Unlike the current two HD broadcast formats (1920 x 1080i 50/60 and 1280 x 720p 50), pictures are no longer transmitted as fields in interlaced scanning mode but rather in progressive mode at 50 or 60 frames per second (3840 x 2160p 50/60).
UHD-1 is commonly also known as 4K, but this term originates from the fields of professional production and cinema, where digital projection requires a resolution of 4096 x 2160 pixels (so about 4000 pixels horizontally) and a refresh rate of 24 frames per second for standard films or 48 frames per second for high frame rate (HFR).
High Dynamic Range (HDR)
Many Ultra high definition televisions will include HDR functionality, which provide a more realistic and immersive viewing experience. This is achieved by increasing the colour space representation with wide colour gamut and increasing the range of luminosity.
Wide colour gamut ITU-R Rec. 2020 (also called BT. 2020) defines the screen resolution, refresh rate, chroma subsampling, colour depth, and colour space that are used for the various UHD formats.
Colour depth refers to the number of luminance or chroma components per pixel. UHD TV broadcast increases the colour depth to 10 bits (1024 shades) for luminance (Y) and for the two colour difference signals (Cb and Cr) because otherwise the transitions would be visible if only 8 bits (256 shades) were used on the latest UHD TV displays. This effect is known as banding or posterisation.
Rec. 2020 provides for enhancement up to 12 bits (4096 shades). In contrast, SD and HDTV use a colour depth of only 8 bits.
The colour space defines the displayable or visible colours in a colour model. Both of the figures below show colour space CIE1931 with the positions of the primary colours, while the triangle represents the colours that can be displayed. Rec. 601 represents standard definition (SD) and Rec. 709 high definition (HD).
The white point is defined in line with D65 and corresponds to about 6500 Kelvin, or average daylight.
As a result, the colour space from Rec. 2020 can display colours that cannot be displayed with Rec. 601/709, and it covers about 75.8 % of the CIE1931 colour space as compared to 35.9 % for Rec. 601/709.
Increased Dynamic Range of Luminosity
Besides colour rendition, contrast is also critical for HDR to allow dark and bright details to be seen in the picture that would otherwise be invisible. As shown in the following figure, HDR-ready TVs can achieve darker black levels and brighter white levels, making details visible that would normally disappear in a grey or white picture area on legacy TVs.
Modern TVs no longer use CRTs and so allow the use of different EOT functions for recording, post processing and playback with better rendition and utilisation of the displayable colour space and dynamic range. The standard gamma curve can display up to 7 f-stops (100 %). Modern CMOS image converters on the recording side on the other hand can capture a dynamic range of up to 14 f-stops (1300 %). However, clipping in the bright picture areas can become a problem in this situation. A number of different EOTFs have therefore been developed for different camera and display combinations and applications.
One group of these EOTFs, based on hyper-gamma curves (HG), have a dynamic range of up to 800 % and are already widely used in digital cinematography.
The class of S-Log curves (Sony) can display up to 14 f-stops, which corresponds to about 1300 % of the dynamic range of the standard gamma curve.
Dolby’s perceptual quantiser (PQ) curve was defined for the DolbyVision HDR process. A dynamic range of up to 10 000 nits should be possible with a colour depth of 10 bits to 12 bits. This curve takes advantage of the fact that the eye cannot perceive details in very light areas.
HDR Metadata Standards
The following documents describe the HDR extensions for the entire broadcast chain:
SMPTE ST 2084 – High Dynamic Range ElectroOptical Transfer Function of Mastering Reference Displays
SMPTE ST 2085 – Color Differencing for High Luminance and Wide Color Gamut Images
SMPTE ST 2086 – Mastering Display Color Volume Metadata Supporting High Luminance and Wide Color Gamut Images
SMPTE ST 2094 – Content-Dependent Metadata for Colour Volume Transformation of High Luminance and Wide Colour Gamut Image