What is HDR ?

HDR stands for High Dynamic Range.

By opposition to SDR (Standard Dynamic Range), HDR refers to a set of techniques and standards that allow a wider range of brightness and color : HDR videos can display more vibrant colors and better contrast, providing a more realistic and immersive viewing experience.

The cameras and displays electronics are design in a way that video systems can represent only a restricted sample of the variety of colors and of the range of brightness that illuminates the real-life world and that the human eye is able to perceive.

Originally, most aspects of video norms were linked to the capabilities and the behavior of CRT displays, and that is where SDR video dynamics formulas and characteristics come from.

Then, with the evolution of video display technologies, several vendors – or consortiums of vendors – moved forward in parallel on the definition and the implementation of different HDR ecosystem. As a result, nowadays different HDR standards co-exist, each one with its specificities, pros and cons. Popular HDR standards include HDR10, Dolby Vision, and HLG.

Some improvements are common to all HDR systems :

• Brightness : HDR allows brightness upper than 1000 nits and lower than 1 nit
• Color gamut : HDR usually adopts P3 or Rec.2020 color gamut, while SDR uses Rec.709. That allows representing a set of colors closer to what the human eye can see. One talks about WCG – Wide Color Gamut
• Color depth : HDR is generally sampled in 10-bit and 12-bit color depth, while SDR is usually conveyed in 8-bit or sometimes 10-bit. The bit-depth directly define the amount of different colors that a system can represent :
  • An 8-bit video system can handle 2⁸ = 256 shades per component, meaning 16 millions of colors in total
  • A 10-bit video system handle 2¹⁰ = 1024 shades per component, meaning 1 billion of colors in total
  • A 12-bit video system raises it to 2¹² = 4096 shades per component, meaning 68 billion of colors in total

As mentioned, different HDR formats co-exist in the industry, with different characteristics and capabilities. They allow brightness ranges wider than what televisions or projectors can display, hence most HDR format also make use of metadata that give useful information like the average scene brightness and the maximum pixel brightness of the video feed, so that the display devices can implement algorithms to map that maximum brightness to the highest shade the display equipment can produce.

Some standards provide single brightness information for the complete video feed by the way of so called “static metadata”. Some other HDR formats provide such information scene by scene in the form of “dynamic metadata”.

Here follow short descriptions of the most commonly found HDR systems :

HDR10

HDR10 is an open-standard, royalty-free, HDR system. It is hence the most widespread format.

It is more a list of recommendations than a rigid set of strict rules, hence HDR10 implementations can vary from manufacturer to manufacturer.

HDR10 requires 10-bit video sampling, and the use of the Rec.2020 color space. It uses static metadata.

Dolby Vision

Dolby Vision is a proprietary and licensed technology, supported by premium devices and based on dynamic metadata.

Also using the Rec.2020 color space, it extends sampling depth to 12-bit and allows higher brightness (up to 10000 nits in theory, but generally up to 4000 nits in actual equipment).

HDR10+

HDR10+ is an open-standard evolution of HDR10 made to compete with Dolby Vision.

It extends bit-depth to 12-bit and add support for dynamic metadata.

HLG

HLG stands for Hybrid Log Gamma.

It consists of a different technology than HDR10 and Dolby Vision, and it notably aims at backward compatibility with SDR displays, by using the standard broadcast gamma curve on the lower half of the signal values, and a logarithmic curve in the upper half. Unlike the other HDR formats, it does not require additional metadata.

That way, it can be rendered on SDK displays supporting 10-bit depth and Rec.2020 color spaces, and HLG-compatible equipment can correctly interpret the logarithmic portion of the signal curve to provide a wider dynamic range.

HDR solutions in SDI

HDR video content can be carried on over SDI. Most methods and norms ensure transparent transport over SDR infrastructures.
An SDI link typically carries on 10-bit content, and in some cases it can also convey 12-bit video.
HDR format information and metadata are transported in the ancillary data (ANC) of the SDI signal :

• The Video Payload Identifier (VPID) information includes fields like the transfer characteristics (SDR, PQ, HLG), the colorimetry (BT.709, BT.2020) and the bit-depth
• SMPTE ST 2108 defines additional signaling information in ANC packets to convey HDR/WCG mastering static and dynamic metadata

All DELTACAST SDI video I/O cards support 10-bit formats and HDR signaling in VPID and ANC data.

HDR solutions in HDMI/DP

HDR video transport over HDMI interfaces is specifically linked to versions 2.0a, 2.0b and 2.1, which respectively introduced support for HDR10, HLG, and then HDR10+ and Dolby Vision.
Static metadata (HDR10) is carried via the HDMI Vendor Specific InfoFrame (VSIF), and dynamic metadata (HDR10+ and Dolby Vision) is transmitted using the Dynamic HDR Metadata Extensions.
Moreover, HDR reception and display capabilities are also signaled through the EDID information provided by the receiver to the emitter of the signal.
DELTACAST HDMI I/O cards and FLEX modules support bit-depth of 10-bit and more, and handle HDR InfoFrame blocks.