Wider colour gamut
Wider colour gamut

What are the main competing HDR standards for broadcast?

High dynamic range (HDR) can massively improve image quality, but there are different standards being developed for broadcast applications. The latest Rohde & Schwarz report sheds light on the two main standards and gives you guidance on the merits of both.

The principles of HDR are well known from still photography applications, but there are many issues to standardise video systems that can capture, edit, transmit and display HDR content across a wide variety of equipment capabilities.

The complexity is such that no single proposed HDR standard can currently optimally address all applications. For example, the 16bit depth SLog3 from Sony is widely used in post production but is considered too complex for consumer broadcast, while two very different approaches to HDR have been proposed for broadcast applications: “scene referred” and “display referred”.

Broadcast HDR standards

In July 2016 the ITU published ITU-R BT.2100, which contained a number of recommendations for future broadcast systems. These included two different High Dynamic Range systems, one scene referred (HLG from the BBC/NHK) and the other display referred (PQ from Dolby Vision). Both systems require the use of gamma curves in order to compensate for the non-linearity of brightness perception by the human eye.

The EBU have issued similar recommendations endorsing both HLG and PQ, while the ARIB B67 standard specifies that only HLG can be used for broadcast applications in Japan.

Hybrid Log Gamma (HLG)

HLG was developed by the BBC in the UK and NHK in Japan with the goal of maintaining as much backward compatibility as possible within the constraints of maximising the picture quality of a live broadcast chain. HLG has also been standardised by the Japanese ARIB standards body for HDR broadcasting and currently in use for 4K and 8K HDR transmissions in Japan.

It is a scene-referred system requiring no metadata as the output values are effectively a percentage of the original scene brightness. This also simplifies the conversion to the actual brightness of any particular display.

As the name suggests, HLG combines a gamma curve at lower brightness levels and a logarithmic curve at higher brightness.

The gamma curve part is very similar to the SDR gamma curve, which ensures backwards compatibility with SDR displays. It was also chosen to be similar to the gamma curve used in most existing High Definition video cameras (not to be confused with true HDR cameras!) for lower luminosities.

HLG uses a logarithmic curve for higher luminosities since the human eye responds logarithmically to increasing brightness.

Many HD cameras used a linear response for higher luminosities starting from a “knee” point on the SDR gamma curve. Although not a perfect match, it is a reasonable approximation to the HLG curve to assist in backwards compatibility.

As HLG does not need metadata to be generated at each stage of the broadcast chain it is expected to be used in live broadcast applications as well as where backwards compatibility is required.

Perceptual Quantiser (PQ)

The PQ EOTF curve was developed by Dolby Vision via a series of experiments to investigate the response and sensitivity of the human eye to changes in display brightness.

PQ is a display referred system supporting a maximum reference display brightness of 10000nits. Changes in brightness beyond 10,000nits cannot be easily and consistently discerned by the human eye and in practice much lower brightness reference displays (up to ~4000nits) are used during the mastering process.

PQ therefore requires the use of metadata at each stage of the production chain to indicate the maximum brightness and colour space used during the mastering process. Metadata is generated by a colour grader during the post-production process and is defined in SMPTE ST.2086 for the PQ EOTF.

Currently, only fixed/default or no metadata is defined for broadcast PQ applications. However, the PQ/ST.2084 EOTF can be used with static metadata as part of the HDR10 standard and with dynamic metadata as part of the Dolby Vision Home standard as used on HDR Blu-rays.

Posted by David Smith, Technology Marketing Manager, Rohde & Schwarz. 30th September 2017.