SMPTE issues call to arms for HDR tech and metadata

The SMPTE has issued the recommendations of its High Dynamic Range (HDR) Imaging Ecosystem technical committee (10E SG). Comprised of some 170 experts, the report examines the issues at stake in the current HDR ecosystem, and identifies areas which need to be tackled.

With a hefty set of appendixes that contain the (sometimes conflicting) views of key stakeholders, the report aims to propose definitions for HDR and its related technologies, describe the problems in the current HDR ecosystem, and identify the existing standards that may be impacted by the rise of HDR – with a view to identifying areas that require further investigation.

First off the bat, the report calls for the standardization of the reference panels that the content creators will be using to grade HDR content, so that there is an industry standard approach. Part of this process might include raising the brightness level for grading content, which is still mastered at 100 nits. Modern TVs are typically capable of outputting 400 nits, with the high-end models making it into triple figures.

In addition, the SMPTE is pushing for a system to automatically detect the brightness of the viewing environment, so that the panel can adjust its brightness to match the room – and achieve a better and more accurate contrast. This would potentially allow for TVs to save energy by dynamically altering their power consumption, but more importantly, it would produce a better picture.

It concludes that the SMPTE needs new standards documents to establish ways of carrying HDR and WCG imagery over SDI; standardizing how to do the same with the emerging video over IP technologies; standardized metadata for content-dependent color transforms of HDR content to SDR displays; standardized file and code stream wrappers for carrying that content-dependent metadata if it is needed for signal interpretation for alternate display capabilities.

“The HDR Ecosystem needs imaging performance requirements that must be met with sufficient precision to ensure that high-quality color reproduction can be achieved on displays with different capabilities without introducing unacceptable artifacts. Properly designed HDR systems will dramatically improve the available creative palette and directly enhance the consumer experience.”

In total, the SMPTE says the parameters that comprise HDR include a higher peak luminance, lower minimum luminance, greater contrast range, and improved precision minimizing quantization errors that are beyond the capabilities of existing standards to deliver.

The SMPTE also notes that the wider color gamuts (WCG) that will be used alongside HDR mean that implementers of both technologies should be aware of the impact that will arise from video with higher frame rates. It notes that with 4K and 8K resolutions, new workflow infrastructure will be required. HDR and WCG can be accommodated on the existing 1.5Gbps and 3Gbps multilink interfaces, or 10Gbps optical links, but not at 4K or 8K resolutions.

In a similar vein, the report pushes for the consideration of video over IP as an alternative to legacy Serial Digital Interface (SDI). The SMPTE is looking at revising its ST 2022 standards to incorporate the HDR and WCG content and the higher frame rates and resolutions into its existing video over IP framework. The report also calls for technologies capable of supporting both real-time (linear) and non-real time (VoD) delivery and display.

A significant aspect of the conversation surrounding HDR and WCG is the use of metadata to augment a signal. Content-dependent metadata is the term used to describe the data that describes changes to an image in order to output an image on a different screen to the one it was mastered on.

With HDR, there’s (essentially) a list of functions per frame that can tell a HDR-ready TV how to transform a still to create a HDR image. The HDR metadata typically tells the panel how to cope with HDR’s smaller peak white capability than the source, and how to interpret the difference in colors between the HDR image and the screen’s capability.

This metadata is relational, and depends on the content. It is also dynamic, as HDR will be using different transform functions for different scenes, represented in the metadata, and they need to be considered when telling an SDR screen how to respond to those changes in the source information.

The SMPTE adds that it is still unclear whether future HDR workflow infrastructure and consumer interfaces will be capable of supporting content-dependent metadata, and adds that downstream ad-insertion and graphic overlays are additional factors that must be considered.

The SMPTE argues that while HDR metadata should be signaled in interfaces, the use of multiple HDR datasets should be discouraged, due to the risk of fragmentation at both the creation and consumption level – which will delay or perhaps even prevent the adoption of HDR.

The report notes that the Advanced Television Systems Committee (ATSC) was considering how to implement HDR in the next generation of its terrestrial standard, with the Society of Cable and Telecommunications Engineers (SCTE) also beginning work on HDR for cable transmissions.

The MPEG Committee is also evaluating whether HEVC needs to be modified to support HDR, and over in Japan, NHK has announced a time table for the delivery of HDR and WCG content in homes. DVB in Europe is also looking, while the ITU has its Rapporteur Group (RG24) looking at HDR recommendations for broadcast TV. The Blu-ray Disc Association has already included three versions of HDR in its UHD Blu-ray standard, supporting Dolby, Technicolor, and (of course) the SMPTE.

In its summary, the Study Group’s report recommendations are:

  • Investigate how content-dependent metadata can be transported persistently through professional media workflow.
  • The identification requirements for HDR systems in the SDI payload ID scheme should be examined, due to the limited number of reserved bits in SDI.
  • The use of multiple HDR metadata sets should be discouraged, as it would cause fragmentation in workflows, and delays in adoption by content creators and users.
  • Automatic brightness control would be useful for matching a display brightness to the brightness of a room, and that a study of these corrections might be a useful SMPTE effort.
  • HDR reference monitor target parameters need to be standardized for content creation interoperability.
  • Encourage members to standardize their WCG and HDR interface and metadata solutions.
  • Investigate the effects of content conversion on HDR material, for converting SDR to HDR.