ATSC 3.0 to Support Both HLG and PQ Transfer Functions for HDR, Says Richer
ATSC's Technology Group 3, which supervises the framing of ATSC 3.0, approved a ballot this week that would elevate the A/341 document on ATSC 3.0 video and high dynamic range to the status of a proposed standard, ATSC President Mark Richer emailed us Thursday through a spokesman. "The document includes support for HLG and PQ transfer functions,“ Richer said of the competing hybrid log-gamma and perceptual quantization approaches to HDR. It was Richer's first known public acknowledgement that ATSC 3.0 will support both HDR technologies.
Sign up for a free preview to unlock the rest of this article
Communications Daily is required reading for senior executives at top telecom corporations, law firms, lobbying organizations, associations and government agencies (including the FCC). Join them today!
Richer's disclosure came after a BBC engineer described HDR for ATSC 3.0 as "quite a controversial issue” among the framers of the next-generation standard. Within ATSC, there's support for both the HLG and the PQ HDR approach of Dolby Vision and HDR10, said Tim Borer, lead engineer at BBC Research and Development, on a Society of Motion Picture and TV Engineers webinar Thursday about the features of the HLG technology that BBC co-developed with NHK.
ATSC 3.0's framers are “about to go to a proposed standard, and I’m hoping that will include both of the formats,” Borer said of HLG and PQ. “I believe they’re having some votes in the near future to try and make decisions,” said Borer, apparently unaware that TG3 already had taken such a vote in meetings this week. The BBC-NHK proposal on HLG is one of six technical HDR systems in the running for ATSC 3.0 (see 1605200031).
Borer, who has been monitoring. ATSC 3.0 developments closely for BBC, hopes “we should come to some kind of decision within the ATSC within the next few weeks, hopefully in time for NAB,” he said of the NAB Show, which opens late April in Las Vegas. “I can’t be much more specific, with issues of confidentiality here.” The candidate standard period on ATSC 3.0's A/341 video document is scheduled to expire Jan. 30 but has been postponed several times over the stalemate in reaching a decision on HDR (see 1611290030).
When BBC engineers viewed the first Dolby Vision PQ HDR demos in 2013, “we were blown away by the picture quality,” said Andrew Cotton, principal technologist at BBC Research and Development, echoing comments he and other engineers made when we visited BBC's labs in West London in October (see 1610030004). “They were undoubtedly the best TV pictures I had ever seen,” Cotton said of Dolby Vision. “But we couldn’t see how we could deploy the system as it was described across a complex television production infrastructure” because the system’s reliance on metadata was “of great concern to us,” said Cotton. That’s when BBC teamed with NHK to develop the metadata-free HLG approach “with TV in mind, to enable an easy migration to HDR television production and distribution,” he said.
BBC wants to emphasize that HLG “delivers a high-quality HDR picture,” Cotton said. “Just because it doesn’t require metadata doesn’t mean that in any way that its performance is inferior to PQ. And indeed, in many practical situations, we believe it can be better than PQ.” Throughout the presentation, Borer and Cotton repeatedly positioned PQ as optimal for the dim environments of commercial movie theaters and for watching packaged-media content on a home theater where viewing environments can be tightly controlled. By comparison, HLG, Borer said at one point, is needed for TV and its “Wild West” diversity of viewing environments that can’t be controlled and that artistic creators can’t account for. Dolby Labs representatives didn’t comment.
HLG’s developers “believe it’s essential” that HDR TVs be geared to all “home viewing environments,” said Cotton. The “absolute-brightness approach of PQ is well suited to cinema and where all viewing environments are the same,” and that doesn't describe TV in the home, he said. “We believe you should be able to view HDR pictures in a whole range of consumer viewing environments. We shouldn’t have to close our curtains or drapes in the daytime if you want to watch a football match in the afternoon."
BBC thinks "the relative brightness of HLG is well suited to most diverse home viewing environments," said Cotton. "To preserve detail in the blacks, the presentation needs to be brighter than in the grading suite” used for PQ HDR production, he said. “And to preserve the impact of highlights, consumer screens may need to be brighter than the professional grading screens” used for PQ, he said. By design, “as the HLG display gets brighter, so does the entire image, enabling HDR viewing in brighter environments,” Cotton said.
Cotton is “amazed” with the peak brightness of “the very latest screens” he has seen on tablets and smartphones, he said in Q&A when asked about the practicality of HDR on mobile devices. Cotton has been told the screens on some premium models of smartphones are now capable of approaching 600 nits peak brightness, he said. “You can get a stunning HDR image” on a screen of that quality when riding the London Underground, he said. “What that looks like when you take it outdoors is another question. But certainly I think that in controlled viewing environments, we’re not very far away.”