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‘HLG or PQ-10’: will the DVB’s new UHD spec set the standard outside Europe?

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The agreement last month by DVB to approve a specification for UHD-1 Phase 2 which supports the use of both ‘flavours’ of High Dynamic Range (HDR) – namely Hybrid Log Gamma (HLG) and the ten-bit variant of Perceptual Quantizer (PQ-10) – could pave the way for a similar approach to be adopted in both the USA and Japan, according to DVB Chairman Peter MacAvock.

“The DVB’s work in this area has been ground-breaking in the sense that it’s been particularly difficult to gain agreement in the different standardization bodies on HDR,” said MacAvock. “I think we’re the first and it remains to be seen whether others will follow. We’ve certainly, as part of the DVB process, been very careful to notify our brethren in ATSC [the US TV standards body] and in the Japanese standards forum ARIB about the work, to make sure that it’s going down the same road and that they can benefit from some of the work that we’ve done. […] I suspect they will [follow the DVB’s lead].”

HDR, said to deliver extra ‘sparkle’ to UHD images both through higher contrast and its associated wider colour gamut, is regarded as one of the key elements next-generation UHD services will require in order to create the ‘wow’ factor that will drive mass-market adoption.

However, the industry has been be-devilled by arguments over the pros and cons of each of the two ‘flavours’ and their variants – as late as September last year, there were reportedly five separate HDR solutions in contention (one HLG and four PQ) within DVB.

PQ, developed by Dolby as part of its Dolby Vision standard, is favoured by the Hollywood studios and OTT providers such as Netflix. However, it is not backwards-compatible with existing receivers which use Standard Dynamic Range (SDR). HLG, meanwhile, which was created by the BBC in collaboration with Japanese broadcaster NHK, was specifically designed to be backwards-compatible with SDR sets and to favour live production workflows.

MacAvock emphasises the fact that the DVB has now effectively side-stepped that debate. “We’re simply saying you can do it either way,” he declares. “Either has their merits, but if I’m doing, for example, live content or if I’m wishing to target a set of receivers that require backwards compatibility, then I’m in HLG. Otherwise, I have a choice and, in general, I want to choose for whatever way the content comes in, [HLG or PQ-10].”

MacAvock suggests that in any case it is a straightforward matter to transcode from PQ into HLG, adding that, “what we have seen in the industry is that [manufacturers] construct panels with both options included.”

The new Phase 2 UHD specification also defines the use of Higher Frame Rates (HFR), which go beyond the current 50/60 Hz; and the latest Next Generation Audio (NGA) schemes, which support immersive object- or scene-based audio schemes.

Interestingly, the specification provides for such new features to be used with HD as well as UHD/4K resolution receivers, the HD+HDR solution being favoured as a ‘bridging’ solution to UHD by companies such as Ericsson.

MacAvock stresses that “overwhelmingly, the commercial requirements in the DVB Project have called for 4K sets with appropriate colorimetry and HDR: that’s the sweet point, if you like, […] and if I look at the pioneers in UHD content where, typically, it’s subscription and pay-TV operators, then that’s where it is.”

However, concedes MacAvock, “what we’re seeing is [that] the need to be at one and half times picture height in order to get the full impact of the higher [4K] spatial resolution, is going to be a factor.” This is closer to the screen than for HD, which implies that either viewers would need to re-arrange their living-rooms, or buy a bigger (4K) set – and there’s a limit to how much bigger: MacAvock notes that a recent study suggests that a screen size of 65 inches is the limit because nothing larger can fit into the average lift. “That’s why, for example, the public service broadcasters in some countries are saying, ‘well, no, let’s concentrate on HD and HDR, particularly if we can get that operating point in the TV sets, because that affords us an opportunity to create the ‘wow’ factor without having to upgrade all our installations to 4K.’”

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