While most AV fans embrace the picture quality you get with new HDR (High Dynamic Range) technology, most people could do without the brain-melting complexity and barrage of acronyms that come with it.
Unfortunately, HDR has only got more complicated in the last couple of years, as we find ourselves having to get to grips with a wide range of HDR formats, from Dolby Vision to HDR10+, and Hybrid Log Gamma.
What is Hybrid Log Gamma (HLG)?
It’s hard to think of a less friendly name for anything than Hybrid Log Gamma (which, for the sake of everyone’s sanity, we’ll refer to as HLG from here on in). Yet it’s a term that you shouldn't ignore if you're thinking about buying a 4K HDR TV.
At its heart, HLG is mercifully more simple than it sounds. The result of a joint research project between the BBC in the UK and Japanese national broadcaster NHK, it’s designed to deliver a more convenient HDR solution for the broadcast world than the HDR10 system used by most current HDR sources and displays.
Why is HLG more convenient? It combines standard dynamic range and high dynamic range images into just one video signal that can play as SDR on SDR screens, or HDR on HLG HDR-compatible screens, ensuring viewers aren't left in the dark, no matter what TV they have.
These ‘one size fits all’ signals can be delivered in just one bitstream in the VP-9 or HEVC compression formats, or over HDMI; won’t eat up precious broadcast bandwidth in the way that separate SDR and HDR signals would; and can even fit within the existing 10-bit production workflows broadcasters are already using.
These are all hugely helpful features for broadcasters who may well, of course, find themselves needing to deliver HDR efficiently and consistently on live feeds.
Hopefully you’re following so far, as there’s one more horribly technical phrase to introduce in order to explain how HLG achieves its seemingly miraculous triumvirate of broadcast HDR benefits: the Electro-Optical Transfer Function. Or EOTF for short.
The simplest explanation for EOTF is that it defines the relationship between a recorded electrical video signal and image brightness. Video displays can then use this information to convert digital signal data into visible light. What HLG does is use a ‘hybrid’ EOTF that employs two different types of light coding.
With low-light content in an image, the HLG system employs the typical gamma curve approach to rendering picture brightness that’s been a feature of TV playback for decades. This means that these parts of the HLG signal can be recognised by SDR TVs and played back normally.
However, the HLG signal also applies a logarithmic curve to the high-brightness parts of its image data that’s ignored by SDR TVs but can be recognised and worked with by compatible HDR TVs, opening up an image with a much wider brightness range.
MORE: Best 4K HDR TVs 2018
How can you watch HLG?
So does this mean that HLG will work on any HDR-capable TV? Sadly not. An HDR TV will need to recognise the logarithmic curve HLG uses.
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More after the break
Will HLG deliver better pictures?
Naturally, the whole point here is to enable 4K HDR TV broadcasts, which deliver better quality pictures provided you have the appropriate kit. However, there are already some question marks over just how good HLG content will look.
For instance, many of the HLG demos shown so far have featured content that uses a resolution lower than native 4K Ultra HD - often ‘just’ Full HD.
Some industry commentators also believe that the way HLG works will lead to a compromised HDR performance versus other HDR formats and possibly even a compromised SDR performance too.
What HLG content is available?
Live HLG trials have been shown at technology shows and press events for many months now, and for a while it looked like HLG had gone mainstream in December 2016 when the BBC delivered a four-minute clip of Planet Earth II to BBC iPlayer, which was streamed in 4K and HLG.
However, most people would have seen this 4K clip in SDR, as not many TVs at the time would have supported the HLG format. HLG updates were rolled out for a lucky few during the trial, though.
In December 2017, the BBC launched the seven-episode Blue Planet II in 4K and HLG HDR on its iPlayer service. It was the first time a full series had been shown in both technologies - no mean feat.
What's the future for HLG?
While HLG may be trickling rather than flooding onto our TV screens, there’s little doubt that the BBC is intending to use it more and more – even if that’s more likely to be through iPlayer than live broadcasts for now.
Elsewhere, Mediapro/Overon has announced that it intends to transmit the Spanish Football League in HLG, while Google has stated that YouTube’s HDR videos will support HLG encoding.
Digital UK included support for HLG in its 2017 specifications for its Freeview Play platform. Eutelsat's Hot Bird video service includes the HLG-enabled Travelxp 4K channel. And Sky has hinted that HLG could form part of future HDR plans being studied for its Sky Q platform.
HLG is certainly not the only HDR format still waiting to properly take off – after all, HDR10+ is still in its early days – but it’s still the one to watch when it comes to 4K HDR TV broadcasts.