Soon, there'll be no fewer than four different types of HDR (High Dynamic Range) doing the rounds with 4K content. But, it's HLG (Hybrid Log Gamma) that's arguably the most important, given that it's set to accompany TV broadcasts...

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, though, HDR is potentially set to become more rather than less complicated in 2017 as we find ourselves having to get to grips with yet another new HDR system called - terrifyingly - 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 an HDR TV in 2017.

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.

So if you're wondering when we're going to get HDR TV from the likes of the BBC and other broadcasters? This is the technology to keep an eye on.

MORE: LG and BBC conduct HDR broadcast trial

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.

These ‘one size fits all’ signals could 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 would 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 HDR TVs 2017

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 - and at the time of writing there are no TVs out there right now that do this.

However, most big-name manufacturers have confirmed that their 2017 sets will be HLG ready, with Samsung and LG also confirming that their 2016 HDR TVs will receive HLG support via a firmware update. Yes, it can come with an update to some 4K TVs, which is good news.

Other brands may add HLG support to 2016 TVs too, but at the time of writing they haven’t formally confirmed this.

In the projector world, the already-available Sony VPL-VW550ES 4K projector has confirmed HLG support, and JVC’s upcoming DLA-Z1, X9500, X7500 and X5500 projectors will definitely handle HLG following their launch.

Version 7.0 of Google’s Android TV platform will also incorporate HLG, as will the recently announced HDMI 2.0b connection update.

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.

MORE: The best TVs of CES 2017: LG, Panasonic, Samsung, Sony

What HLG content is available?

We can’t yet discuss HLG’s potential quality with any certainty for the simple reason that no finalised HLG content is currently 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 4K clip of Planet Earth II to iPlayer, which was said to be available in HLG.

However, closer inspection proved that while the Planet Earth clip was in 4K, it was SDR rather than HDR. So no HLG in sight. 

MORE: Yes, you can watch Planet Earth II in 4K Ultra HD (briefly)

What's the future for HLG?

There’s no doubt, though, that HLG is coming. The BBC is clearly intending to use it. Mediapro/Overon has announced that it intends to transmit the Spanish Football League in it. Google has stated that YouTube’s recently launched HDR videos will support HLG encoding.

Digital UK has included support for HLG in its 2017 specifications for its Freeview Play platform. Eutelsat has revealed that its Hot Bird video service will include 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 new HDR format still waiting in the wings or starting to emerge in 2017, but it looks as if it could be the one to watch when it comes to watching 4K HDR video TV broadcasts. Watch this space.

MORE: HDR TV - What is it? How can you get it?