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Hybrid Log-Gamma: Why it's the next big thing in 4K TV


Hybrid Log-Gamma – What is it? 7
Hybrid Log-Gamma – What is it?

There’s a very good reason why the BBC, Sky and BT to name but three broadcasters haven’t rushed to compete with UHD HDR services from Netflix and Amazon. But Hybrid Log-Gamma will soon change all that…

If you’ve been looking to buy a new TV, you’ll have no doubt been on a crash course of techy acronyms – UHD (Ultra High Definition), HDR (High Dynamic Range), WCG (Wide Colour Gamut) and HFR (High Frame Rate). These terms are fast becoming more commonplace, even if some of the technologies aren’t really ready for prime time (we’re looking at you, HFR). The good thing about all of these labels is that they’re relatively self-explanatory. No PhD required.

But there’s a new acronym a’coming – and it’s deliciously incomprehensible. Hybrid Log-Gamma (or HLG, as the cool kids call it) is on the verge of becoming incredibly important. It essentially opens the door to broadcast HDR. But how does it differ from the HDR we can currently enjoy through streaming services or get from UHD Blu-ray – and is that TV you’re about to buy (or have just taken home) already redundant if it doesn’t already support it?

Related: What is HDR TV?

Hybrid Log-Gamma: What is it and why do we need it?

Hybrid Log-Gamma is different to existing HDR technologies in a number of cool ways.

It’s been jointly developed by the BBC and Japanese state broadcaster NHK, and sets out to resolve the conundrum of how broadcast UHD TV services with HDR can be made backwards compatible with regular HD TVs.

Lack of any backwards compatibility is a major stumbling block for TV companies. Without it they would need to simulcast channels or programmes, and that has huge cost implications, both from a studio workflow point of view and for bandwidth. Even better, it allows much of the existing production ecosystem for SDR TV to be used for HDR production. It also provides a way to broadcast live HDR – universally seen as a vital viewing upgrade for sports coverage.

The “hybrid” part of Hybrid Log-Gamma refers to the fact that HLG doesn’t reinvent the wheel. It adapts traditional picture-making techniques which have been a part of TV image lore since the early days of the CRT TV, slapping a bit of new take-it-or-leave-it information on top – the so-called Log file. Consequently, HLG HDR signals are inherently backwards compatible with all standard dynamic range TVs.

Hybrid Log-Gamma – What is it? 9

Hybrid Log-Gamma: How does it differ from HDR 10 and Dolby Vision?

HDR already comes in multiple flavours. HDR 10 is the standard for pre-produced content; it’s used on UHD Blu-ray and by streaming services such as Netflix and Amazon. HDR 10 uses what’s known as “static metadata” to relay the HDR information to a compatible display. It’s guide to what’s about to be received, as it were. Dolby Vision is a proprietary alternative that uses frame-by-frame dynamic metadata, which means there’s signal information in every frame, for supposedly greater picture accuracy.

But both HDR systems still rely on metadata, and that doesn’t work for TV transmissions and, in particular, live broadcasts. It only suits pre-produced content. For example, HDR 10 contains Maximum Content Light Level and Maximum Frame Average Light Level as part of the mastering metadata information, neither of which exist in a live context, as there isn’t any, because, well, it’s live.Hybrid Log-Gamma – What is it? 1

Another problem with a metadata-based HDR broadcast solution happens when content gets re-distributed around a network or supplied to affiliate broadcasters who may need to top-and-tail. The metadata HDR info in the signal could simply vanish in an editing suite.

The attraction of Hybrid Log-Gamma is that there are no such metadata complications.

To reiterate, streaming services – Netflix, Amazon, etc – don’t need to worry about errant metadata, as they only deal in pre-produced and encoded HDR content, streaming from their own servers to a TV or other device. Streaming services control the entire chain. Broadcast TV with live programming just doesn’t have that luxury.

Related: Best 4K TV 2016

Hybrid Log-Gamma: How does it work?

Unlike rival HDR systems, which rely on pre-encoded signalling metadata to explain an HDR picture to a compatible telly, HLG is a scene-referred system. That’s to say, the delivered signal reflects the light in the original scene, just like it was shot. Exactly what’s been photographed in the studio or outside broadcast is delivered to your home TV or projector, without any need for pesky metadata.

From a broadcast point of view, there’s simply a flag which lets a receiving set-top box know that it needs to tell your TV via HDMI that there’s some lovely HLG UHD images coming over with 10-bit colour depth.

All that any HLG-compatible TV at home needs to be aware of is its own technical capabilities and of the environment the content is being viewed in.

This latter point is a big deal. HLG has the ability to alter the HDR image based on the viewing environment – light room, dark room, even specific brightness levels measured in nits. This feature makes HLG fundamentally different from HDR 10, and potentially could offer everyone a far more satisfying viewing experience.

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Currently, some HDR 10 TVs don’t let the user alter the Average Picture Level of HDR content, which means in an averagely bright living room, HDR material looks too dark. Hybrid Log-Gamma adjusts the picture according to the surround ambient light level, to ensure the image always looks acceptable.

Trials of HLG HDR have reputedly been very successful, particularly when shooting live sporting events in stadiums where one half of the pitch is bathed in sunlight, and the other in darkness.

The recent broadcast technology show IBC was awash with Hybrid Log-Gamma test transmissions delivered by satellite operator SES. Visitors to IFA for the past two years have been able to see live demonstrations of HLG.

Hybrid Log-Gamma: When can I buy an HLG 4K TV and is my current TV upgradeable?

HLG is currently included in the ITU-R BT.2100 specification for programme production and exchange. It’s expected to be ratified into the DVB specification before the end of the year, at which point broadcasters and screen manufacturers can begin to firm up road maps. But where does that leave TV buyers in the meantime. Can you expect 4K screens bought today to be firmware updateable?

We put out a call. Understandably, currently TV manufacturers have little to say on the subject as yet. Only Samsung, out of all those polled, maintained that all its UHD HDR 2016 models could be firmware updated to support HLG. “All of Samsung’s 2016 HDR TVs will be firmware upgradeable to support Hybrid Log-Gamma broadcast HDR once it is widely available,” we were told emphatically.

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Sony was more circumspect, saying: “Once the standard is confirmed, we will comment then…” However, Sony’s projector division has announced that the new VPL-VW55ES HDR 4K home cinema projector will be firmware upgradable to HLG, at some point.

Philips was non-committal: “The implementation of Hybrid Log-Gamma HDR for Philips TVs is currently under investigation.”

LG, despite leading demonstrations on HLG at trade events, did not offer any official comment. Neither did Panasonic. Although to be fair, until any specification is ratified, official replies are probably unlikely.

When it comes to HLG on 2016 screens, the watch phrase has to be caveat emptor (or buyer beware). After all, Hybrid Log-Gamma probably won’t join the official roster of TV logos until 2017, when the likes of the BBC and Sky will likely announce broadcast HDR services.

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November 4, 2016, 4:56 pm

Well good on Samsung. Less impressed with the others, and if my 2015 LG OLED isn't supported then I'll be mightily miffed.
Don't buy a TV expecting HDR to work across the board, or to look any good when it does work. The standards are still up in the air and HDR mastering is still all over the place and that probably won't change any time soon.


November 4, 2016, 6:54 pm

Gives the phrase '2020' vision, more dimension.
Every innovation to resolve an issue creates another hurdle for manufacturers and consumers and creates more landfill.
I hope my current TV lasts long enough.

Neil Richardson

November 5, 2016, 10:26 am

TV tech is moving explosively fast. Too fast in my opinion. A TV used to be such an investment and one that could last a decade or more but with emerging technology and in particular changing cabling standards, redundancy is being achieved a lot faster. TVs are becoming the new Smartphones only with high multiples of the cost to maintain 'flagship' technology. Broadcasters can't keep up. There are 4K TVs that are currently outdated before 4K broadcasts are even mainstream. It's unsustainable.

Swift John

November 7, 2016, 5:07 pm

I'm fine with my 6 year old tech(G10 plasma), works as a I need it to. You save a fortune and get a big leap forward technology when it's time to upgrade. Take what you own for granted, make it last.


November 13, 2016, 3:58 pm

If Samsung can commit to supporting this for all their 2016 models, specifically the UN70KU6300F, which I'm about to get, then more respect to them, because Sharp sucks for product support!


January 2, 2017, 4:11 pm

Well don't count on Samusng delivering to its promises. They also promised Evolution Kits for its 2013 TVs "through 2016" and haven't delivered for the last two years. And they also promised to enable Netflix HDR in its top line 2015 SUHD models (even giving a release deadline), and haven't delivered.

I much prefer an honest "we still don't know" stance rather than empty promises.

John Fak

January 8, 2017, 4:59 pm

Intro ... intro ... intro, then you "re-iterate". Reiterate what, you didnt actually said anything.

Do you really need 10 pages to say "no metadata"? And that doesnt actually mean anything. You could have just said: I dont know what HLG is.

Piece of shiat inflated article with zero information.

John Fak

January 8, 2017, 5:03 pm

Idiots who call spending a huge amount of money on toys "investment" bother me greatly.

No its never an "investment". Its just entertainment. That money is gone. POOF!

And thats ok, sometimes its nice to throw money around on fun things. But dont call it "investment" you buffoon.


February 3, 2017, 8:58 pm

samsung tends to overpromise. all of the other companies just don't want to say yes now and get caught with their pants down if their current hardware can't support the undefined future standard.

Toby Piller

March 17, 2017, 3:10 pm

I think HDR makes LCD tech look better than it is, but HDR doesn't look so good on LCD with edge lit or even back lit led panel. Yes you can get very high brightness levels (2000nits) that blind you in a dark room, but your "black" will be gray as wel. OLED is superior in contrast and eventhough you cannot get further that 500 or 500 nits (300 on average), may seem low, but your black stays black and it's too low for HDR, but who cares, you have the best picture. So if you don't know which tv to get for HDR, get an LG. They have HDR10, Dolby vision and maybe HLG in the future.


March 17, 2017, 3:51 pm

LG announced they will update their (good) 2016 tv's with HLG. (all of the OLEDs and UH77, UH85, UH95)

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