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Explainer TVs

All about HLG: What Hybrid log gamma means for your TV

'Hybrid log gamma' is yet another a new video format that promises better picture quality for broadcast and live TV. So how is it different from Dolby Vision and HDR10?

BBC

Hybrid log gamma, or HLG, is another kind of high dynamic range (HDR), similar to HDR10 and Dolby Vision. All three promise better picture quality for compatible TVs than standard dynamic range material -- aka the TV shows and movies you're probably watching now. HDR images on good HDR TVs have brighter highlights, more impact and improved color.

So how is it different? While HDR10 and Dolby Vision are restricted to streaming (Netflix, Amazon Video, etc.), disc (4K Blu-ray) or video games, HLG is designed for broadcast cable, satellite and live TV.

What is HDR for TVs?

At this point you're probably muttering, "Great, another HDR format. Something else to worry about when I buy a new TV." Relax. 

Unlike the other HDR formats, no HLG TV shows or movies are available yet, and given the slow pace of broadcast innovations, I'm not holding my breath. Still, many new HDR TVs either have HLG built in already, or the ability to add it via a firmware update. It's even backward compatible with SDR TVs! 

So while it's possible you'll be hearing more about HLG in the future, it's not something you'll need to worry about now -- or regret missing if you recently bought a TV that lacks HLG.

Still, maybe you're curious what it's all about. I'm here to help.

H-L-G could be B-I-G (eventually)

Hybrid log gamma was co-developed by the BBC in Britain and NHK in Japan. At issue was how the two currently available HDR formats, HDR10 and Dolby Vision, were difficult to broadcast. Essentially, the metadata is a problem. Metadata is the additional information that travels alongside the visible video signal, and an HDR TV requires metadata to make HDR look like HDR.

sdr-vs-hdr-via-sony

An illustration of the difference between SDR and HDR. Remember, you're viewing this on an SDR display, so the real-world difference will be different (and likely, more obvious). 

Sony

With broadcast, there's too much chance for this metadata to get lost or become out of sync with the image, causing all sorts of issues. It also requires more bandwidth, not something broadcasters have an unlimited amount of.

Additionally, there's the issue of HDR not being easily backward compatible with non-HDR TVs (colloquially referred to as "SDR" TVs, or standard dynamic range). Generally speaking, if you send an SDR TV an HDR signal it won't know what to do with it.

Those of you old enough, or those familiar with broadcast TV history, will see parallels with the advent of color broadcasting. Early TVs were black and white, and broadcasters couldn't just flip a switch and start sending color. The B&W TVs wouldn't work. There wasn't the bandwidth to have a B&W channel AND a color channel either. The same is true of broadcasting HDR. There can't be an HDR channel and an SDR channel. Well, there could be, but that's a lot of repeated content (especially since many current systems already have duplicate standard-def and high-def TV channels).

Interestingly, and perhaps not surprisingly, HLG's clever method to broadcast HDR is a lot like how they got color working in the '50s. Essentially, the broadcast is a signal all TVs can use, but there's additional info in that signal that HDR TVs can use to show HDR. Regular TVs will ignore this, but HDR TVs will look extra pretty. It's not metadata, merely a different way of broadcasting a signal. SDR and HDR in the same signal; no extra bandwidth needed.

How is that possible? It involves a different way of encoding the signal, essentially cramming in more information in the bright parts of the signal. An SDR TV would just compress this down and show it as if it was "normal," while an HDR TV that's HLG-compatible would stretch it out and show an HDR image.

hybrid-log-gamma

A graphical representation of an SDR and HLG signal. The vertical axis is the signal, from broadcast, cable or satellite for example. "0" is black, "1" is bright white. The horizontal axis is the physical brightness coming out of your television. An SDR TV would see the HLG signal and think it was "normal" showing an image that had, perhaps better highlight detail. An HDR TV that's HLG-compatible would understand what to do with the HLG signal, and show that brighter information as a physically brighter part of the image (i.e. how HDR normally works). 

Public Domain/Creative Commons

One way to think about it is like stairs to get to a second floor. SDR TVs would be, say, 8 stairs. Big steps, but manageable. HDR10/Dolby Vision would be like having 12 stairs. Much easier to walk up. HLG is like having 10 steps, but the extra two are added only near the top. The rest are roughly the same as SDR.

(For this analogy to be truly accurate, the second floor on the HDR10/DV/HLG version would be physically higher than the SDR version, but you get the idea). 

How HLG manages the trick requires a description gamma (an article in itself) and/or a lot of math. For those of you so inclined, you can check out the Wiki page. The BBC's HDR page also has a lot of detail as well.

Compromises, solutions and next steps

HLG is primarily about showing the bright highlights not possible with SDR, while HDR10/Dolby Vision are about expanding the entire dynamic range.

HLG is a great solution to a problem, but it's not as good as the dedicated HDR methods, such as HDR10 or Dolby Vision. For one example, bright, saturated colors possible with HDR10/Dolby Vision just aren't possible with HLG (though the color range is just as wide). However, overall it works, and that's more important. It doesn't matter how good a method is -- if you can't get it to someone's TV it's pretty useless.

So what does this mean for you? Well, at the moment, not much. TVs are starting to get HLG updates, but we're a long way away from 4K over-the-air broadcasts, which will likely carry HDR in HLG form, too.

Eventually, though, you'll be able to watch live sports, car racing and so on with the color and dynamic range of HDR. Will it look as good as that same program recorded, mastered in HDR10/Dolby Vision, and sent to your house via 4K Blu-ray? Maybe or maybe not, but doesn't seeing it live offset that? And as yet, we don't know how big a difference in picture quality this will be. Likely, for most people, it won't be a huge difference. 

It's also possible that those of you watching on SDR displays will get a little better image, since the HLG is sending more brightness detail. Your TV will show this detail (textures in clouds on a bright day, for example), it just won't show it as brighter since you'd need an HDR TV for that.

HDR is the future of TV, as the vast majority of people on both sides of the screen are excited about it (unlike, say, 3D, which most hated). With hybrid log gamma, the amount of HDR content will grow considerably, and that's always a good thing.


Got a question for Geoff? First, check out all the other articles he's written on topics like why all HDMI cables are the sameTV resolutions explainedLED LCD vs. OLED, and more. Still have a question? Tweet at him @TechWriterGeoff then check out his travel photography on Instagram. He also thinks you should check out his best-selling sci-fi novel and its sequel.