Even if you specialize in tech, it seems inevitable that you’re going to run across unfamiliar jargon. No brain can absorb everything, and if you don’t need to know a term, there’s less incentive to learn about it. I’m a naturally curious person — but I’m sure I’d push some important information out of my head if I started learning the intricacies of, say, motherboard design on the smartphones and PEVs I use. I barely understand what a MOSFET is.
HLG is a term I’ve encountered repeatedly when scanning the HDR specs of TVs, yet never needed to know much more about than that. I decided to probe a little deeper, and as it turns out, there’s a good reason it’s largely sidelined next to technologies like Dolby Vision and HDR10+. Regardless, you may have already experienced HLG without being aware.
What is HLG, and who supports it?
A solution to a very specific problem
HLG is short for Hybrid Log-Gamma. It was co-developed in 2014 by two public broadcasters, specifically the UK’s BBC and Japan’s NHK. As that suggests, the standard was developed to provide HDR (high dynamic range) over broadcast TV. It’s a very unusual implementation of HDR, though, and it’s not strictly relegated to over-the-air signals.
The tech addresses a few problems. First, it allows HDR to ride on the same channels as SDR (standard dynamic range). You might remember that earlier in the HD era, some broadcasters maintained separate SD and HD channels, and that may still be a problem where you live. That’s a burden not just for stations, but for people trying to watch — it means having to flip past low-resolution channels to get to something that actually exploits 4K, 1080p, or just 720p.
HLG’s trick is that it uses an SDR signal as its foundation, but adds a logarithmic curve with extra brightness information, hence hybrid log-gamma — the base gamma curve provides standard luminance data.
More importantly, technologies like Dolby Vision, HDR10, and HDR10+ were once distinctly unsuited to broadcast for one reason: metadata. If this gets lost or out-of-sync during a transmission, it can produce terrible results. Vision is particularly challenging, since it’s not only a dynamic HDR format like HDR10+ — meaning it can adjust on a per-frame basis — but more bandwidth-intensive.
HLG’s trick is that it uses an SDR signal as its foundation, but adds a logarithmic curve with extra brightness information, hence “hybrid log-gamma” — the base gamma curve provides standard luminance data. TVs that can detect this extra info will produce HDR effects without any special metadata required. SDR TVs may actually benefit as well in the form of improved highlight details, although they’ll encounter color distortion if they don’t support the BT.2020/Rec. 2020 color space. All modern TVs do, so there’s no real worry there.
This stealth approach means that HLG can show up in surprising places. It’s supported not just by BBC iPlayer and Freeview Play in the UK, but also DirecTV in the US, and YouTube worldwide. As far as TVs go, HLG is present on many models released in the last decade, including ones from brands like LG, Sony, Samsung, Panasonic, and Hisense. You’ll be hard-pressed to find a new smart TV without HLG support.
Why HLG is the forgotten HDR mode
A casualty of the streaming era, or a companion?
For one, it’s less technically capable than the other major HDR standards. It doesn’t support the range of HDR10, much less Dolby Vision — while it can enhance highlights and make images more vivid overall, it can’t improve shadows the way other formats can. Those details remain largely unchanged from SDR, so it’s not going to help with those ultra-dark scenes in titles like The Witch or Game of Thrones. Also, while HLG is dynamic in a manner of speaking, the dynamic metadata in HDR10+ and Dolby Vision shouldn’t be undersold. This allows studios to exercise more precise control over how shots appear, something that’s only going to expand once Dolby Vision 2 and HDR10+ Advanced make their debut.
The biggest issue is more fundamental: the shift away from broadcast TV. Many people now subscribe to purely on-demand streaming services like Netflix, Hulu, and Disney+, where the concerns HLG was meant to address don’t apply. Metadata doesn’t really fall out of sync in that arena, and if anything, the priority is quality over efficiency. Though there is compression, the highest-quality streams mix 4K, Dolby Vision, and Dolby Atmos surround sound. People are, after all, turning to these services to replace not just Blu-ray and broadcasts, but often the theatrical experience.
Many people now subscribe to purely on-demand streaming services like Netflix, Hulu, and Disney+, where the concerns HLG was meant to address don’t apply.
Even broadcasting is beginning to adopt more advanced HDR formats. Dolby Vision is supported over NextGen TV (ATSC 3.0) signals, and in fact NBC’s live sports broadcasts combine Vision and Atmos. You can also get Vision during live events on purely internet-based services like Apple TV, Peacock, and HBO Max. HDR10+ doesn’t seem to be popular in the broadcasting world yet, but it already has a foothold with France 2 UHD, which is applying it to its own sports coverage.
There’s a distinct chance, then, that HLG could be relegated to YouTube, but that’s no safe-haven either. There, HDR10+ is an option, if probably more effort than it’s worth for the average creator. It’s comparatively easy to master content in HLG.
Will HLG disappear from TVs?
Outlook is hazy, try again later
Not anytime soon, certainly. There are too many major players invested in it, and buyers would naturally be upset if some videos on iPlayer, YouTube, or DirecTV looked worse on a newer TV than an old one. Perhaps more sigificantly, HLG is both simple and royalty-free. With no real cost to implement it, there’s no short-term incentive to drop it, now that it’s widespread. I’m sure Samsung in particular is clinging to it, since the company refuses to pay royalties for Dolby Vision, even on TVs costing five or six digits.
It would be tremendously useful to have something equivalent to Dolby Vision or HDR10+ baked into every video, yet my suspicion is that there’s only so much you can do without attaching larger and larger amounts of metadata to the things we watch.
It’s the long term that’s doubtful. Given the move away from over-the-air broadcasting, and growing support for more advanced HDR formats everywhere, HLG could silently fade away at some point, much in the same way that no one is particularly concerned about supporting 3D TV anymore. I’d expect this to happen sooner rather than later if there’s some clear advantage to mastering in another format that creates a conflict with HLG workflows.
Could the HLG format itself evolve to stay relevant? That’s beyond my experience to say. It would be tremendously useful to have something equivalent to Dolby Vision or HDR10+ baked into every video, yet my suspicion is that there’s only so much you can do without attaching larger and larger amounts of metadata to the things we watch.
