If you’ve been shopping for a TV recently, you’ve probably noticed that nearly any TV larger than 40 inches offers 4K Ultra HD resolution — that’s almost a given. But what about High Dynamic Range (HDR), the cool, new thing when it comes to TVs? It offers up darker blacks, brighter highlights, and generally richer, more vivid color. But unlike 4K, which is a standard resolution you can easily measure, there are multiple different kinds of HDR, and you don’t always get access to all of them with a given TV. The TV you get might support HDR10, but not Dolby Vision, for example.
The latest format you’re probably beginning to hear about Hybrid Log Gamma (HLG). Why do we need yet another HDR standard? As we’ll explore in this article, HLG offers a more flexible approach than other standards, and it is the HDR format of choice for broadcast media.
What is Hybrid Log Gamma?
Hybrid Log Gamma aims to solve two problems that exist with other HDR technologies. First, existing HDR solutions like Dolby Vision and HDR10 aren’t suitable for broadcast signals, for reasons we’ll explain later. Second, no other existing HDR technologies are easily backward compatible, meaning that not only are owners of older TVs out of luck, but content needs to be produced in both HDR and non-HDR varieties.
Hybrid Log Gamma began in 2014 as a collaboration between the BBC and Japanese broadcaster NHK, which is no stranger to pushing new broadcast technologies. Both were intent on creating a new broadcast-friendly HDR solution. Looking to avoid the same sort of troubles that we saw in the early days of HDTV, when some networks would have SD and HD versions on different channels, the two broadcasters were looking to create a more all-in-one solution.
How is HLG different?
In order to show something in HDR, your TV needs to know how to display the signal. Both HDR 10 and Dolby Vision use metadata to tell your TV how to display colors and assign brightness parameters. The problem, at least from a backward-compatibility standpoint, is that older TVs and modern TVs that don’t support HDR don’t see this metadata and wouldn’t know what to do with it if they did.
Solving the compatibility problem
HLG takes a different approach. Instead of starting with an HDR signal, HLG begins with a standard dynamic range (SDR) signal that any TV can use. The extra information for HDR rendering is added on, so an HDR TV that knows to look for this information can use it to display a broader range of colors and a wider range of brightness. To get a little more technical, HLG uses the same gamma curve that an SDR signal uses but adds a logarithmic curve with extra brightness over the top of the signal, hence the “log” and “gamma” in Hybrid Log Gamma.
This is what makes HLG especially well-suited for broadcast. Early on, the BBC and NHK decided against using a metadata-based approach, since metadata could be lost, or in the case of Dolby Vision, which uses a more complicated approach, out of sync with the image on screen, causing colors to display incorrectly. The ability to display on any TV — theoretically even your old non-HD TV — is another bonus for broadcast.
Advantages & disadvantages
There are a few more advantages to HLG aside from its compatibility. While other HDR solutions can require an extensive overhaul of the production workflow when it comes to the people making the shows and movies you watch, HLG allows studios to use most of their existing equipment, with only a few changes.
Since HLG doesn’t use metadata, it can also dynamically alter colors based on your environment, though how much of a difference this could really make literally remains to be seen. Since there is more color information in the signal, regardless of the TV displaying it, this also could make for better color reproduction even on SDR displays, though this advantage is mostly theoretical at this point.
Not everything about HLG is perfect. Due to the way HLG handles low light, it can’t enhance darker parts of an image the way that HDR10 or Dolby Vision can. Basically, HLG can make images brighter and more vivid, but the blacks remain the same as an SDR image.
There is also one issue with compatibility. Despite being backward-compatible with SDR TVs, it actually isn’t compatible with non-HLG HDR TVs. This can be fixed via firmware updates, but if your TV doesn’t support HLG out of the box and the manufacturer doesn’t issue an update, you’re out of luck.
How can I see HLG in action?
If you’re in a hurry to start watching HDR content, you’ll want to look elsewhere, as HLG is still in its relative infancy. Because of this, the video actually using HLG is currently scarce. Support for the standard does seem to be on the way from quite a few of the major players in the industry.
Who’s on board?
Obviously, the BBC and NHK are moving forward with support for HLG. In the case of the BBC, the technology is already in use in the BBC iPlayer. For the time being, the U.K. seems to be leading the HLG charge, at least when it comes to broadcasters, with British BT and Sky UK indicating planned support for HLG in the future.
When it comes to hardware and software, the number of companies supporting the standard grows significantly. Both Samsung and Sony support HLG in their newer TVs, with many models dating back to 2016 able to add support via a firmware update. Panasonic’s 4K OLED TVs, recent offerings from Hisense, and LG TVs like the impressive C7 support HLG as well. Sony also offers HLG support on some of its newer projectors, as does JVC. HLG seems to be picking up steam, with more and more products released with out-of-the-box support, so expect to see it more often moving forward.
Finally, major support for HLG has come from Google. Not only does the company’s Android TV platform offer support for HLG starting with version 7.0, but it also already offers support for HLG as one of the HDR options for YouTube.
Where’s the content?
For the time being, you’re going to have a tough time finding HLG content to watch anywhere. While the technology is used in the BBC iPlayer and can be used for HDR videos on YouTube, the presence of technologies that have already been adopted like Dolby Vision or HDR10 means that most content providers aren’t jumping up and down to support another one.
As mentioned above, some major players have announced plans to use HLG, and 2018 could be the year that we begin to see them rolling out support. Widespread mainstream support could be further off, considering that major TV providers are only just starting to roll out the occasional 4K Ultra HD content. After years of 4K TVs being available, we’re finally starting to see providers like DirecTV ramp up 4K content availability, but the generally slow pace could be an indicator of how long we may have to wait for HLG — or any HDR technology — to be widely used in broadcasts.
Will Hybrid Log Gamma become the HDR standard moving forward? It’s too early to say. What we can say for now is that if you’re interested in watching broadcast TV in HDR, you might want to make sure that your TV supports HLG or can in the future. In the meantime, if you want to know more about this subject as a whole, check our guide to all things HDR.