As one of the many new content formats found on modern TVs, you may support Hybrid Log Gamma (or 'HLG') on your TV and you don't even know it.. But what is HLG and why should you care? The answer is in HDR (High Dynamic Range): a video format that improves the brightness, sharpness, and color gamut of an image beyond SDR (Standard Dynamic Range).
- What is HDR TV? Learn more in our complete TV format guide
HDR already comes in several different forms, from the original HDR10 standard to the more unique Dolby Vision, to the advanced HDR still emerging from Technicolor and, for broadcasters, to Hybrid Log Gamma. To help you understand this brave new world of color and clarity, we've put together an overview of the new Hybrid Log Gamma HDR format and everything that sets it apart from competing standards.
So what is Hybrid Log Gamma?
Hybrid Log Gamma (or HLG) is an HDR format developed by the UK's own BBC, in collaboration with NHK, Japan's national broadcaster.
The problem with traditional broadcasters is that many of their viewers still have old SDR TVs, which can't display the increasingly popular HDR standard. SDR is also much cheaper to film, and the likes of the BBC are naturally reluctant to abandon a profitable format where tens of thousands of viewers still count. The HLG format overcomes this hurdle by encoding the SDR and HDR streams on the same broadcast signal. It's a much more efficient process for broadcasters, who don't have to provide twice the bandwidth to get their programming across the country in two separate formats.
When it reaches your home TV, the HLG signal will be displayed in HDR if your TV supports the HLG HDR format.. Otherwise, it's displayed in classic SDR, with a careful update to bring it closer to its HDR counterpart.
The HLG transmission format is slightly different from the HLG photo mode, which displays HDR still images from Panasonic Lumix cameras., and is only available on OLED TVs from the 2019 Panasonic TV family.
How does Hybrid Log Gamma work?
Hybrid Log Gamma uses something called the "opto-optical transfer function", which is the process used to convert a transmission signal into light that appears on your TV screen. Both SDR and HDR are converted to two types of light encoding, which can then be decompressed separately depending on the TV's compatibility.
Hybrid in Hybrid Log Gamma refers to this dual SDR and HDR encoding. "Gamma" refers to low-light image data encoded into the signal, while "log" is short for "logarithmic curve" that transmits over the wider brightness range of HDR.
And why does anyone need Hybrid Log Gamma?
Although having a single dominant format would certainly be simpler, for users who just want to continue, you know, watching TV,
competition is undoubtedly raising the standard picture quality we expect from our television screens.
HLG is just one of many available HDR formats, all of which do different things and are suitable for different purposes. HDR10 is the most common of these, and this is generally what people mean when they say "HDR" (any HDR TV will come with it built in). Like HLG, it's an open source format, meaning anyone can use it, and it offers a broader color palette than SDR, with a 10-bit color depth and a maximum brightness of 4000 nits. Dolby Vision is a well-known alternative, which can achieve up to 10,000 nits of brightness and 12-bit color depth, and generally delivers a better picture. However, it requires fees from content providers to use, therefore it's far less ubiquitous, already facing competition from an updated HDR10+ standard with equivalent bells and whistles. Both Dolby Vision and HDR10+ use a type of "dynamic metadata" in real time, optimizing brightness and contrast based on the images displayed on the screen in each shot.
HLG is specially designed to facilitate transmission, which means you forgo metadata that could be lost or out of sync during a live stream.
So what can I watch HLG content on?
As with any new format, HLG will only be as strong as the players that support it. It is essentially its own HDR format. Therefore, an HDR TV will need to have the ability to recognize and read the format.
All recent HDR TVs from LG, Sony or Samsung, manufactured in 2016 or later, have included HLG support in firmware updates. Panasonic has also supported the format in several of its premium HDR packages. Support is scattered elsewhere, including the odd Sony and JVC projectors, but it's not as widespread as HDR10 and you'll need to make sure it's compatible with your specific TV brand or another.
What's the next step for Hybrid Log Gamma?
The BBC began testing the technology in 2017. and, after strong public reaction, rolled it out to flagship shows like Blue Planet II, as well as major sporting events like this year's FIFA World Cup. One of the BBC's blog articles on HLG says: “The public comment on last year's trial was fantastic and exceeded our expectations. Even with just four minutes of content, we were delighted to learn that there is a real public appetite for better quality images, and more."
HLG is compatible with BBC iPlayer, of course, as well as on YouTube, Freeview Play and DirecTV. Hybrid Log Gamma will no doubt start showing up on more TVs, content platforms and the like, though in a highly competitive market, you never know what will last and what will fall.