Hybrid Log-Gamma (HLG): Everything you need to know about broadcast HDR

If you’re looking for a new TV, you’d have no doubt been on a crash course of tech acronyms – UHD, HDR, WCG and more are all terms to grips with. Well, here’s another. HLG.

HLG – or Hybrid Log-Gamma as it’s also known as – is another HDR format, but how does it differ from the HDR offered through streaming services or on UHD Blu-ray. Read on to discover what it is and what it means to your TV streaming experience.

What is Hybrid Log-Gamma?

Hybrid Log-Gamma is different to related HDR technologies in several ways.

It was jointly developed by the BBC and Japanese state broadcaster NHK and was intended to resolve the conundrum of how broadcast UHD TV services with HDR can be made backwards compatible with regular HD TVs.

A of backwards compatibility was a major stumbling block for TV companies. Without it they would need to simulcast channels or programmes, and that has huge cost implications from a studio workflow point of view and for bandwidth.

Even better, HLG allows the existing production ecosystem for SDR TV to be used for HDR production – it also provides a way to broadcast live HDR.

The “hybrid” part of Hybrid Log-Gamma refers to the fact that HLG doesn’t reinvent the wheel. It adapts traditional picture-making techniques that 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.

How does HLG differ from HDR10 and Dolby Vision?

HDR already comes in multiple flavours. HDR10 is the industry standard, used on UHD Blu-ray and by streaming services. HDR10 uses “static metadata” to relay the HDR information to a compatible display – a 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 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, HDR10 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.

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 (trim off the top and bottom). 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, Prime Video, Disney+, 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.

How does Hybrid Log-Gamma 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 can 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 HDR10.

Currently, some HDR10 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.

Where can I watch programming in HLG?

For this question, we’ll focus on the UK as we’re not too sure about HLG’s use in other countries. We can say that in the UK, BBC and Sky have taken up the HLG baton.

For the BBC HLG HDR content is only available through its iPlayer streaming service. It still seems to be running trials in HLG since 2016 – this is apparently due to government legislation on what the Beeb can do – so you won’t find every programme available in HLG HDR, but since the end of 2022/beginning of 2023, there’s been a marked increase in HDR content on the service.

Other places include Sky, which currently broadcasts sports such as the English Premier League, F1, cricket, and golf in HLG HDR. If you’re a Sky Cinema subscriber, you’ll be able to stream in HDR10 (similar, but not the same as HLG). You will need to sign up to Sky’s UHD package to be able to view its programming in HDR, though.

While TNT Sports (formerly BT Sports) broadcasts in HDR, we’re haven’t been able to confirm (yet) whether it broadcasts in HDR10 or HLG. We do know that from attending the first ever 8K HDR broadcast at the Emirates Stadium in 2020 that it was in HDR10, and it’s our understanding that nothing has changed on that front.

The summer of 2024 will see Wimbledon Tennis tournament and the Paris Olympics broadcast in HLG HDR; while the Euro 2024 football tournament is slated to be produced in 1080p HDR to be streamed to people’s homes and mobile devices.