All you wanted to know about Hybrid Log Gamma (HLG)

Amidst the quagmire of information about UHD and more specifically, about HDR, HLG seems a truly widespread HDR standard. Let us look into a bit more detail what HLG is and the direction it may take.

By Yeray Alfageme, Service Manager Olympic Channel

Most TV professionals and viewers agree that within the UHD (Ultra High Definition) universe, HDR (High Dynamic Range) is what really makes a difference. Research empirically shows that an HD-HDR image is to be preferred over a 4K-SDI image. An increased dynamic range in an image is visually much more attractive than a four-fold increase in resolution.

While parameters such as definition -HD or 4K- or color –BT.709 or BT.2020- have standards in place, this is not the case with HDR and therefore we find several proposals which try to make it through, such as:

  • Dolby Vision
  • PQ10
  • Sony S-Log 3
  • HDR10 and HDR 10+

Within all these, HLG (Hybrid Log Gamma) seems to be becoming a widely established standard, at least in the audiovisual industry. Let us try to find out why.

BBC and NHK

HLG was born from a joint initiative by BBC (England) and NHK (Japan) as their technical interpretation of the new HDR space. Once this origin is understood it is easier to grasp that this is a backward compatible standard, as it enormously simplifies productions for both broadcasters and producers.

Manufacturers in which both companies were highly influential, such as Ikegami, quickly joined this initiative. In collaboration with manufacturers, development was initiated of an open standard that was compatible with any equipment meeting the required technical specifications.

Sunset comparison HLG technology

Backward compatibility

HLG is not the best standard in regard to image quality. It seems that in this regard Dolby Vision would be the right choice if only quality were considered. However, there are other issues to take into account. One that is quite important -even crucial- in an economics-driven world, its backward compatibility. Under any other standard, generating an HDR signal –for a live event, for example- means creating two signals, one SD signal and one HDR signal. This is not the case when HLG is used.

The HLG gamma curve extends further beyond SDR luminance parameters, thus allowing such higher dynamic range sought in an HDR image. However, it has a peculiarity making it backward compatible. In the standard luminance range -SDR- both SDR and HLG gamma curves are identical, which makes it possible to extract from the same HDR-HLG signal a full HDR image and a SDR image with no quality loss.

This turns it into a more than convenient choice in production environments requiring both signal types.

An open standard

In an environment in which the standard is not established, there are brands and competitors trying to impose their solution with the clear intention of setting the trend in the market, thus gaining share in sales. The same that happened between Betamax and VHS or between HD-DVD and Blu-Ray is now taking place in the HDR arena.

Dolby proposes its Dolby Vision system, which is widely used in cinema environments. In order to implement this standard, the whole production chain must be able to handle a Dolby Vision signal, which is seldom the case. Starting with the camera, the recording format, the edition and post-production system, as well as final playback must be Dolby-compatible and Dolby-certified. And the latter is the key point. Dolby must certify all Dolby Vision systems upon prior payment of royalties, of course. This makes its expansion limited in environments in which the whole chain is not under control, such as normal TV distribution.

The same did Sony with S-Log 3, but only in professional environments. All HDR equipment manufactured by Sonly –cameras, monitors, converters and signal processing systems natively work in S-Log 3 and only in said system, thus avoiding multiple compatibility and configuration issues. But -there is always a ‘but’- only Sony works in S-Log 3. No other manufacturer has this technology, which makes complicated mixing Sony’s HDR equipment with devices from any other manufacturer.  There are converters –by Sony itself- that make this scenario possible, but in some instances introducing an additional failure point in the chain is not the most advisable thing to do.

HLG was not conceived by a specific manufacturer, and to me this is a real strength for a standard. As it is an initiative born from a partnership between BBC and NHK which other broadcasters and manufacturers have been gradually joining, it is widely accepted and has the potential of adding more adopters in the future.

TV simulating HLG technology

How and what to see in HLG

Nearly all 4K TV manufacturers in the market are implementing support for HLG in their systems. Even Samsung and LG have announced that their HDR TV sets manufactured from 2016 onwards will be HLG-compatible through a software update.

Other minor manufacturers such as Hisense, Loewe, Philips or Toshiba also hopped to the HLG bandwagon during this year 2019. Fortunately, HLG support is becoming a must, which is good for this technology and for consumers.

In regard to content, of course, BBC is one of the main promoters of this technology. Its iPlayer, launched in December 2017, supports HLG and its broadly known Blue Planet II documentary had all seven episodes shot in 4K-HLG. It also produced the royal wedding, the FA Cup and Wimbledon in HLG, which helped to disseminate this standard, especially in England.

As for the Spanish market, Mediapro announced its intention to broadcast the Spanish La Liga football league championship in HLG. On the other hand, Google has assured that YouTube’s HDR videos will support HLG coding. Even some satellite TV providers such as Sky or Eutelsat, will have some channels dedicated to HLG.

What about its future

There is no doubt that there is a long way before the HLG system becomes something normal in broadcasts in all countries. Even more so if we are talking about public TV.

Other HDR formats, such as HDR10+ or Dolby Vision may again draw our attract our attention, especially in regard to streaming services or recording formats, but sure that HLG will be the standard of choice in broadcast format.

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