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Hybrid Log Gamma (HLG) – A new HDR Format for Broadcasting

by on March 13, 2017

As we know, the main focus of TV in 2016 is HDR where there are two most popular HDR format which used by most of TVs that released in 2016, HDR 10 and Dolby Vision. HDR 10 is royalty-free HDR format developed by UHD Alliance while Dolby Vision is proprietary HDR format developed by Dolby Laboratories. But as the note, these HDR format only involve a subscription to a video on-demand service such as Netflix and Amazon or Physical source such as Blu-Ray player. This means none of these HDR formats involve video sources from broadcasting. For this case, Hybrid Log Gamma (HLG) come to complete existing HDR formats. Now, with HLG format, you can enjoy live TV in HDR quality. The advantage, to enjoy HLG contents, you may still need a TV with 4K resolution but don’t have to have HDR ability, so you don’t need to buy the latest TV.

What is Hybrid Log Gamma (HLG)?

Hybrid Log Gamma (HLG)

As we have mentioned above, Hybrid Log Gamma is an HDR standard that specifically developed for broadcasting. It is royalty-free HDR format that was jointly developed by 2 big names of TV broadcasters, BBC and NHK, and was approved by ARIB STD-B67. As we know, two previously HDR format, HDR 10 and Dolby Vision use metadata as the additional signal for remapping color and brightness, HDR 10 uses static metadata while Dolby Vision uses frame-by-frame dynamic metadata.  But the use of metadata signals for TV transmission will make susceptible to interference or get lost during the production process especially when they are re-distributed around a network or supplied to affiliate broadcasters who may need to top-and-tail. That’s why both of HDR 10 and Dolby Vision format are not suitable for TV transmission especially for live broadcast and they only suit for pre-produced contents.

For solve this problem, BBC and NHK have jointly developed HDR system that suitable for TV transmission where it didn’t require metadata HDR info. For this case, Hybrid Log Gamma (HLG) came. HLG is designed refer to scene system, where this system is used by conventional TV. Because it is designed with the system which used by conventional TV, with its own internal technology, your TV at home should be able to read HLG signals and display them with quality as expected without additional equipments. This means, your old 4K TV that don’t have HDR ability also can read HLG signals and display them, despite perhaps with limited quality.

How Does Hybrid Log Gamma Work?

As we have mentioned above, HLG is a scene-referred system, the same system which used by conventional TV broadcast. Unlike other HDR format where they require pre-encoded metadata signals to explain the receivers, how the picture will be displayed. Because it is a scene-referred system, it doesn’t need metadata. This means, the signal that was delivered refers to the lights level in the original scene, just like when the picture was shot. Put simply, what have been taken in the studio or outside broadcast was delivered with the same artistic effect in on brighter or darker to the screens like TV or projectors in your home. The display itself only needs the information about its own technical capabilities and environment to precisely render HLG signal that is received. Why is environment? This becomes one of the advantages of HLG than the other formats like HDR 10 and Dolby Vision. You need to know, HLG has an ability to adjust HDR Images that produced based on the condition of environment where the contents is being watched, whether it is dark room or bright room. This means, HLG will send different brightness for a dark room or bright room to get the maximum result. This allows you to enjoy HDR contents with a far more satisfying viewing experience.

The other advantage, HLG is designed to be compatible with Standard Dynamic Range (SDR) TVs. Unlike HDR or Dolby Vision where they have different color format, so TVs which support these formats that only can display. This means TVs which don’t support HDR 10 or Dolby Vision certainly cannot display them.  Meanwhile, because HLG has the same color format to conventional TV, of course all TVs including SDR TVs can play HLG format. But even so, at least they must have 4K resolution. But of course, because old SDR TVs don’t have the ability to alter the viewing environment, of course they just can display HLG. Meanwhile, the quality of HDR picture displayed is depend on each of their display specs themselves

When will Hybrid Log Gamma (HLG) able to be enjoyed at home?

Unfortunately, there is no certainty about when will Hybrid Log Gamma be available at Home?  Currently, HLG is currently included in the ITU-R BT.2100 specification for programme production and exchange. But even so, it has not been ratified into DVB specification, so the Television Manufactures and Broadcasters cannot begin to firm road maps. But it is expected, before the end of this year, it has been ratified.

While the standards of HLG has not been confirmed, all of the TV manufactures has started to release their 2017 models. This means, all of them have not equipped with HLG ability and you cannot enjoy HLG at your home for this time or a few months ahead. If we by TV before that standard of HLG is confirmed, we certainly only can expect each of the manufacturers will provide update firmware for supporting HLG.  Fortunately, most of TV manufacturers have confirmed that they will provide update firmware which already support HLG, even not for 2017 model but also for 2016 model. So you do not need to worry for it, although you have bought earlier.

Why does their firmware need to be update while HLG is already compatible to all UHD TV even for SDR 4K TVs? As we have discussed, to display HLG contents, a display needs the information about its own capabilities and environment to alter the average of picture level of HLG contents. This means, without a firmware that already support HLG, a TV especially SDR UHD TV can display HLG contents only but it can’t get the advantages that offered by HLG like the ability to alter the HDR image based on the viewing environment.


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