Will Hybrid Log-Gamma Present An HDR Log Jam?

As the nascent market for 4K Ultra TVs with high dynamic range (HDR) continues to accelerate, a number of competing standards have emerged, promising to deliver peak luminance, color grading picture improvement benefits, and backward compatibility with standard dynamic range (SDR) TV sets.

Looking to avoid confusion in the market, television set manufacturers have shied away from calling the assortment of competing HDR formats now surfacing “a format war.” However, that hasn’t stopped some press outlets from sounding the battle drums. Whether this will have any impact on the accelerating demand for next-generation HDR displays warrants further monitoring.

The arrival of the HDR-10 and Dolby Vision HDR platforms has been much discussed over the past two years as both systems gain traction for consumer applications. On top of this, a joint system between Technicolor and Philips was promoted at CES 2016 and NAB 2016 as an alternative with additional SDR-compatibility benefits.

The latest, and perhaps least known, HDR format in the consumer products sphere is called Hybrid Log Gamma (HLG). It is also the only one of the four competing HDR formats that is not based on the perceptual quantitizer (PQ) gamma curve/Electro-Optical Transfer Function (EOTF) ST2084 platform. It was developed by the BBC and NHK for use in live on-the-fly and pre-recorded 4K UHD content with HDR for future over-the-air broadcasts.

The HLG HDR format was approved as ARIB STD-B67 by Japan’s Association of Radio Industries and Businesses (ARIB) and adopted in the Phase A guidelines for commercially deployable Ultra HD real-time linear service with live and pre-recorded content in 2016—along with SDR and PQ—by the Ultra HD Forum. The Forum is an international association of broadcasters, mastering houses, workflow producers and others.

HLG is an open technology that doesn’t require use of metadata, since it is backward compatible with SDR TVs, although Pat Griffis, Dolby Labs office of the CTO, VP of Technology, pointed out that the backward compatibility only applies if the SDR is Rec. 2020 color (the latest color primaries) and not legacy Rec .709 color primaries. This would make the backward compatibility benefit less useful in most applications. HLG can be used with HEVC Main 10, Level 5.1 compression, but is not yet supported in any consumer TVs available in the U.S. market.

HLG has been suggested as a format for use in the next-generation U.S. broadcast standard known as ATSC 3.0, now in development—though ATSC executives have said no HDR formats have been selected for that system yet. In addition to delivering HDR, the next-generation ATSC 3.0 digital broadcast standard will be expandable and will soon give broadcasters in supporting countries the ability to present up to 4K Ultra HD capability, high frame rates and many other things.

Thus far, most consumer television manufacturers have been mum about their positions on HLG. Both LG and Sony are part of the Ultra HD Forum, which has adopted HLG. LG told us that HLG will be supported in LG 4K UHD TVs as soon as the HDMI signaling standards are finalized, possibly as soon as this fall. Sony, which has shown applications using HLG in some of its prototype equipment for the broadcasting industry, did not return repeated requests for comment.

Understandably, other manufacturers with vested interests in selling this year’s crop of 4K Ultra HDTVs have been less forthcoming about their positions on HLG. Samsung, which is a major backer of the baseline HDR-10 format now used in Ultra HD Blu-ray Discs and some HDR streaming and download applications, had no comment when we asked about their plans for HLG.

One of the problems with HLG, as well as the Technicolor/Philips HDR format also under consideration, and various applications of Dolby Labs’ Dolby Vision HDR system, is that they will require or benefit further from a next-generation digital interface connection (possibly a future HDMI 2.1 version?).

In a Philips whitepaper about its HDR proposal, HDMI 2.0a covers HDR EOTF signaling and static metadata like that used in HDR-10, but the Philips HDR system and Dolby Vision, support so-called dynamic metadata, which enables luminance and color grading on a scene-by-scene basis. Dolby Vision can be adapted for use with existing HDMI versions, but Philips, like HLG, will require a new connector, that “is to be covered in HDMI 2.1.”

LG has demonstrated HLG capability at IFA 2015, CES 2016 and NAB 2016. It said that it expects the existing HDMI interface in its current TVs to be upgraded via a firmware update to incorporate the required new HDMI signaling, and possibly without the need of additional hardware. Sources said that it is generally expected that the CTA’s Profile for Uncompressed Digital Interfaces (CTA 861) will be revised this fall to accommodate the new HDMI signaling. Meanwhile, LG has already demonstrated HLG capability at IFA 2015, CES 2016 and NAB 2016.

Hybrid Log-Gamma has a number of different benefits including a generally less disruptive ability to upgrade or integrate with established broadcast equipment. It is presumed to provide one of the more efficient means of presenting at least some of the benefits of wider colors and luminance levels associated with the other HDR formats in live broadcasts.

Since HLG does not need to use metadata, like the HDR-10 and Dolby Vision HDR formats, it is compatible with both standard dynamic range (SDR) displays and HDR displays, and can be used for presenting different brightness levels in a wide range of viewing environments.

HLG extends the capacity to capture and deliver highlights in any type of content by adding a log function to the top half of a traditional camera gamma curve. The dynamic range that can be perceived by the human eye in a single image is around 14 stops. SDR video with a 2.4 gamma curve and a bit depth of 8-bits per sample has a dynamic range of about 6 stops. Professional SDR video with a bit depth of 10-bits per sample has a dynamic range of about 10 stops. When HLG is displayed on a 2,000 cd/m2 display with a bit depth of 10-bits per sample it has a dynamic range of 200,000:1 or 17.6 stops.

Unlike the ST2084 based formats (HDR-10, Dolby Vision, Technicolor/Philips), the HLG HDR standard is not based on a reference display with a specified maximum luminance value. HLG, instead, changes the EOTF (the EOTF is the method of transforming digital code into visible light) gamma curve based on any given display’s actual peak luma value, as well as the display’s surround illumination. The HLG standard has an EOTF gamma curve and shows that only a small portion of the image dynamic range would use the extended brightness capability, and brightness of the majority of the surrounding image would remain relatively low.

The HLG format, like each of the other HDR systems, provides a range of benefits that will enhance broadcast video imagery as we know it, but whether or not the selection of multiple formats will create a boon to Ultra HDTV acceptance or a logjam of confusion is a conundrum standards bodies will need to carefully consider in the months ahead.