Hybrid Log-Gamma (HLG) is a high dynamic range (HDR) standard that was jointly developed by the BBC and NHK. The HLG standard is royalty-free and was approved as ARIB STD-B67 by the Association of Radio Industries and Businesses (ARIB). HLG is compatible with standard dynamic range (SDR) displays.
The BBC/NHK approach to HDR is to replace the gamma of Rec. 709 (which defines the relationship between the signal input coming into the display and the brightness of the output) with a new Electrical Opto Transfer Function (EOTF) that “splices” a log-curve onto the “high end” of Rec. 709. There is a white paper with more technical detail on the BBC R&D website.
The BBC points out that the BT.709 gamma works pretty well at low to mid levels of brightness, as seen on traditional CRTs. However, to match the HDR TVs that are starting to be available, better performance at high brightness is needed. The BBC believes that its proposal would support displays up to around 5,000 cd/m² with 10-bit grey scale data (it also has a 12-bit proposal for content creation). (Adam said that 1,000 cd/m² is likely to be a practical maximum brightness for a while for consumer TVs)
There are some huge advantages of the BBC proposal for broadcasters, no metadata is needed, which would be an advantage for live broadcast.
For more information see:
ARIB STD-B67 (PDF)
BBC WHP 309: A “Display Independent” High Dynamic Range Television System (PDF)
Technical Details
HLG defines a nonlinear transfer function in which the lower half of the signal values use a gamma curve and the upper half of the signal values use a logarithmic curve.
where E is the signal normalized by the reference white level and E’ is the resulting nonlinear signal
where r is the reference white level and has a signal value of 0.5
where a = 0.17883277, b = 0.28466892, and c = 0.55991073
The signal value is 0.5 for the reference white level while the signal value for 1 has a relative luminance that is 12 times higher than the reference white level. ARIB STD-B67 has a nominal range of 0 to 12. HLG uses a logarithmic curve for the upper half of the signal values due to Weber’s law.
HLG does not need to use metadata since it is compatible with both SDR displays and HDR displays. HLG can be used with displays of different brightness in a wide range of viewing environments.
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.
HLG increases the dynamic range of the video compared to a conventional gamma curve by using a logarithmic curve for the upper half of the signal values. HLG also increases the dynamic range by not including the linear part of the conventional gamma curve used by Rec. 601 and Rec. 709. The linear part of the conventional gamma curve was used to limit camera noise in a low light video but is no longer needed with HDR cameras.
HLG is supported in HEVC with a formula that is mathematically equivalent to ARIB STD-B67 but has a nominal range of 0 to 1 instead of 0 to 12:
where Lc has a nominal range of 0 to 1 and V is the resulting nonlinear signal
where a = 0.17883277, b = 0.28466892, and c = 0.55991073
HLG teststream & HDR Tests
SES has launched a 24/7 Ultra-HD test channel for transmitting High Dynamic Range (HDR) content, HLG teststream.
HLG stands for Hybrid Log-Gamma.SES is, with industry partners, demonstrating several technologies for enhancing the quality of Ultra-HD television with HDR, which provides much higher contrast and offers a more realistic viewing experience. The transmission is in 4K, with a resolution of 3840 x 2160 pixels, 60p and HLG Both HDR10 and Dolby Vision is being tested.
Hybrid Log-Gamma (HLG) is a high dynamic range (HDR) standard that was jointly developed by the BBC and NHK. The HLG standard is royalty-free and was approved as ARIB STD-B67 by the Association of Radio Industries and Businesses (ARIB). HLG is compatible with standard dynamic range (SDR) displays.
The broadcasting industry is currently in discussions to formulate a standard for HDR transmission. The goal of SES’ HDR demo channel is to allow industry partners to test the various technology candidates.
At present, the channel carries Ultra-HD content from LG Electronics with a backwards-compatible HDR technology called Hybrid Log Gamma (HLG). HLG HDR content will be demoed on LG’s E6 4K OLED TV.
Satellite: Astra 1M at 19.2°E
Transponder: 49
Frequency: 10.714
Polarization: Horizontal
SR: 23500
FEC: 3/4
Modulation: 8PSK
Coverage: Europe
See the HLG teststream on YouTube in 4K 2160p
History of HLG
On May 15, 2015, the BBC announced that they had begun work with the NHK to develop a joint HDR proposal that would be proposed to the International Telecommunication Union (ITU).
On June 9, 2015, HLG was proposed to the JCT-VC for High-Efficiency Video Coding (HEVC) and added to the June 2015 draft of the screen content coding extensions.
On October 26, 2015, Sony showed HLG video on a modified HDR display at the SMPTE 2015 conference.
On October 27, 2015, Colorfront announced that their Transkoder 2016 software would support HDR output using HLG.
On November 4, 2015, SKY PerfecTV! announced that they will use HLG to transmit 4K UHDTV HDR programming to their satellite subscribers in Japan.
On November 6, 2015, LG announced that their 2015 4K OLED TVs would support HDR from HLG and Perceptual Quantizer (PQ).
On December 3, 2015, Harmonic Inc. and NASA announced the HDR capture of an Atlas V launch which was broadcast the next day on NASA TV using HLG.[15] UHDTV HDR video clips will be published early next year in both HDR10 and HLG.
On December 14, 2015, Vatican Television Center broadcast the ceremony of the Holy Door using HLG and the Rec. 2020 colour space.
On December 23, 2015, Blackmagic Design released an update for DaVinci Resolve that added support for HLG.
2016
On January 12, 2016, Leader Electronics Corporation announced their 12G-SDI waveform monitors with support for HLG.
On January 29, 2016, Avid Technology released an update for Media Composer that added support for HLG.
On March 29, 2016, Harmonic Inc. released an update for the ViBE 4K UHD encoder that added support for HLG.
On April 13, 2016, Canon Inc. announced that they will release firmware updates for the DP-V2410 and DP-V3010 reference displays to add support for HLG.
On April 15, 2016, Dome Productions announced that they will begin trials of HLG to deliver HDR content.
On April 18, 2016, Sony announced the PVM-X550 OLED monitor with support for HLG.[24] Sony also announced a firmware update for the BVM-X300 OLED monitor to add support for HLG.
On April 18, 2016, the Ultra HD Forum announced their guidelines for UHD Phase A which includes support for HLG. The Ultra HD Forum also defined HLG10 as HLG, a bit depth of 10-bits, and the Rec. 2020 colour space.
On July 6, 2016, the ITU announced Rec. 2100 which defines two HDR transfer functions which are HLG and PQ.
On July 29, 2016, SKY Perfect JSAT Group announced that on October 4 they will start the world’s first 4K HDR broadcasts using HLG.
On August 3, 2016, Sony announced that in October they will release a firmware update to add HLG to their BVM-X300 OLED monitor.
