HDR and Dolby Vision

HLG

Hybrid Log-Gamma

HLG is the HDR that arrives from a broadcaster rather than from a disc. On the Diva it turns up in three separate places: the EDID flags, the custom HDR modes and the disable options.

HLG stands for Hybrid Log-Gamma. It was developed jointly by the BBC and NHK and standardised in ITU-R BT.2100 in 2016, for live broadcast, where the problems are different from those of a graded film. The name describes the curve: an ordinary gamma curve over the lower part of the signal range, and a logarithmic curve above it, the log section carrying the highlights.

Two consequences follow. A standard dynamic range television shown an HLG signal produces something reasonable rather than a flat grey picture, because the bottom of the curve is close to what it already expects, which is what lets a broadcaster send one stream to every viewer. And HLG is relative rather than absolute: it carries no mastering metadata describing peak brightness, so there is nothing per programme to transmit, unlike HDR10 with its static metadata. Each display maps the signal to whatever it can actually produce.

HLG on the HDfury Diva

The Diva reads what is arriving and reports it: the OLED signal line names the HDR flavour in use, and CAPTURED HDR METADATA in the HDR and DV section shows the infoframe deciphered into readable form. When you build your own metadata there, the EOTF dropdown offers HLG ITU-R BT.2100-0 alongside SDR Luminance Range, HDR Luminance Range and SMPTE ST 2084. Three of the custom HDR modes in that section are specific to HLG.

Use custom HDR for TX0 and TX1 when input is HLG

Ignores the original HLG metadata and sends yours instead, which is how you render HLG content on an HDR display. The IP command is set hdrcustomhlgonly on.

Use custom HDR for TX0 and TX1 when input is HLG and display has no HLG support

The same, but conditional on the television as well. With one HLG capable set and one without connected, turning the HLG set off changes the metadata automatically. The IP command is set hdrcustomhlgonlyauto on.

Disable output HDR for TX0 and TX1 when input is HLG

Intercepts and cancels incoming HLG metadata while HDR10 still passes through untouched. The IP command is set hdrdisablehlgonly on.

The other place HLG appears is the EDID section. Automix carries an HLG flag among its fourteen forceable flags: tick it and the mixed EDID reports HLG support whatever the connected sinks claim, so a source will send HLG through a chain that would otherwise refuse it. Over IP that is set edid hlg on, and the command only adds the capability, it does not take it away from a sink that already advertises it.

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