To Metadata or Not to Metadata

That is the question – on many people’s minds at the SMPTE Conference. There is a clear diversity of opinions about the use of metadata in live broadcast and file-based workflows.

Dolby says they are suggesting that a new HDR format called PQ10 is the answer for live broadcast. PQ10 is essentially HDR10 but without metadata. That means it uses the PQ display referred EOTF, BT.2020 color coordinates and container and 10-bits per color.

But Sony’s Hugo Gaggioni (pictured) is not a supporter. For one, there is near universal agreement that trying to use metadata in a broadcast environment is going to be a very difficult sell. The problem with metadata in live broadcast is that there is no standard for the carriage with video. There is a metadata standard for audio in broadcast, apparently, but we were told that it is highly problematic as it gets dropped in passing through various pieces of equipment, so the industry has essentially given up on using it.

But even with PQ10, Gaggioni’s problem is the use of the PQ curve primarily. This is a display referred transfer curve with specific code values pegged to absolute luminance values. In a broadcast environment, each camera is set up for the range of luminance values it expects to see. For example, camera 1 may set the luminance range from 0.5 to 600 cd/m² while camera two may set it from 1 cd/m² to 1200 cd/m².

In an SDR or HLG workflow, these camera set ups are called “scene referred” and the normalized code values go from 0 to 1 – regardless of the actual luminance values. This allows a switcher to change from camera 1 to camera 2 and create program content that has a consistent look from camera to camera.

Gaggioni notes that if these cameras use the PQ curve, then content on camera 2 will be brighter than desired compared to camera 1. Metadata is needed to normalize these signals to create the scene referred images that are better for switching, but that is a non-starter.

The whole idea behind HLG is that there is no metadata as it is a scene-referred format. This is supported by the BBC and NHK and has been standardized in ITU BT.2100.

Technicolor and Dolby want to add metadata to HDR content once it is ready for distribution to affiliates or other distribution points. But distribution has not historically supported metadata before, so the same reticence will exist to now including it. And so the debate continues. -CC