HDMI HDR signaling

HDMI HDR signaling communicates relevant interpretation of the active output picture, separately from HDR metadata in the source and HDR capabilities declared by the sink.

HDR information appears at several stages of playback. A source file describes its encoded picture; a display declaration describes accepted inputs; output signaling describes the presentation actually being sent.

Keeping those stages separate avoids a common misunderstanding: a movie being HDR does not guarantee that every later stage is currently carrying the same HDR representation.

Three descriptions, three scopes

Information What it describes
Source color tags and mastering/content metadata Interpretation and creation/content context of the encoded media
Sink capability, including relevant CTA blocks What the connected path declares it can accept
Output signal and applicable InfoFrames Interpretation/information supplied with the active presentation

CTA’s static HDR extension defines EDID capability information and a relevant HDR metadata InfoFrame relationship. Capability and output information have different directions and purposes.

CTA-861 blocks develops the declaration side.

A compatible display can still receive SDR

This original readable example describes one valid kind of adapted route:

{
  "source": {"transfer": "PQ", "primaries": "BT.2020"},
  "sink": {"acceptsPQ": true},
  "output": {
    "transfer": "SDR",
    "primaries": "BT.709",
    "toneMapped": true
  }
}

It is a teaching descriptor, not a native HDMI packet or captured Emby report.

The sink could accept PQ, yet the selected route sends an SDR target after tone mapping. The source’s PQ tags should not then be copied blindly as though they described unchanged PQ output.

The active output must describe the values actually being presented. Otherwise the display may interpret an adapted SDR picture through the wrong transfer/color relationship.

The renderer and output system must agree

A decoder reconstructs picture values in their encoded interpretation. A renderer can scale, convert or tone-map those values. The presentation/output system then establishes the appropriate surface and signal interpretation.

A PQ-tagged source interpreted as SDR can look wrong without any missing video bytes. A surface carrying changed values but retaining a stale interpretation can also be wrong.

Display color management explains those responsibilities. The server’s Direct Play decision establishes the original media delivery choice, while local client output preparation still needs its own correct route.

Metadata is not a measurement of the current panel

Mastering display metadata describes a creation context. Content-light information describes properties of the content. Sink declarations can provide target information within their defined scope.

None is automatically a measurement of the light emitted by the panel at that moment.

A TV reporting HDR recognition is useful evidence that its input identified a relevant signal arrangement. Its tone mapping, local dimming and brightness limits still determine the actual output.

An AV receiver or converter can also affect whether the required signaling reaches the display. Successful ordinary video transport through that device does not establish preservation of every HDR packet or format.

See Also

References