Overview
ISO/IEC 23091-3:2018 - part of the Coding‑independent Code Points series - defines coding‑independent code points for audio. The standard specifies a central set of metadata fields and enumerated values that describe audio stream properties independent of compression format or bit rate. These code points cover loudspeaker geometry and positions, loudness metrics, peak levels, and dynamic range compressor (DRC) characteristics. The goal is consistent, interoperable audio interpretation across codecs, containers, broadcast and streaming systems.
Key topics and requirements
- Coding‑independent metadata: Centralized definitions to avoid duplicated or conflicting code point assignments across specifications.
- OutputChannelPosition (loudspeaker index): Enumerated positions for loudspeaker locations (type: unsigned integer; range example provided in the standard).
- ChannelConfiguration / LoudspeakerLayout: Identifiers for common loudspeaker constellations used during authoring or playback.
- Loudspeaker geometry: Azimuth, elevation and layout descriptors for 3D/immersive audio rendering.
- Loudness and peak metrics: Code points for ProgramLoudness, AnchorLoudness, LoudnessRange, ShortTerm and Momentary loudness maxima, SamplePeakLevel and TruePeakLevel-aligned with referenced practices (ITU‑R BS.1771, EBU R128, EBU Tech 3341/3342).
- DRC characteristics: Indexed descriptors to indicate compressor behaviour applicable to content.
- Reference and encoding guidance: Recommendations on value types, ranges, default handling and use of a URN format (defined in ISO/IEC 23091‑1) for unambiguous referencing.
Practical applications
- Improve interoperability of audio metadata across codecs, containers and delivery networks.
- Enable consistent loudness management for broadcast, streaming, and OTT workflows by carrying format‑agnostic loudness and peak information.
- Support immersive audio authoring and rendering by standardizing loudspeaker positions and layouts for multichannel and object‑based systems.
- Facilitate encoder and decoder implementations that need to understand pre‑ or post‑encoded signal characteristics (e.g., automated level control, DRC application, downmixing).
- Provide a stable central registry for vendors and standards writers to reference without embedding duplicated enumerations.
Who should use this standard
- Audio codec and container spec authors
- Broadcast and streaming engineers (loudness normalization & metadata)
- Immersive audio system integrators and game/audio engine developers
- Metadata tool and middleware vendors
- Standards bodies and implementers seeking consistent, versionable audio descriptors
Related standards
- ISO/IEC 23091‑1 (Systems / URN format for code points)
- ITU‑R BS.1771, EBU R128, EBU Tech 3341 / 3342 (normative references for loudness and metering)
Using ISO/IEC 23091‑3 helps ensure consistent interpretation of audio across production, distribution and playback chains by providing clear, encoding‑agnostic metadata for loudspeaker layout, loudness, peaks and DRC behavior.
