Ambisonics in an Ogg Opus Container

Ambisonics is a representation format for three dimensional sound fields which can be used for surround sound and immersive virtual reality playback. See and for technical details on the ambisonics format. For the purposes of the this document, ambisonics can be considered a multichannel audio stream. Ogg is a general purpose container, supporting audio, video, and other media. It can be used to encapsulate audio streams coded using the Opus codec. See and for technical details on the Opus codec and its encapsulation in the Ogg container respectively. This document extends the Ogg format by defining a new channel mapping family for encoding ambisonics. The Ogg Opus format is extended indirectly by adding an item with value 2 to the IANA "Opus Channel Mapping Families" registry. When 2 is used as the Channel Mapping Family Number in an Ogg stream, the semantic meaning of the channels in the multichannel Opus stream is the ambisonics layout defined in this document. This mapping can also be used in other contexts which make use of the channel mappings defined by the Opus Channel Mapping Families registry.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

Ambisonics MAY be encapsulated in the Ogg format by encoding with the Opus codec and setting the Channel Mapping Family value to 2 in the Ogg Identification Header. A demuxer implmentation encountering Channel Mapping Family 2 MUST interpret the Opus stream as containing ambisonics with the format described in .

Allowed numbers of channels: (1 + n)^2 for n = 0...14. Explicitly 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225. Periphonic ambisonics from zeroth to fourteenth order. This channel mapping uses the same channel mapping table format used by channel mapping families 1 and 255. Each output channel is assigned to an ambisonic component in Ambisonic Channel Number (ACN) order. The ambisonic component with order n and degree m corresponds to channel (n * (n + 1) + m). The reverse correspondence can also be computed for a channel with index k.

Channels are normalized with Schmidt Semi-Normalization (SN3D). The interpretation of the ambisonics signal as well as detailed definitions of ACN channel ordering and SN3D normalization are described in Section 2.1.

An Ogg Opus player MAY use the matrix in Figure to implement downmixing from multichannel files using Channel Mapping Family 2 , which is known to give acceptable results for stereo. The first and second ambisonic channels are known as "W" and "Y" respectively.

The first ambisonic channel (W) is a mono audio stream which represents the average audio signal over all directions. Since W is not directional, Ogg Opus players MAY use W directly for mono playback.

Implementations of the Ogg container need take appropriate security considerations into account, as outlined in Section 10 of . The extension defined in this document requires that semantic meaning be assigned to more channels than the existing Ogg format requires. Since more allocations will be required to encode and decode these semantically meaningful channels, care should be taken in any new allocation paths. Implementations MUST NOT overrun their allocated memory nor read from uninitialized memory when managing the ambisonic channel mapping.

This document updates the IANA Media Types registry "Opus Channel Mapping Families" to add a new assignment. ValueReference 2This Document

Thanks to Timothy Terriberry and Marcin Gorzel for their guidance and valuable contributions to this document.