Ambisonics in an Ogg Opus Container
draft-graczyk-codec-ambisonics-02

Abstract

This document defines an extension to the Ogg format to encapsulate ambisonics coded using the Opus audio codec.

Status of This Memo

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Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Ambisonics With Ogg Opus
• 3.1. Channel Mapping Family 2
• 3.2. Downmixing
• 4. Security Considerations
• 5. IANA Considerations
• 6. Acknowledgments
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address

1. Introduction

Ambisonics is a representation format for three dimensional sound fields which can be used for surround sound and immersive virtual reality playback. See [gerzon75] and [daniel04] 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 [RFC6716] and [RFC7845] 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.

2. Terminology

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 [RFC2119].

3. Ambisonics With Ogg Opus

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 Section 3.1.

3.1. Channel Mapping Family 2

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.

order   n = ceil(sqrt(k)) - 1,
degree  m = k - n * (n + 1).

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 [ambix] Section 2.1.

3.2. Downmixing

An Ogg Opus player MAY use the matrix in Figure 1 to implement downmixing from multichannel files using Channel Mapping Family 2 Section 3.1, which is known to give acceptable results for stereo. The first and second ambisonic channels are known as "W" and "Y" respectively.

/   \   /                  \ /  W  \
| L |   | 0.5  0.5 0.0 ... | |  Y  |
| R | = | 0.5 -0.5 0.0 ... | | ... |
\   /   \                  / \ ... /