RFC 8486
Ambisonics in an Ogg Opus Container
Ambisonics in an Ogg Opus Container 🎀✨
Okay so here's the header stuff, bestie 🧚: this is RFC 8486 (Standards Track) by J. Skoglund (Google LLC) and M. Graczyk, dated October 2018. It updates RFC 7845 and is published as an ISSN 2070-1721 doc. Main character energy, no cap 👑
Abstract 💅
Okay so this doc extends the Opus audio codec so you can wrap coded Ambisonics inside the Ogg format 🦋 It also tweaks RFC 7845 because the way channel mapping families were described needed a glow-up, periodt 🌈
Status of This Memo 🌸
This one is an Internet Standards Track document, which is literally the most official vibe there is 💖 It's a product of the IETF, it represents the consensus of the IETF community, got public review, and was approved by the IESG. If you want more on Internet Standards, check out Section 2 of RFC 7841 🫶
Errata and feedback stuff lives at https://www.rfc-editor.org/info/rfc8486 ✨
Copyright Notice 🍓
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved 💜
This doc is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) as of the publication date. Read those carefully, bestie, because they're your rights and restrictions 🙈 Code Components pulled out of this doc must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they come without warranty as described in that license 🔥
Table of Contents 🧚
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Ambisonics with Ogg Opus . . . . . . . . . . . . . . . . . . 3
3.1. Channel Mapping Family 2 . . . . . . . . . . . . . . . . 3
3.2. Channel Mapping Family 3 . . . . . . . . . . . . . . . . 4
3.3. Allowed Numbers of Channels . . . . . . . . . . . . . . . 5
4. Downmixing . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Updates to RFC 7845 . . . . . . . . . . . . . . . . . . . . . 7
5.1. Format of the Channel Mapping Table . . . . . . . . . . . 7
5.2. Unknown Mapping Families . . . . . . . . . . . . . . . . 8
6. Experimental Mapping Families . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 101. Introduction 🌷
Ambisonics is a representation format for three-dimensional sound fields, and it's giving surround sound and immersive VR playback 🎧 Check out [fellgett75] and [daniel04] for the technical details on the Ambisonics format. For this doc's purposes, Ambisonics can be treated as a multichannel audio stream. A separate stereo stream can run alongside the Ambisonics in a head-tracked VR experience to provide so-called non-diegetic audio, meaning audio that should stay unchanged when the listener's head rotates, like narration or stereo music 🎶
Ogg is a general-purpose container that handles audio, video, and other media 📦 It can encapsulate audio streams coded with the Opus codec. See [RFC6716] and [RFC7845] for the technical details on the Opus codec and its encapsulation in Ogg, respectively 💕
This doc extends the Ogg Opus format by defining two new channel mapping families for encoding Ambisonics ✨ The Ogg Opus format gets extended indirectly by adding items with values 2 and 3 to the "Opus Channel Mapping Families" IANA registry. When 2 or 3 is used as the Channel Mapping Family Number in an Ogg stream, the semantic meaning of the channels in the multichannel Opus stream is one of the Ambisonics layouts defined here. This mapping can also be used in other contexts that use the channel mappings from the "Opus Channel Mapping Families" registry 🦋
Also, mapping families 240 through 254 (inclusively) are reserved for experimental use 🧪
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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here 🙈
3. Ambisonics with Ogg Opus 🌈
Ambisonics can be encapsulated in Ogg by encoding with Opus and setting the channel mapping family value to 2 or 3 in the Ogg identification (ID) header 🎀 A demuxer implementation that encounters channel mapping family 2 or 3 MUST interpret the Opus stream as containing Ambisonics in the format described in Sections 3.1 or 3.2, respectively 👑
3.1. Channel Mapping Family 2 🦋
This channel mapping uses the same channel mapping table format as channel mapping family 1 🌸 The output channels are Ambisonic components ordered in Ambisonic Channel Number (ACN) order (defined in Figure 1), followed by two optional channels of non-diegetic stereo, indexed (left, right). The terms "order" and "degree" are defined according to [ambix] 💖
ACN = n * (n + 1) + m,
for order n and degree m.
Figure 1: Ambisonic Channel Number (ACN)For the Ambisonic channels, the ACN component corresponds to channel index as k = ACN. The reverse correspondence can also be computed for an Ambisonic channel with index k 🧚
order n = floor(sqrt(k)),
degree m = k - n * (n + 1).
Figure 2: Ambisonic Degree and Order from ACNNote that channel mapping family 2 allows for so-called mixed-order Ambisonic representation, in which only a subset of the full Ambisonic order number of channels is encoded 🍓 By specifying the full number in the channel count field, the inactive ACNs can then be indicated in the channel mapping field using the index 255 ✨
Ambisonic channels are normalized with Schmidt Semi-Normalization (SN3D) 💅 The interpretation of the Ambisonics signal, plus detailed definitions of ACN channel ordering and SN3D normalization, are described in [ambix], Section 2.1 🫶
3.2. Channel Mapping Family 3 💜
In this mapping, C output channels (the channel count) are generated at the decoder by multiplying K = N + M decoded channels with a designated demixing matrix, D, having C rows and K columns (C and K do not have to be equal) 🔥 Here, N is the number of streams encoded, and M is the number of these encoded streams that are coupled to produce two channels. Just like channel mapping family 2, this mapping family also allows encoding and decoding of full-order Ambisonics and mixed-order Ambisonics, plus non-diegetic stereo channels. It also has the added flexibility of mixing channels 🌈 Let X be a column vector containing K decoded channels X1, X2, ..., XK (from N streams), and let S be a column vector containing C output streams S1, S2, ..., SC. Then, S = D X, as shown in Figure 3 🦋
/ \ / \ / \
| S1 | | D11 D12 ... D1K | | X1 |
| S2 | | D21 D22 ... D2K | | X2 |
| ... | = | ... ... ... ... | | ... |
| SC | | DC1 DC2 ... DCK | | XK |
\ / \ / \ /
Figure 3: Demixing in Channel Mapping Family 3The matrix MUST be provided in the channel mapping table part of the identification header; see Section 5.1.1 of [RFC7845] 🎀 The matrix replaces the need for a channel mapping field. For channel mapping family 3, the mapping table has the following layout:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Stream Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Coupled Count | Demixing Matrix :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Channel Mapping Table for Channel Mapping Family 3The fields in the channel mapping table have the following meaning 🧠
- Stream Count "N" (8 bits, unsigned): This is the total number of streams encoded in each Ogg packet. 🌸
- Coupled Stream Count "M" (8 bits, unsigned): This is the number of the N streams whose decoders are to be configured to produce two channels (stereo). 💖
- Demixing Matrix (16*K*C bits, signed): The coefficients of the demixing matrix are stored in column-major order as 16-bit, signed, two's complement fixed-point values with 15 fractional bits (Q15), little endian. If needed, the output gain field can be used for a normalization scale. For mixed-order Ambisonic representations, the silent ACN channels are indicated by all zeros in the corresponding rows of the mixing matrix. This also allows for mixed order with non-diegetic stereo, since the number of columns implies the presence of non-diegetic channels ✨
Note that [RFC7845] specifies that the identification header cannot exceed one "page", which is 65,025 octets. This limits the Ambisonic order, which then MUST be lower than 12 if full order is utilized and the number of coded streams is the same as the Ambisonic order plus the two non-diegetic channels 😭 The total output channel number, C, MUST be set in the third field of the identification header 🔥
3.3. Allowed Numbers of Channels 🌟
For both channel mapping families 2 and 3, the allowed numbers of channels are (1 + n)^2 + 2j for n = 0, 1, ..., 14 and j = 0 or 1. Here n is the (highest) Ambisonic order, and j denotes whether or not there is a separate non-diegetic stereo stream. This corresponds to periphonic Ambisonics from zeroth to fourteenth order, plus potentially two channels of non-diegetic stereo 🧚 Explicitly, the allowed numbers of channels are:
1, 3, 4, 6, 9, 11, 16, 18, 25, 27, 36, 38, 49, 51, 64, 66, 81, 83,
100, 102, 121, 123, 144, 146, 169, 171, 196, 198, 225, and 227.Note again that if full Ambisonic order is used and the number of coded streams is the same as the Ambisonic order plus the two non-diegetic channels, the order MUST then be lower than 12, due to the identification header length limit 🙈
To be continued in part 2 ✨ (Sections 4 through 9, the Acknowledgments, and Authors' Addresses are coming up next, bestie 💖)
4. Downmixing ✨
okay so the matrices in this section are just examples 💅 they're known to give good results when you squash Ambisonics down to stereo, but bestie, other mixing strategies are totally allowed too, like if you wanna emphasize a certain panning 🎀
An Ogg Opus player MAY use the matrix in Figure 5 to downmix multichannel files that use channel mapping families 2 and 3, but only when there's no non-diegetic stereo 🦋 any coefficients left out of the matrix are just 0.0 💖 also the first and second Ambisonic channels are called "W" and "Y", respectively 🌈
/ \ / \ / \
| L | | 0.5 0.5 0.0 ... | | W |
| R | = | 0.5 -0.5 0.0 ... | | Y |
\ / \ / | ... |
\ /Figure 5: Stereo Downmixing Matrix for Channel Mapping Families 2 and 3 - Only Ambisonic Channels
W is the first Ambisonic channel and it's a mono audio stream holding the average signal over all directions 🌸 since W isn't directional, Ogg Opus players MAY use W straight up for mono playback ✨
if there's a non-diegetic stereo track, the player MAY use the matrix in Figure 6 for downmixing 🧚 Ls and Rs are the two non-diegetic stereo channels, btw 💜
/ \ / \ / \
| L | | 0.25 0.25 0.0 ... 0.5 0.0 | | W |
| R | = | 0.25 -0.25 0.0 ... 0.0 0.5 | | Y |
\ / \ / | ... |
| Ls |
| Rs |
\ /Figure 6: Stereo Downmixing Matrix for Channel Mapping Families 2 and 3 - Ambisonic Channels Plus a Non-Diegetic Stereo Stream
5. Updates to RFC 7845 ✨
5.1. Format of the Channel Mapping Table
okay so the language in Section 5.1.1 of [RFC7845] (copied below) kinda implies that the channel mapping table has one fixed format for every single channel mapping family 🙈
The order and meaning of these channels are defined by a channel mapping, which consists of the 'channel mapping family' octet and, for channel mapping families other than family 0, a 'channel mapping table', as illustrated in Figure 3.
this document updates [RFC7845] to clarify that the format of the channel mapping table may depend on the channel mapping family 💅
The order and meaning of these channels are defined by a channel mapping, which consists of the 'channel mapping family' octet and for channel mapping families other than family 0, a 'channel mapping table'. The format of the channel mapping table depends on the channel mapping family. Unless the channel mapping family requires a custom format for its channel mapping table, the RECOMMENDED channel mapping table format for new mapping families is illustrated in Figure 3.
the change above is not meant to change how families 1 and 255 currently work 🦋 so to make sure of that, the first paragraph of Section 5.1.1.2 changes from:
Allowed numbers of channels: 1...8. Vorbis channel order (see below).
to:
Allowed numbers of channels: 1...8, with the mapping specified according to Figure 3. Vorbis channel order (see below).
similarly, the first paragraph of Section 5.1.1.3 changes from:
Allowed numbers of channels: 1...255. No defined channel meaning.
to:
Allowed numbers of channels: 1...255, with the mapping specified according to Figure 3. No defined channel meaning.
5.2. Unknown Mapping Families
the treatment of unknown mapping families gets a little glow-up 🌈 Section 5.1.1.4 of [RFC7845] says:
The remaining channel mapping families (2...254) are reserved. A demuxer implementation encountering a reserved 'channel mapping family' value SHOULD act as though the value is 255.
and that is changed to:
The remaining channel mapping families (2...254) are reserved. A demuxer implementation encountering a 'channel mapping family' value that it does not recognize SHOULD NOT attempt to decode the packets and SHOULD NOT use any information except for the first 19 octets of the ID header packet (Figure 2) and the comment header (Figure 10).
6. Experimental Mapping Families ✨
to make developing new mapping families easier, and to lower the risk of compatibility issues with non-final versions of mapping families, mapping families 240 through 254 (inclusively) are now reserved for experiments and for implementations of in-development families 🧪 these experiments aren't restricted to Ambisonics, fyi 💜 implementers SHOULD attempt to use experimental family numbers that have not recently been used, and SHOULD advertise which experimental numbers they use (e.g., for Internet-Drafts) 🫶
the Ambisonics mapping experiments that led to this document used experimental family 254 for family 2 and experimental family 253 for family 3 🍓
7. Security Considerations 💀
okay bestie, this part is SERIOUS 😭 implementations of the Ogg container need to take appropriate security considerations into account, as outlined in Section 8 of [RFC7845] ⚠️ the extension in this document requires that semantic meaning be assigned to more channels than the existing Ogg format requires. since more allocations will be needed 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 🚨
8. IANA Considerations 🍓
IANA has added 17 new assignments to the "Opus Channel Mapping Families^a registry. here's the table, it's giving organized 📋
+---------+----------------------+----------------------------------+
| Value | Description | Reference |
+---------+----------------------+----------------------------------+
| 0 | Mono, L/R stereo | Section 5.1.1.1 of [RFC7845], |
| | | Section 5 of this document |
| | | |
| 1 | 1-8 channel surround | Section 5.1.1.2 of [RFC7845], |
| | | Section 5 of this document |
| | | |
| 2 | Ambisonics as | Section 3.1 of this document |
| | individual channels | |
| | | |
| 3 | Ambisonics with | Section 3.2 of this document |
| | demixing matrix | |
| | | |
| 240-254 | Experimental use | Section 6 of this document |
| | | |
| 255 | Discrete channels | Section 5.1.1.3 of [RFC7845], |
| | | Section 5 of this document |
+---------+----------------------+----------------------------------+9. References 📚
9.1. Normative References
[ambix] Nachbar, C., Zotter, F., Deleflie, E., and A. Sontacchi, "AMBIX - A SUGGESTED AMBISONICS FORMAT", Ambisonics Symposium, June 2011, <http://iem.kug.ac.at/fileadmin/media/iem/projects/2011/ambisonics11_nachbar_zotter_sontacchi_deleflie.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC6716] Valin, JM., Vos, K., and T. Terriberry, "Definition of the Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716, September 2012, <https://www.rfc-editor.org/info/rfc6716>.
[RFC7845] Terriberry, T., Lee, R., and R. Giles, "Ogg Encapsulation for the Opus Audio Codec", RFC 7845, DOI 10.17487/RFC7845, April 2016, <https://www.rfc-editor.org/info/rfc7845>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[daniel04] Daniel, J. and S. Moreau, "Further Study of Sound Field Coding with Higher Order Ambisonics", Audio Engineering Society Convention Paper, May 2004, <https://www.researchgate.net/publication/277841868_Further_Study_of_Sound_Field_Coding_with_Higher_Order_Ambisonics>.
[fellgett75] Fellgett, P., "Ambisonics. Part one: General system description", Studio Sound vol. 17, no. 8, pp. 20-22, August 1975, <http://www.michaelgerzonphotos.org.uk/articles/Ambisonics%201.pdf>.
Acknowledgments 🫶
thanks to Timothy Terriberry, Jean-Marc Valin, Mark Harris, Marcin Gorzel, and Andrew Allen for their guidance and valuable contributions to this document 💖 you all are literally the reason this is so good 👑
Authors' Addresses 🌸
Jan Skoglund Google LLC 345 Spear Street San Francisco, CA 94105 United States of America
Email: jks@google.com
Michael Graczyk
Email: michael@mgraczyk.com
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