Ambisonics in an Ogg Opus Container
RFC 8486

Internet Engineering Task Force (IETF)                       J. Skoglund
Request for Comments: 8486                                    Google LLC
Updates: 7845                                                 M. Graczyk
Category: Standards Track                                   October 2018
ISSN: 2070-1721

                  Ambisonics in an Ogg Opus Container

Abstract

   This document defines an extension to the Opus audio codec to
   encapsulate coded Ambisonics using the Ogg format.  It also contains
   updates to RFC 7845 to reflect necessary changes in the description
   of channel mapping families.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained 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 document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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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  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Ambisonics is a representation format for three-dimensional sound
   fields that can be used for surround sound and immersive virtual-
   reality playback.  See [fellgett75] and [daniel04] for technical
   details on the Ambisonics format.  For the purposes of the this
   document, Ambisonics can be considered a multichannel audio stream.
   A separate stereo stream can be used alongside the Ambisonics in a
   head-tracked virtual reality experience to provide so-called non-
   diegetic audio -- that is, audio that should remain unchanged by
   rotation of the listener's head, such as narration or stereo music.
   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 Opus format by defining two new channel
   mapping families for encoding Ambisonics.  The Ogg Opus format is
   extended indirectly by adding items with values 2 and 3 to the "Opus
   Channel Mapping Families" IANA registry.  When 2 or 3 are 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 in this document.  This mapping can also
   be used in other contexts that make use of the channel mappings
   defined by the "Opus Channel Mapping Families" registry.

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   Furthermore, 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 the Ogg format by encoding with the
   Opus codec and setting the channel mapping family value to 2 or 3 in
   the Ogg identification (ID) header.  A demuxer implementation
   encountering channel mapping family 2 or 3 MUST interpret the Opus
   stream as containing Ambisonics with 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 used
   by channel mapping family 1.  The output channels are Ambisonic
   components ordered in Ambisonic Channel Number (ACN) order (which is
   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 ACN

   Note 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.

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   Ambisonic 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.  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 denotes the number of streams
   encoded, and M is the number of these encoded streams that are
   coupled to produce two channels.  As for channel mapping family 2,
   this mapping family also allows for the encoding and decoding of
   full-order Ambisonics and mixed-order Ambisonics, as well as non-
   diegetic stereo channels.  Furthermore, it has the added flexibility
   of mixing channels.  Let X denote a column vector containing K
   decoded channels X1, X2, ..., XK (from N streams), and let S denote 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 3

   The 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 3

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   The fields in the channel mapping table have the following meaning:

   1.  Stream Count "N" (8 bits, unsigned):

       This is the total number of streams encoded in each Ogg packet.

   2.  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).

   3.  Demixing Matrix (16*K*C bits, signed):

       The coefficients of the demixing matrix 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 as 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,
   where n denotes 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 number 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.

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4.  Downmixing

   The downmixing matrices in this section are only examples known to
   give acceptable results for stereo downmixing from Ambisonics, but
   other mixing strategies will be allowed, e.g., to emphasize a certain
   panning.

   An Ogg Opus player MAY use the matrix in Figure 5 to implement
   downmixing from multichannel files using channel mapping families 2
   and 3 when there is no non-diegetic stereo.  The first and second
   Ambisonic channels are known as "W" and "Y", respectively.  The
   omitted coefficients in the matrix in the figure have the value 0.0.

                   /   \   /                  \ /     \
                   | 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

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

   If a non-diegetic stereo track is present, the player MAY use the
   matrix in Figure 6 for downmixing.  Ls and Rs denote the two non-
   diegetic stereo channels.

              /   \   /                            \  /     \
              | 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

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5.  Updates to RFC 7845

5.1.  Format of the Channel Mapping Table

   The language in Section 5.1.1 of [RFC7845] (copied below) implies
   that the channel mapping table, when present, has a fixed format for
   all channel mapping families:

      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.  To ensure that, the first paragraph of
   Section 5.1.1.2 is changed 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 is changed 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.

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5.2.  Unknown Mapping Families

   The treatment of unknown mapping families is changed slightly.
   Section 5.1.1.4 of [RFC7845] states:

      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.

   This 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 development of new mapping families easier while reducing the
   risk of creating compatibility issues with non-final versions of
   mapping families, mapping families 240 through 254 (inclusively) are
   now reserved for experiments and implementations of in-development
   families.  Note that these mapping-family experiments are not
   restricted to Ambisonics.  Implementers SHOULD attempt to use
   experimental family numbers that have not recently been used and
   SHOULD advertise what 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

   Implementations of the Ogg container need to take appropriate
   security considerations into account, as outlined in Section 8 of
   [RFC7845].  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.

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8.  IANA Considerations

   IANA has added 17 new assignments to the "Opus Channel Mapping
   Families^?a registry.

   +---------+----------------------+----------------------------------+
   | 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>.

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   [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.

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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