Ambisonics in an Ogg Opus Container
RFC 8486

amp; Graczyk           Standards Track                    [Page 4]
RFC 8486                     Opus Ambisonics                October 2018

   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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RFC 8486                     Opus Ambisonics                October 2018

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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RFC 8486                     Opus Ambisonics                October 2018

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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RFC 8486                     Opus Ambisonics                October 2018

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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RFC 8486                     Opus Ambisonics                October 2018

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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RFC 8486                     Opus Ambisonics                October 2018

   [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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