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Versions: (draft-graczyk-codec-ambisonics) 00 01 02 03 04 05 06 07 08 09 10 RFC 8486

codec                                                         M. Graczyk
Internet-Draft                                               J. Skoglund
Intended status: Standards Track                             Google Inc.
Expires: May 25, 2017                                  November 21, 2016


                  Ambisonics in an Ogg Opus Container
                     draft-ietf-codec-ambisonics-01

Abstract

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on May 25, 2017.

Copyright Notice

   Copyright (c) 2016 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
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   (http://trustee.ietf.org/license-info) in effect on the date of
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   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
   4.  Downmixing  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Channel Mapping Family 2  . . . . . . . . . . . . . . . .   5
     4.2.  Channel Mapping Family 3  . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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.
   A separate stereo stream can be used alongside the ambisonics in a
   head-tracked virtual reality experience to provide so-called non-
   diegetic audio - audio which should remain unchanged by listener head
   rotation; e.g., 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 format by defining two new channel
   mapping families for encoding ambisonics.  The Ogg Opus format is
   extended indirectly by adding an item with value 2 or 3 to the IANA
   "Opus Channel Mapping Families" 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 which make use of the channel mappings
   defined by the Opus Channel Mapping Families registry.








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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 or 3 in
   the Ogg Identification Header.  A demuxer implementation encountering
   Channel Mapping Family 2 or Family 3 MUST interpret the Opus stream
   as containing ambisonics with the format described in Section 3.1 or
   Section 3.2, respectively.

3.1.  Channel Mapping Family 2

   Allowed numbers of channels: (1 + n)^2 + 2j for n = 0...14 and j = 0
   or 1, where n denotes the ambisonic order and j 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, 227.

   This channel mapping uses the same channel mapping table format used
   by channel mapping families 1 and 255.  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).

                         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 + 1.  The reverse correspondence can also be
   computed for an ambisonic channel with index k.

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

               Figure 2: Ambisonic Degree and Order from ACN





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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 are generated at the decoder by
   multiplying N decoded streams with a designated demixing matrix, D,
   having C rows and N columns.  The number of output channels does not
   need to correspond to a full ambisonic order representation.  This
   mapping allows for encoding and decoding of full order ambisonics,
   mixed order ambisonics, and for non-diegetic stereo channels.  Let X
   denote a column vector containing N decoded streams X1, X2, ..., XN,
   and let S denote a column vector containing C output streams S1, S2,
   ..., SC.  Then S = D X, i.e.,

                  /     \   /                   \ /     \
                  | S1  |   | D11  D12  ... D1N | | X1  |
                  | S2  |   | D21  D22  ... D2N | | X2  |
                  | ... | = | ...  ...  ... ... | | ... |
                  | SC  |   | DC1  DC2  ... DCN | | XN  |
                  \     /   \                   / \     /

              Figure 3: Demixing in Channel Mapping Family 3

   The matrix MUST be provided as side information and MUST be stored in
   the channel mapping table part of the identification header, c.f.
   section 5.1.1 in [RFC7845].  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 | Channel Numbering                             :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Demixing Matrix                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       Figure 4: Channel Mapping Table for Channel Mapping Family 3

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

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



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       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.  Output Channel Numbering (8*C unsigned bits):

       This contains one octet per output channel, indicating which
       ambisonic channel the output channel corresponds to.  Let 'index'
       be the value of this octet for a particular output channel.  If
       'index' is less than 254, it equals the ACN number of the
       corresponding channel.  If 'index' is 254, it means the
       corresponding channel contains the left channel of a non-diegetic
       stereo stream.  If 'index' is 255, it means the corresponding
       channel contains the right channel of a non-diegetic stereo
       stream.



   4.  Demixing Matrix (32*N*C bits):

       The coefficients of the demixing matrix stored column-wise as
       little endian floats.

   Note that [RFC7845] specifies that the identification header cannot
   exceed one "page", which is 65,025 octets.  This sets a practical
   maximum ambisonic order of 10, if full order is utilized and the
   number of coded streams is the same as the ambisonic order plus the
   two non-diegetic channels.  Also note that the total output channel
   number, C, MUST be set in the 3rd field of the identification header.

4.  Downmixing

4.1.  Channel Mapping Family 2

   An Ogg Opus player MAY use the matrix in Figure 5 to implement
   downmixing from multichannel files using Channel Mapping Family 2
   Section 3.1, when there is no non-diegetic stereo.  This downmixing
   is known to give acceptable results for stereo downmixing from
   ambisonics.  The first and second ambisonic channels are known as "W"
   and "Y" respectively.




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

     Figure 5: Stereo Downmixing Matrix for Channel Mapping Family 2 -
                          only Ambisonic Channels

   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.

   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 Family 2 -
           Ambisonic Channels Plus a Non-diegetic Stereo Stream

4.2.  Channel Mapping Family 3

   In Channel Mapping Family 3 described in Section 3.2, additional
   side-information stored in the identification header is needed to
   transform the coded streams into ambisonic and non-diegetic stereo.
   It would therefore be reasonable to also utilize this information for
   stereo downmix.  Assume in the following that the output channels
   contain the 0th and 1st order channels, ACN 0 and 1, also known as
   "W" and Y" respectively.  If we also assume that the output channel
   numbering in Figure 4 is structured so that the first output channel
   contains W and the second contains Y, an Ogg Opus player MAY use the
   matrix in Figure 7 to implement downmixing when there is no non-
   diegetic stereo.  If the output channels are ordered differently the
   columns of the downmixing matrix should be rearranged accordingly so
   that only W and Y contribute to the downmix.







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/   \   /                  \  /     \   /                  \  /      \
| L |   | 0.5  0.5 0.0 ... |  |  W  |   | 0.5  0.5 0.0 ... |  |  S1  |
| R | = | 0.5 -0.5 0.0 ... |  |  Y  | = | 0.5 -0.5 0.0 ... |  |  S2  | =
\   /   \                  /  | ... |   \                  /  | ...  |
                              \     /                         \      /

        /                  \  /                   \  /     \
        | 0.5  0.5 0.0 ... |  | D11  D12  ... D1N |  | X1  |
      = | 0.5 -0.5 0.0 ... |  | D21  D22  ... D2N |  | X2  |
        \                  /  | ...  ...  ... ... |  | ... |
                              | DC1  DC2  ... DCN |  | XN  |
                              \                   /  \     /

     Figure 7: Stereo Downmixing Matrix for Channel Mapping Family 3 -
                          only Ambisonic Channels

   Similarly, if a non-diegetic stereo track is present, the player MAY
   use the matrix in Figure 8 for downmixing.  Ls and Rs denote the two
   non-diegetic stereo channels and it is assumed here that they are
   located as the two last channels of the output channels.  If the
   output channels are ordered differently the columns of the downmixing
   matrix should be rearranged accordingly so that only W, Y, Ls, and Rs
   contribute to the downmix.




























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   /   \   /                            \  /     \
   | 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 |
                                           \     /

           /                            \  /      \
           | 0.25  0.25 0.0 ... 0.5 0.0 |  |  S1  |
         = | 0.25 -0.25 0.0 ... 0.0 0.5 |  |  S2  | =
           \                            /  | ...  |
                                           | SC-1 |
                                           |  SC  |
                                           \      /

           /                            \  /                   \ /     \
           | 0.25  0.25 0.0 ... 0.5 0.0 |  | D11  D12  ... D1N | | X1  |
         = | 0.25 -0.25 0.0 ... 0.0 0.5 |  | D21  D22  ... D2N | | X2  |
           \                            /  | ...  ...  ... ... | | ... |
                                           | DC1  DC2  ... DCN | | XN  |
                                           \                   / \     /

     Figure 8: Stereo Downmixing Matrix for Channel Mapping Family 3 -
           Ambisonic Channels Plus a Non-diegetic Stereo Stream

5.  Security Considerations

   Implementations of the Ogg container need take appropriate security
   considerations into account, as outlined in Section 10 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.

6.  IANA Considerations

   This document updates the IANA Media Types registry "Opus Channel
   Mapping Families" to add two new assignments.









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                   +-------+---------------------------+
                   | Value | Reference                 |
                   +-------+---------------------------+
                   | 2     | This Document Section 3.1 |
                   |       |                           |
                   | 3     | This Document Section 3.2 |
                   +-------+---------------------------+

7.  Acknowledgments

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

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://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, <http://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, <http://www.rfc-editor.org/info/rfc7845>.

   [ambix]    Nachbar, C., Zotter, F., Deleflie, E., and A. Sontacchi,
              "AMBIX - A SUGGESTED AMBISONICS FORMAT", June 2011,
              <http://iem.kug.ac.at/fileadmin/media/iem/projects/2011/
              ambisonics11_nachbar_zotter_sontacchi_deleflie.pdf>.

8.2.  Informative References

   [gerzon75]
              Gerzon, M., "Ambisonics. Part one: General system
              description", August 1975,
              <http://www.michaelgerzonphotos.org.uk/articles/
              Ambisonics%201.pdf>.

   [daniel04]
              Daniel, J. and S. Moreau, "Further Study of Sound Field
              Coding with Higher Order Ambisonics", May 2004,
              <http://pcfarina.eng.unipr.it/Public/phd-thesis/
              aes116%20high-passed%20hoa.pdf>.



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Authors' Addresses

   Michael Graczyk
   Google Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   USA

   Email: mgraczyk@google.com


   Jan Skoglund
   Google Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   USA

   Email: jks@google.com

































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