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Versions: (draft-graczyk-codec-ambisonics) 00 01 02 03

codec                                                        J. Skoglund
Internet-Draft                                               Google Inc.
Intended status: Standards Track                              M. Graczyk
Expires: November 3, 2017                                    May 2, 2017


                  Ambisonics in an Ogg Opus Container
                     draft-ietf-codec-ambisonics-03

Abstract

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

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 3, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Ambisonics With Ogg Opus  . . . . . . . . . . . . . . . . . .   3
     3.1.  Channel Mapping Family 2  . . . . . . . . . . . . . . . .   3
     3.2.  Channel Mapping Family 3  . . . . . . . . . . . . . . . .   4
   4.  Downmixing  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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 Opus 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.

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].



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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 header (ID).  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 (highest) 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 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).

                         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 where only a subset of the full ambisonic
   order number of channels.  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 as well as



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   detailed definitions of ACN channel ordering and SN3D normalization
   are described in [ambix] Section 2.1.

3.2.  Channel Mapping Family 3

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

   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.  Here, N
   denotes the number of streams encoded and M the number of these which
   are coupled to produce two channels.  As for channel mapping family 2
   this mapping family also allows for encoding and decoding of full
   order ambisonics, mixed order ambisonics, and for non-diegetic stereo
   channels, but also 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, i.e.,

                  /     \   /                   \ /     \
                  | 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 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].  The matrix replaces the need for a
   channel mapping field and for channel mapping family 3 the mapping
   table has the following layout:











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      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

   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 column-wise 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 allows also 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 to 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.  Also note that the total output channel number,
   C, MUST be set in the 3rd field of the identification header.








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4.  Downmixing

   An Ogg Opus player MAY use the matrix in Figure 5 to implement
   downmixing from multichannel files using Channel Mapping Family 2 and
   3, 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.

                   /   \   /                  \ /     \
                   | 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 and 3
                         - 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 and 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




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   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.

                   +-------+---------------------------+
                   | 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







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   [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>.

Authors' Addresses

   Jan Skoglund
   Google Inc.
   345 Spear Street
   San Francisco, CA  94105
   USA

   Email: jks@google.com


   Michael Graczyk

   Email: michael@graczyk.com

























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