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Versions: 00 01 02 draft-ietf-codec-ambisonics

codec                                                         M. Graczyk
Internet-Draft                                               Google Inc.
Intended status: Standards Track                            May 31, 2016
Expires: December 2, 2016


                  Ambisonics in an Ogg Opus Container
                   draft-graczyk-codec-ambisonics-02

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 December 2, 2016.

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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   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.  Downmixing  . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

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.
   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 a new channel
   mapping family for encoding ambisonics.  The Ogg Opus format is
   extended indirectly by adding an item with value 2 to the IANA "Opus
   Channel Mapping Families" registry.  When 2 is used as the Channel
   Mapping Family Number in an Ogg stream, the semantic meaning of the
   channels in the multichannel Opus stream is the ambisonics layout
   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 MAY be encapsulated in the Ogg format by encoding with the
   Opus codec and setting the Channel Mapping Family value to 2 in the
   Ogg Identification Header.  A demuxer implmentation encountering
   Channel Mapping Family 2 MUST interpret the Opus stream as containing
   ambisonics with the format described in Section 3.1.

3.1.  Channel Mapping Family 2

   Allowed numbers of channels: (1 + n)^2 for n = 0...14.  Explicitly 1,
   4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225.
   Periphonic ambisonics from zeroth to fourteenth order.

   This channel mapping uses the same channel mapping table format used
   by channel mapping families 1 and 255.  Each output channel is
   assigned to an ambisonic component in Ambisonic Channel Number (ACN)
   order.  The ambisonic component with order n and degree m corresponds
   to channel (n * (n + 1) + m).  The reverse correspondence can also be
   computed for a channel with index k.

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

   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.  Downmixing

   An Ogg Opus player MAY use the matrix in Figure 1 to implement
   downmixing from multichannel files using Channel Mapping Family 2
   Section 3.1, which is known to give acceptable results for stereo.
   The first and second ambisonic channels are known as "W" and "Y"
   respectively.

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

                    Figure 1: Stereo Downmixing Matrix

   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.



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4.  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.

5.  IANA Considerations

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

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

6.  Acknowledgments

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

7.  References

7.1.  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>.







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   [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>.

7.2.  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>.

Author's Address

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

   Email: mgraczyk@google.com























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