Internet DRAFT - draft-graczyk-codec-ambisonics
draft-graczyk-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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 2, 2016.
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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
Graczyk Expires December 2, 2016 [Page 5]
