codec M. Graczyk Internet-Draft Google Inc. Updates: 7845 (if approved) May 24, 2016 Intended status: Standards Track Expires: November 25, 2016 Ambisonics in an Ogg Opus Container draft-graczyk-codec-ambisonics-00 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 25, 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 Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Graczyk Expires November 25, 2016 [Page 1]
Internet-Draft Opus Ambisonics May 2016 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Ambisonics With Ogg Opus . . . . . . . . . . . . . . . . . . 2 3.1. Channel Mapping Family 2 . . . . . . . . . . . . . . . . 3 3.2. Downmixing . . . . . . . . . . . . . . . . . . . . . . . 3 4. Security Considerations . . . . . . . . . . . . . . . . . . . 3 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. The Ogg format is a container for transmission and storage of audio. It can be used to encapsulate 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. 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 in the Ogg Identification Header. A demuxer implmentation encountering Graczyk Expires November 25, 2016 [Page 2]
Internet-Draft Opus Ambisonics May 2016 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 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225. 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) with no Condon-Shortley phase factor. In SN3D, the spherical harmonic of order n and degree m is normalized according to sqrt((2 - delta(m)) * ((n - m)! / (n + m)!)), where delta(0) = 1 and delta(m) = 0 otherwise. The interpretation of the ambisonics signal as well as the channel order and normalization are described in [ambix]. 3.2. Downmixing Implementations 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. / \ / \ / W \ | L | | 0.5 0.5 0.0 ... | | Y | | R | = | 0.5 -0.5 0.0 ... | | ... | \ / \ / \ ... / Figure 1: Stereo Downmixing Matrix 4. Security Considerations Implementations of the Ogg container need take appropriate security considerations into account, as outlined in Section 10 of [RFC7845]. The extension definied in this document requires that semantic Graczyk Expires November 25, 2016 [Page 3]
Internet-Draft Opus Ambisonics May 2016 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>. [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>. Graczyk Expires November 25, 2016 [Page 4]
Internet-Draft Opus Ambisonics May 2016 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 November 25, 2016 [Page 5]