





Internet Engineering Task Force (IETF)                         JM. Valin
Request for Comments: 6569                                       Mozilla
Category: Informational                                       S. Borilin
ISSN: 2070-1721                                               SPIRIT DSP
                                                                  K. Vos

                                                           C. Montgomery
                                                     Xiph.Org Foundation
                                                                 R. Chen
                                                    Broadcom Corporation
                                                              March 2012


      Guidelines for Development of an Audio Codec within the IETF

Abstract

   This document provides general guidelines for work on developing and
   specifying an interactive audio codec within the IETF.  These
   guidelines cover the development process, evaluation, requirements
   conformance, and intellectual property issues.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6569.

Copyright Notice

   Copyright (c) 2012 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



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   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.

Table of Contents

   1. Introduction ....................................................2
   2. Development Process .............................................2
   3. Evaluation, Testing, and Characterization .......................5
   4. Specifying the Codec ............................................6
   5. Intellectual Property ...........................................8
   6. Relationship with Other SDOs ...................................10
   7. Security Considerations ........................................12
   8. Acknowledgments ................................................12
   9. References .....................................................12
      9.1. Normative References ......................................12
      9.2. Informative References ....................................12

1.  Introduction

   This document describes a process for work in the IETF codec WG on
   standardization of an audio codec that is optimized for use in
   interactive Internet applications and that can be widely implemented
   and easily distributed among application developers, service
   operators, and end users.

2.  Development Process

   The process outlined here is intended to make the work on a codec
   within the IETF transparent, predictable, and well organized, in a
   way that is consistent with [PROCESS].  Such work might involve
   development of a completely new codec, adaptation of an existing
   codec to meet the requirements of the working group, or integration
   of two or more existing codecs that results in an improved codec
   combining the best aspects of each.  To enable such procedural
   transparency, the contributor of an existing codec must be willing to
   cede change control to the IETF and should have sufficient knowledge
   of the codec to assist in the work of adapting it or applying some of
   its technology to the development or improvement of other codecs.
   Furthermore, contributors need to be aware that any codec that
   results from work within the IETF is likely to be different from any
   existing codec that was contributed to the Internet Standards
   Process.







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   Work on developing an interactive audio codec is expected to proceed
   as follows:

   1.  IETF participants will identify the requirements to be met by an
       Internet codec in the form of an Internet-Draft.

   2.  Interested parties are encouraged to make contributions proposing
       existing or new codecs, or elements thereof, to the codec WG as
       long as these contributions are within the scope of the WG.
       Ideally, these contributions should be in the form of Internet-
       Drafts, although other forms of contributions are also possible,
       as discussed in [PROCESS].

   3.  Given the importance of intellectual property rights (IPR) to the
       activities of the working group, any IPR disclosures must be made
       in a timely way.  Contributors are required, as described in
       [IPR], to disclose any known IPR, both first and third party.
       Timely disclosures are particularly important, since those
       disclosures may be material to the decision process of the
       working group.

   4.  As contributions are received and discussed within the working
       group, the group should gain a clearer understanding of what is
       achievable within the design space.  As a result, the authors of
       the requirements document should iteratively clarify and improve
       their document to reflect the emerging working group consensus.
       This is likely to involve collaboration with IETF working groups
       in other areas, such as collaboration with working groups in the
       Transport area to identify important aspects of packet
       transmission over the Internet and to understand the degree of
       rate adaptation desirable and with working groups in the RAI area
       to ensure that information about and negotiation of the codec can
       be easily represented at the signaling layer.  In parallel with
       this work, interested parties should evaluate the contributions
       at a higher level to see which requirements might be met by each
       codec.

   5.  Once a sufficient number of proposals has been received, the
       interested parties will identify the strengths, weaknesses, and
       innovative aspects of the contributed codecs.  This step will
       consider not only the codecs as a whole, but also key features of
       the individual algorithms (predictors, quantizers, transforms,
       etc.).

   6.  Interested parties are encouraged to collaborate and combine the
       best ideas from the various codec contributions into a
       consolidated codec definition, representing the merging of some
       of the contributions.  Through this iterative process, the number



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       of proposals will reduce, and consensus will generally form
       around one of them.  At that point, the working group should
       adopt that document as a working group item, forming a baseline
       codec.

   7.  IETF participants should then attempt to iteratively add to or
       improve each component of the baseline codec reference
       implementation, where by "component" we mean individual
       algorithms such as predictors, transforms, quantizers, and
       entropy coders.  The participants should proceed by trying new
       designs, applying ideas from the contributed codecs, evaluating
       "proof of concept" ideas, and using their expertise in codec
       development to improve the baseline codec.  Any aspect of the
       baseline codec might be changed (even the fundamental principles
       of the codec), or the participants might start over entirely by
       scrapping the baseline codec and designing a completely new one.
       The overriding goal shall be to design a codec that will meet the
       requirements defined in the requirements document [CODEC-REQ].
       Given the IETF's open standards process, any interested party
       will be able to contribute to this work, whether or not they
       submitted an Internet-Draft for one of the contributed codecs.
       The codec itself should be normatively specified with code in an
       Internet-Draft.

   8.  In parallel with work on the codec reference implementation,
       developers and other interested parties should perform evaluation
       of the codec as described under Section 3.  IETF participants
       should define (within the PAYLOAD working group) the codec's
       payload format for use with the Real-time Transport Protocol
       [RTP].  Ideally, application developers should test the codec by
       implementing it in code and deploying it in actual Internet
       applications.  Unfortunately, developers will frequently wait to
       deploy the codec until it is published as an RFC or until a
       stable bitstream is guaranteed.  As such, this is a nice-to-have
       and not a requirement for this process.  Lab implementations are
       certainly encouraged.

   9.  The group will produce a testing results document.  The document
       will be a living document that captures testing done before the
       codec stabilized, after it has stabilized, and after the codec
       specification is issued as an RFC.  The document serves the
       purpose of helping the group determine whether the codec meets
       the requirements.  Any testing done after the codec RFC is issued
       helps implementers understand the final performance of the codec.
       The process of testing is described in Section 3.






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3.  Evaluation, Testing, and Characterization

   Lab evaluation of the codec being developed should happen throughout
   the development process because it will help ensure that progress is
   being made toward fulfillment of the requirements.  There are many
   ways in which continuous evaluation can be performed.  For minor,
   uncontroversial changes to the codec, it should usually be sufficient
   to use objective measurements (e.g., Perceptual Evaluation of Speech
   Quality (PESQ) [ITU-T-P.862], Perceptual Evaluation of Audio Quality
   (PEAQ) [ITU-R-BS.1387-1], and segmental signal-to-noise ratio)
   validated by informal subjective evaluation.  For more complex
   changes (e.g., when psychoacoustic aspects are involved) or for
   controversial issues, internal testing should be performed.  An
   example of internal testing would be to have individual participants
   rate the decoded samples using one of the established testing
   methodologies, such as MUltiple Stimuli with Hidden Reference and
   Anchor (MUSHRA) [ITU-R-BS.1534].

   Throughout the process, it will be important to make use of the
   Internet community at large for real-world distributed testing.  This
   will enable many different people with different equipment and use
   cases to test the codec and report any problems they experience.  In
   the same way, third-party developers will be encouraged to integrate
   the codec into their software (with a warning about the bitstream not
   being final) and provide feedback on its performance in real-world
   use cases.

   Characterization of the final codec must be based on the reference
   implementation only (and not on any "private implementation").  This
   can be performed by independent testing labs or, if this is not
   possible, by testing labs of the organizations that contribute to the
   Internet Standards Process.  Packet-loss robustness should be
   evaluated using actual loss patterns collected from use over the
   Internet, rather than theoretical models.  The goals of the
   characterization phase are to:

   o  ensure that the requirements have been fulfilled

   o  guide the IESG in its evaluation of the resulting work

   o  assist application developers in understanding whether the codec
      is suitable for a particular application

   The exact methodology for the characterization phase can be
   determined by the working group.  Because the IETF does not have
   testing resources of its own, it has to rely on the resources of its
   participants.  For this reason, even if the group agrees that a
   particular test is important, if no one volunteers to do it, or if



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   volunteers do not complete it in a timely fashion, then that test
   should be discarded.  This ensures that only important tests be done
   -- in particular, the tests that are important to participants.

4.  Specifying the Codec

   Specifying a codec requires careful consideration regarding what is
   required versus what is left to the implementation.  The following
   text provides guidelines for consideration by the working group:

   1.  Any audio codec specified by the codec working group must include
       source code for a normative software implementation, documented
       in an Internet-Draft intended for publication as a Standards
       Track RFC.  This implementation will be used to verify
       conformance of an implementation.  Although a text description of
       the algorithm should be provided, its use should be limited to
       helping the reader in understanding the source code.  Should the
       description contradict the source code, the latter shall take
       precedence.  For convenience, the source code may be provided in
       compressed form, with base64 [BASE64] encoding.

   2.  Because of the size and complexity of most codecs, it is possible
       that even after publishing the RFC, bugs will be found in the
       reference implementation, or differences will be found between
       the implementation and the text description.  As usual, an errata
       list should be maintained for the RFC.  Although a public
       software repository containing the current reference
       implementation is desirable, the normative implementation would
       still be the RFC.

   3.  It is the intention of the group to allow the greatest possible
       choice of freedom in implementing the specification.
       Accordingly, the number of binding RFC 2119 [KEYWORDS] keywords
       will be the minimum that still allows interoperable
       implementations.  In practice, this generally means that only the
       decoder needs to be normative, so that the encoder can improve
       over time.  This also enables different trade-offs between
       quality and complexity.

   4.  To reduce the risk of bias towards certain CPU/DSP (central
       processing unit / digital signal processor) architectures,
       ideally the decoder specification should not require "bit-exact"
       conformance with the reference implementation.  In that case, the
       output of a decoder implementation should only be "close enough"
       to the output of the reference decoder, and a comparison tool
       should be provided along with the codec to verify objectively
       that the output of a decoder is likely to be perceptually
       indistinguishable from that of the reference decoder.  An



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       implementation may still wish to produce an output that is bit-
       exact with the reference implementation to simplify the testing
       procedure.

   5.  To ensure freedom of implementation, decoder-side-only error
       concealment does not need to be specified, although the reference
       implementation should include the same packet-loss concealment
       (PLC) algorithm as used in the testing phase.  Is it up to the
       working group to decide whether minimum requirements on PLC
       quality will be required for compliance with the specification.
       Obviously, any information signaled in the bitstream intended to
       aid PLC needs to be specified.

   6.  An encoder implementation should not be required to make use of
       all the "features" (tools) in the bitstream definition.  However,
       the codec specification may require that an encoder
       implementation be able to generate any possible bitrate.  Unless
       a particular "profile" is defined in the specification, the
       decoder must be able to decode all features of the bitstream.
       The decoder must also be able to handle any combination of bits,
       even combinations that cannot be generated by the reference
       encoder.  It is recommended that the decoder specification shall
       define how the decoder should react to "impossible" packets
       (e.g., reject or consider as valid).  However, an encoder must
       never generate packets that do not conform to the bitstream
       definition.

   7.  Compressed test vectors should be provided as a means to verify
       conformance with the decoder specification.  These test vectors
       should be designed to exercise as much of the decoder code as
       possible.

   8.  While the exact encoder will not be specified, it is recommended
       to specify objective measurement targets for an encoder, below
       which use of a particular encoder implementation is not
       recommended.  For example, one such specification could be: "the
       use of an encoder whose PESQ mean opinion score (MOS) is better
       than 0.1 below the reference encoder in the following conditions
       is not recommended".












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5.  Intellectual Property

   Producing an unencumbered codec is desirable for the following
   reasons:

   o  It is the experience of a wide variety of application developers
      and service providers that encumbrances such as licensing and
      royalties make it difficult to implement, deploy, and distribute
      multimedia applications for use by the Internet community.

   o  It is beneficial to have low-cost options whenever possible,
      because innovation -- the hallmark of the Internet -- is hampered
      when small development teams cannot deploy an application because
      of usage-based licensing fees and royalties.

   o  Many market segments are moving away from selling hard-coded
      hardware devices and toward freely distributing end-user software;
      this is true of numerous large application providers and even
      telcos themselves.

   o  Compatibility with the licensing of typical open source
      applications implies the need to avoid encumbrances, including
      even the requirement to obtain a license for implementation,
      deployment, or use (even if the license does not require the
      payment of a fee).

   Therefore, a codec that can be widely implemented and easily
   distributed among application developers, service operators, and end
   users is preferred.  Many existing codecs that might fulfill some or
   most of the technical attributes listed above are encumbered in
   various ways.  For example, patent holders might require that those
   wishing to implement the codec in software, deploy the codec in a
   service, or distribute the codec in software or hardware need to
   request a license, enter into a business agreement, pay licensing
   fees or royalties, or adhere to other special conditions or
   restrictions.  Because such encumbrances have made it difficult to
   widely implement and easily distribute high-quality codecs across the
   entire Internet community, the working group prefers unencumbered
   technologies in a way that is consistent with BCP 78 [IPR] and BCP 79
   [TRUST].  In particular, the working group shall heed the preference
   stated in BCP 79: "In general, IETF working groups prefer
   technologies with no known IPR claims or, for technologies with
   claims against them, an offer of royalty-free licensing."  Although
   this preference cannot guarantee that the working group will produce
   an unencumbered codec, the working group shall follow and adhere to
   the spirit of BCP 79.  The working group cannot explicitly rule out
   the possibility of adopting encumbered technologies; however, the




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   working group will try to avoid encumbered technologies that require
   royalties or other encumbrances that would prevent such technologies
   from being easy to redistribute and use.

   When considering license terms for technologies with IPR claims
   against them, some members of the working group have expressed their
   preference for license terms that:

   o  are available to all, worldwide, whether or not they are working
      group participants

   o  extend to all essential claims owned or controlled by the licensor

   o  do not require payment of royalties, fees, or other consideration

   o  do not require licensees to adhere to restrictions on usage
      (though, licenses that apply only to implementation of the
      standard are acceptable)

   o  do not otherwise impede the ability of the codec to be implemented
      in open-source software projects

   The following guidelines will help to maximize the odds that the
   codec will be unencumbered:

   1.  In accordance with BCP 79 [IPR], contributed codecs should
       preferably use technologies with no known IPR claims or
       technologies with an offer of royalty-free licensing.

   2.  As described in BCP 79, the working group should use technologies
       that are perceived by the participants to be safer with regard to
       IPR issues.

   3.  Contributors must disclose IPR as specified in BCP 79.

   4.  In cases where no royalty-free license can be obtained regarding
       a patent, BCP 79 suggests that the working group consider
       alternative algorithms or methods, even if they result in lower
       quality, higher complexity, or otherwise less desirable
       characteristics.

   5.  In accordance with BCP 78 [TRUST], the source code for the
       reference implementation must be made available under a BSD-style
       license (or whatever license is defined as acceptable by the IETF
       Trust when the Internet-Draft defining the reference
       implementation is published).





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   Many IPR licenses specify that a license is granted only for
   technologies that are adopted by the IETF as a standard.  While
   reasonable, this has the unintended side effect of discouraging
   implementation prior to RFC status.  Real-world implementation is
   beneficial for evaluation of the codec.  As such, entities making IPR
   license statements are encouraged to use wording that permits early
   implementation and deployment.

   IETF participants should be aware that, given the way patents work in
   most countries, the resulting codec can never be guaranteed to be
   free of patent claims because some patents may not be known to the
   contributors, some patent applications may not be disclosed at the
   time the codec is developed, and only courts of law can determine the
   validity and breadth of patent claims.  However, these observations
   are no different within the Internet Standards Process than they are
   for standardization of codecs within other Standards Development
   Organizations (SDOs) (or development of codecs outside the context of
   any SDO); furthermore, they are no different for codecs than for
   other technologies worked on within the IETF.  In all these cases,
   the best approach is to minimize the risk of unknowingly incurring
   encumbrance on existing patents.  Despite these precautions,
   participants need to understand that, practically speaking, it is
   nearly impossible to _guarantee_ that implementers will not incur
   encumbrance on existing patents.

6.  Relationship with Other SDOs

   It is understood that other SDOs are also involved in the codec
   development and standardization, including but not necessarily
   limited to:

   o  The Telecommunication Standardization Sector (ITU-T) of the
      International Telecommunication Union (ITU), in particular Study
      Group 16

   o  The Moving Picture Experts Group (MPEG) of the International
      Organization for Standardization and International
      Electrotechnical Commission (ISO/IEC)

   o  The European Telecommunications Standards Institute (ETSI)

   o  The 3rd Generation Partnership Project (3GPP)

   o  The 3rd Generation Partnership Project 2 (3GPP2)

   It is important to ensure that such work does not constitute
   uncoordinated protocol development of the kind described in [UNCOORD]
   in the following principle:



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      [T]he IAB considers it an essential principle of the protocol
      development process that only one SDO maintains design authority
      for a given protocol, with that SDO having ultimate authority over
      the allocation of protocol parameter code-points and over defining
      the intended semantics, interpretation, and actions associated
      with those code-points.

   The work envisioned by this guidelines document is not uncoordinated
   in the sense described in the foregoing quote, since the intention of
   this process is that two possible outcomes might occur:

   1.  The IETF adopts an existing audio codec and specifies that it is
       the "anointed" IETF Internet codec.  In such a case, codec
       ownership lies entirely with the SDO that produced the codec, and
       not with the IETF.

   2.  The IETF produces a new codec.  Even if this codec uses concepts,
       algorithms, or even source code from a codec produced by another
       SDO, the IETF codec is a specification unto itself and under
       complete control of the IETF.  Any changes or enhancements made
       by the original SDO to the codecs whose components the IETF used
       are not applicable to the IETF codec.  Such changes would be
       incorporated as a consequence of a revision or extension of the
       IETF RFC.  In no case should the new codec reuse a name or code
       point from another SDO.

   Although there is already sufficient codec expertise available among
   IETF participants to complete the envisioned work, additional
   contributions are welcome within the framework of the Internet
   Standards Process in the following ways:

   o  Individuals who are technical contributors to codec work within
      other SDOs can participate directly in codec work within the IETF.

   o  Other SDOs can contribute their expertise (e.g., codec
      characterization and evaluation techniques) and thus facilitate
      the testing of a codec produced by the IETF.

   o  Any SDO can provide input to IETF work through liaison statements.

   However, it is important to note that final responsibility for the
   development process and the resulting codec will remain with the IETF
   as governed by BCP 9 [PROCESS].








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   Finally, there is precedent for the contribution of codecs developed
   elsewhere to the ITU-T (e.g., Adaptive Multi-Rate Wideband (AMR-WB)
   was standardized originally within 3GPP).  This is a model to explore
   as the IETF coordinates further with the ITU-T in accordance with the
   collaboration guidelines defined in [COLLAB].

7.  Security Considerations

   The procedural guidelines for codec development do not have security
   considerations.  However, the resulting codec needs to take
   appropriate security considerations into account, as outlined in
   [SECGUIDE] and in the security considerations of [CODEC-REQ].  More
   specifically, the resulting codec must avoid being subject to denial
   of service [DOS] and buffer overflows, and should take into
   consideration the impact of variable bitrate (VBR) [SRTP-VBR].

8.  Acknowledgments

   We would like to thank all the other people who contributed directly
   or indirectly to this document, including Jason Fischl, Gregory
   Maxwell, Alan Duric, Jonathan Christensen, Julian Spittka, Michael
   Knappe, Timothy B. Terriberry, Christian Hoene, Stephan Wenger, and
   Henry Sinnreich.  We would also like to thank Cullen Jennings and
   Gregory Lebovitz for their advice.  Special thanks to Peter Saint-
   Andre, who originally co-authored this document.

9.  References

9.1.  Normative References

   [IPR]              Bradner, S., "Intellectual Property Rights in IETF
                      Technology", BCP 79, RFC 3979, March 2005.

   [PROCESS]          Bradner, S., "The Internet Standards Process --
                      Revision 3", BCP 9, RFC 2026, October 1996.

   [TRUST]            Bradner, S. and J. Contreras, "Rights Contributors
                      Provide to the IETF Trust", BCP 78, RFC 5378,
                      November 2008.

9.2.  Informative References

   [BASE64]           Josefsson, S., "The Base16, Base32, and Base64
                      Data Encodings", RFC 4648, October 2006.

   [CODEC-REQ]        Valin, J. and K. Vos, "Requirements for an
                      Internet Audio Codec", RFC 6366, August 2011.




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   [COLLAB]           Fishman, G. and S. Bradner, "Internet Engineering
                      Task Force and International Telecommunication
                      Union - Telecommunications Standardization Sector
                      Collaboration Guidelines", RFC 3356, August 2002.

   [DOS]              Handley, M., Rescorla, E., and IAB, "Internet
                      Denial-of-Service Considerations", RFC 4732,
                      December 2006.

   [ITU-R-BS.1387-1]  "Method for objective measurements of perceived
                      audio quality", ITU-R Recommendation BS.1387-1,
                      November 2001.

   [ITU-R-BS.1534]    "Method for the subjective assessment of
                      intermediate quality levels of coding systems",
                      ITU-R Recommendation BS.1534, January 2003.

   [ITU-T-P.862]      "Perceptual evaluation of speech quality (PESQ):
                      An objective method for end-to-end speech quality
                      assessment of narrow-band telephone networks and
                      speech codecs", ITU-T Recommendation P.862,
                      October 2007.

   [KEYWORDS]         Bradner, S., "Key words for use in RFCs to
                      Indicate Requirement Levels", BCP 14, RFC 2119,
                      March 1997.

   [RTP]              Schulzrinne, H., Casner, S., Frederick, R., and V.
                      Jacobson, "RTP: A Transport Protocol for Real-Time
                      Applications", STD 64, RFC 3550, July 2003.

   [SECGUIDE]         Rescorla, E. and B. Korver, "Guidelines for
                      Writing RFC Text on Security Considerations",
                      BCP 72, RFC 3552, July 2003.

   [SRTP-VBR]         Perkins, C. and JM. Valin, "Guidelines for the Use
                      of Variable Bit Rate Audio with Secure RTP",
                      RFC 6562, March 2012.

   [UNCOORD]          Bryant, S., Morrow, M., and IAB, "Uncoordinated
                      Protocol Development Considered Harmful",
                      RFC 5704, November 2009.









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Authors' Addresses

   Jean-Marc Valin
   Mozilla
   650 Castro Street
   Mountain View, CA  94041
   USA

   EMail: jmvalin@jmvalin.ca


   Slava Borilin
   SPIRIT DSP

   EMail: borilin@spiritdsp.com


   Koen Vos

   EMail: koen.vos@skype.net


   Christopher Montgomery
   Xiph.Org Foundation

   EMail: xiphmont@xiph.org


   Raymond (Juin-Hwey) Chen
   Broadcom Corporation

   EMail: rchen@broadcom.com



















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