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rfc:rfc3802

Network Working Group G. Vaudreuil Request for Comments: 3802 Lucent Technologies Obsoletes: 2422 G. Parsons Category: Standards Track Nortel Networks

                                                             June 2004
  Toll Quality Voice - 32 kbit/s Adaptive Differential Pulse Code
           Modulation (ADPCM) MIME Sub-type Registration

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).

Abstract

 This document describes the registration of the MIME sub-type
 audio/32KADPCM Adaptive Differential Pulse Code Modulation for toll
 quality audio.  This audio encoding is defined by the ITU-T in
 Recommendation G.726.

1. Introduction

 This document describes the registration of the MIME sub-type
 audio/32KADPCM for toll quality audio.  This audio encoding is
 defined by the ITU-T in Recommendation G.726.  This document
 obsoletes an earlier sub-type registration contained in RFC 1911.
 This document also obsoletes RFC 2422.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [REQ].

Vaudreuil, et al. Standards Track [Page 1] RFC 3802 32 kbit/s ADPCM June 2004

2. ITU-T Definition

 Recommendation G.726 [G726] defines the characteristics that are
 recommended for the conversion of a 64 kbit/s A-law or m-law pulse
 code modulation (PCM) channel at 8000 samples/second to and from a
 40, 32, 24 or 16 kbit/s channel.  The conversion is applied to the
 PCM bit stream using an adaptive differential pulse code modulation
 (ADPCM) transcoding technique.  This Recommendation obsoletes G.721
 which only defined the 32 kbit/s characteristics.
 Recommendation G.726 was prepared by Study Group 15 of the
 Telecommunications Standardization Sector of the International
 Telecommunication Union (ITU-T) and was approved under the ITU's
 Resolution No. 2 procedure on the 14 of December 1990.

3. MIME Definition

3.1. audio/32KADPCM

 CCITT Recommendation G.726 [G726] describes the algorithm recommended
 for conversion of a 64 kbit/s A-law or u-law PCM channel to and from
 a 32 kbit/s channel (this is the same algorithm as described in the
 deprecated G.721).  The conversion is applied to the PCM stream using
 an Adaptive Differential Pulse Code Modulation (ADPCM) transcoding
 technique.
 The MIME sub-type audio/32KADPCM is defined to hold binary audio data
 encoded in 32 kbit/s ADPCM exactly as defined by ITU-T Recommendation
 G.726.  No header information shall be included as part of the audio
 data.  The content transfer encoding is typically either binary or
 base64.
 An additional consideration that this document defines for clarity is
 the choice of little endian ordering of the four bit code words. This
 default ordering is defined in ITU-T Recommendation X.420 [X420] for
 the equivalent X.400 body part, but is also detailed below in the
 IANA Registration.

3.2. VPIM Usage

 The audio/32KADPCM sub-type is a primary component of the VPIM
 specification [VPIM].  In this context, the Content-Description and
 Content-Disposition headers are used to succinctly describe the
 contents of the audio body.  As well, only the little endian bit
 ordering is valid.  Refer to the VPIM Specification for proper usage.

Vaudreuil, et al. Standards Track [Page 2] RFC 3802 32 kbit/s ADPCM June 2004

4. IANA Registration

    To: ietf-types@iana.org
    Subject: Registration of MIME media type audio/32KADPCM
    MIME media type name: audio
    MIME subtype name: 32KADPCM
    Required parameters: none
    Optional parameters: none
    Encoding considerations:
       Binary or Base-64 generally preferred
    Security considerations:
       There are no known security risks with the sending or playing
       of raw audio data  Audio data is typically interpreted only by
       an audio codec.  Unintended information introduced into the
       data stream will result in noise.
    Interoperability considerations:
       The four bit code word ordering within a byte may differ
       between existing implementations of G.726 codecs.  Since this
       content only permits the little endian ordering, codecs that
       support the opposite ordering must reorder the code words
       before storing to or retrieving from this content type.
    Published specification:
       ITU-T G.726 with little endian ordering
    Applications which use this media type:
       Primarily voice messaging
    Additional information:
       Magic number(s): ? File extension(s): .726 Macintosh File Type
       Code(s):  APCM

Vaudreuil, et al. Standards Track [Page 3] RFC 3802 32 kbit/s ADPCM June 2004

        Little Endian Ordering:
        The 4-bit code words of the G.726 encoding MUST be packed into
        octets/bytes as follows:  the first code word (A) is placed in
        the four least significant bits of the first octet, with the
        least significant bit (LSB) of the code word (A0) in the least
        significant bit of the octet;  the second code word (B) is
        placed in the four most significant bits of the first octet,
        with the most significant bit (MSB) of the code word (B3) in
        the most significant bit of the octet. Subsequent pairs of the
        code words shall be packed in the same way into successive
        octets, with the first code word of each pair placed in the
        least significant four bits of the octet.  It is preferred
        that the voice sample be extended with silence such that the
        encoded value comprises an even number of code words.
        However, if the voice sample comprises an odd number of code
        words, then the last code word shall be discarded.
                +--+--+--+--+--+--+--+--+
                |B3|B2|B1|B0|A3|A2|A1|A0|
                +--+--+--+--+--+--+--+--+
        MSB ->  | 7| 6| 5| 4| 3| 2| 1| 0|  <- LSB
                +--+--+--+--+--+--+--+--+
                32K ADPCM / Octet Mapping
    Person & email address to contact for further information:
      Glenn W. Parsons gparsons@NortelNetworks.com
      Gregory M. Vaudreuil GregV@ieee.org
    Intended usage: COMMON
    Author/Change controller:
      Glenn W. Parsons & Gregory M. Vaudreuil

5. Security Considerations

 There are no known security risks with the sending or playing of raw
 audio data  Audio data is typically interpreted only by an audio
 codec.  Unintended information introduced into the data stream will
 result in noise.

Vaudreuil, et al. Standards Track [Page 4] RFC 3802 32 kbit/s ADPCM June 2004

6. References

6.1. Normative References

 [G726]     CCITT Recommendation G.726 (1990), General Aspects of
            Digital Transmission Systems, Terminal Equipment - 40, 32,
            24,16 kbit/s Adaptive Differential Pulse Code Modulation
            (ADPCM).
 [VPIM2R2]  Vaudreuil, G., and G. Parsons, "Voice Profile for Internet
            Mail - version 2 (VPIMv2)", RFC 3801, June 2004.
 [REQ]      Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.

6.2. Informative References

 [RFC 3023] Murata, M., St. Laurent, S. and D. Kohn, "XML Media
            Types", RFC 3023, January 2001.
 [VPIM1]    Vaudreuil, G., "Voice Profile for Internet Mail", RFC
            1911, February 1996.
 [VPIM2]    Vaudreuil, G., and G. Parsons, "Voice Profile for Internet
            Mail - version 2", RFC 2421, September 1998.
 [X420]     ITU-T Recommendation X.420 (1996) - ISO/IEC 10021-7:1996,
            Message handling systems: Interpersonal messaging.

7. Changes from RFC 2422

 Only editorial and boilerplate changes from RFC 2422 have been made
 to this document.

Vaudreuil, et al. Standards Track [Page 5] RFC 3802 32 kbit/s ADPCM June 2004

8. Authors' Addresses

 Gregory M. Vaudreuil
 Lucent Technologies
 7291 Williamson Rd
 Dallas, TX  75214
 United States
 EMail: gregv@ieee.org
 Glenn W. Parsons
 Nortel Networks
 P.O. Box 3511, Station C
 Ottawa, ON  K1Y 4H7
 Canada
 Phone: +1-613-763-7582
 Fax:   +1-613-763-2697
 EMail: gparsons@nortelnetworks.com

Vaudreuil, et al. Standards Track [Page 6] RFC 3802 32 kbit/s ADPCM June 2004

9. Full Copyright Statement

 Copyright (C) The Internet Society (2004).  This document is subject
 to the rights, licenses and restrictions contained in BCP 78, and
 except as set forth therein, the authors retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
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 this standard.  Please address the information to the IETF at ietf-
 ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Vaudreuil, et al. Standards Track [Page 7]

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