Network Working Group                                         G. Vaudreuil
Request for Comments: 2422                             Lucent Technologies
Obsoletes: 1911                                                 G. Parsons
Category: Standards Track                                 Northern Telecom
                                                           September 1998


                 Toll Quality Voice - 32 kbit/s 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 (1998).  All Rights Reserved.

Overview

  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.

1.  Abstract

  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 refines
  an earlier sub-type registration in RFC 1911.

  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].

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






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  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 Specifcation for proper usage.












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4.  IANA Registration

  To: [email protected]
  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




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    Macintosh File Type Code(s):  APCM

     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
    [email protected]

    Gregory M. Vaudreuil
    [email protected]

  Intended usage: COMMON

  Author/Change controller:

    Glenn W. Parsons & Gregory M. Vaudreuil







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5. Authors' Addresses

  Glenn W. Parsons
  Northern Telecom
  P.O. Box 3511, Station C
  Ottawa, ON  K1Y 4H7
  Canada

  Phone: +1-613-763-7582
  Fax: +1-613-763-4461
  EMail: [email protected]


  Gregory M. Vaudreuil
  Lucent Technologies
  17080 Dallas Parkway
  Dallas, TX  75248-1905
  United States

  Phone/Fax: +1-972-733-2722
  EMail:[email protected]

6. 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).

  [MIME4] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
          Mail Extensions (MIME) Part Four: Registration Procedures",
          RFC 2048, November 1996.

  [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.

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








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7.  Full Copyright Statement

  Copyright (C) The Internet Society (1998).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implmentation may be prepared, copied, published and
  distributed, in whole or in part, without restriction of any kind,
  provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS 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."
























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