    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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    RFC 2422                    32 kbit/s ADPCM               September 1998

       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: 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

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    RFC 2422                    32 kbit/s ADPCM               September 1998

         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
         Glenn.Parsons@Nortel.ca

         Gregory M. Vaudreuil
         GregV@Lucent.Com

       Intended usage: COMMON

       Author/Change controller:

         Glenn W. Parsons & Gregory M. Vaudreuil

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    RFC 2422                    32 kbit/s ADPCM               September 1998

    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: Glenn.Parsons@Nortel.ca

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

       Phone/Fax: +1-972-733-2722
       EMail:GregV@Lucent.Com

    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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