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rfc:rfc2422

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

Vaudreuil & Parsons Standards Track [Page 1] 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.

Vaudreuil & Parsons Standards Track [Page 2] RFC 2422 32 kbit/s ADPCM September 1998

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

Vaudreuil & Parsons Standards Track [Page 3] 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

Vaudreuil & Parsons Standards Track [Page 4] 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.

Vaudreuil & Parsons Standards Track [Page 5] RFC 2422 32 kbit/s ADPCM September 1998

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."

Vaudreuil & Parsons Standards Track [Page 6]

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