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Network Working Group                                         G. VaudreuilRequest for Comments: 2422                             Lucent TechnologiesObsoletes:1911                                                 G. ParsonsCategory: Standards Track                                 Northern Telecom                                                            September 1998Toll Quality Voice - 32 kbit/s ADPCMMIME Sub-type RegistrationStatus 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 inRFC 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 PCMVaudreuil & 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 Definition3.1 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 19984.  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): .726Vaudreuil & 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. VaudreuilVaudreuil & Parsons         Standards Track                     [Page 4]

RFC 2422                    32 kbit/s ADPCM               September 19985. 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.Com6. 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 19987.  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]