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

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.

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
          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.

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.

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

9. Full Copyright Statement

Copyright © 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.

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Acknowledgement

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