Toll Quality Voice - 32 kbit/s Adaptive Differential Pulse Code Modulation (ADPCM) MIME Sub-type Registration
RFC 3802

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

Vaudreuil, et al.           Standards Track                     [Page 1]
RFC 3802                    32 kbit/s ADPCM                    June 2004

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.

Vaudreuil, et al.           Standards Track                     [Page 2]
RFC 3802                    32 kbit/s ADPCM                    June 2004

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