Toll Quality Voice - 32 kbit/s ADPCM MIME Sub-type Registration
RFC 2422

Document Type RFC - Proposed Standard (September 1998; No errata)
Obsoleted by RFC 3802
Obsoletes RFC 1911
Was draft-ema-vpim-32kadpcm (individual)
Last updated 2013-03-02
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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.
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