RFC 3802 (rfc3802) - Page 2 of 7

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

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