RFC 3802 (rfc3802) - Page 2 of 7
Toll Quality Voice - 32 kbit/s Adaptive Differential Pulse Code Modulation (ADPCM) MIME Sub-type Registration
Alternative Format: Original Text Document
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