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 (C) The Internet Society (2004).
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
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
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
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
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
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
Subject: Registration of MIME media type audio/32KADPCM
MIME media type name: audio
MIME subtype name: 32KADPCM
Required parameters: none
Optional parameters: none
Binary or Base-64 generally preferred
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.
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.
ITU-T G.726 with little endian ordering
Applications which use this media type:
Primarily voice messaging
Magic number(s): ? File extension(s): .726 Macintosh File Type
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.
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
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.
6.1. 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
[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.
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