audio/32KADPCM: Toll Quality Voice - 32 kbit/s Adaptive Differential Pulse Code Modulation (ADPCM) MIME Sub-type Registration
Registers the MIME type that carries 32 kbit/s ADPCM telephone-quality voice, with a fixed bit order.
Vaudreuil, G. and G. Parsons, "Toll Quality Voice - 32 kbit/s Adaptive Differential Pulse Code Modulation (ADPCM) MIME Sub-type Registration", RFC 3802, DOI 10.17487/RFC3802, June 2004, <https://www.rfc-editor.org/info/rfc3802>.
@misc{rfc3802,
series = {Request for Comments},
number = 3802,
howpublished = {RFC 3802},
publisher = {RFC Editor},
doi = {10.17487/RFC3802},
url = {https://www.rfc-editor.org/info/rfc3802},
author = {G. Vaudreuil and G. Parsons},
title = {{Toll Quality Voice - 32 kbit/s Adaptive Differential Pulse Code Modulation (ADPCM) MIME Sub-type Registration}},
pagetotal = 7,
year = 2004,
month = jun,
} RFC text
Contents
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].
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.
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.
Published specification:
ITU-T G.726 with little endian ordering
Applications which use this media type:
Primarily voice messaging
Additional information:
Magic number(s): ? File extension(s): .726 Macintosh File Type
Code(s): APCM
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.
+--+--+--+--+--+--+--+--+
|B3|B2|B1|B0|A3|A2|A1|A0|
+--+--+--+--+--+--+--+--+
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
Author/Change controller:
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. References
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
(ADPCM).
[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.
8. Authors' Addresses
Gregory M. Vaudreuil
Lucent Technologies
7291 Williamson Rd
Dallas, TX 75214
United States
EMail: gregv@ieee.org
Glenn W. Parsons
Nortel Networks
P.O. Box 3511, Station C
Ottawa, ON K1Y 4H7
Canada
Phone: +1-613-763-7582
Fax: +1-613-763-2697
EMail: gparsons@nortelnetworks.com
9. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
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pertain to the implementation or use of the technology described in
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1Introduction
RFC 3802 registers the MIME sub-type audio/32KADPCM for toll quality audio. The encoding itself is defined by the ITU-T in Recommendation G.726. The RFC obsoletes an earlier sub-type registration in RFC 1911 and also obsoletes RFC 2422.
A section titled Changes from RFC 2422 states that only editorial and boilerplate changes were made.
This note gives an official name to one kind of phone-quality sound so email programs know what it is.
§2The ITU-T Definition
Recommendation G.726 defines the recommended conversion of a 64 kbit/s A-law or mu-law pulse code modulation (PCM) channel at 8000 samples/second to and from a 40, 32, 24 or 16 kbit/s channel. The conversion uses an adaptive differential pulse code modulation (ADPCM) transcoding technique.
The Recommendation was prepared by ITU-T Study Group 15 and approved on 14 December 1990. It obsoletes G.721, which defined only the 32 kbit/s characteristics.
Phone calls are turned into numbers many times a second. This method squeezes those numbers so the sound takes about half the space, and the 32 in the name is how many thousand bits per second remain.
§3The MIME Type Definition
The sub-type audio/32KADPCM holds binary audio data encoded in 32 kbit/s ADPCM exactly as defined by G.726. This is the same algorithm as in the deprecated G.721. No header information shall be included in the audio data, and the content transfer encoding is typically binary or base64.
For clarity the RFC adds one rule: little endian ordering of the four-bit code words. That is the default in ITU-T X.420 for the equivalent X.400 body part.
The type is a primary component of the Voice Profile for Internet Mail (VPIM). There, the Content-Description and Content-Disposition headers describe the audio body, and only little endian bit ordering is valid.
The audio is sent as a plain stream of sound numbers with no cover page. The only extra rule is about which half of each byte holds the first number.
§4Packing Code Words into Bytes
The 4-bit code words MUST be packed into octets so that the first code word (A) goes in the four least significant bits of the first octet, and the second code word (B) in the four most significant bits. Later pairs are packed the same way into successive octets.
Encoders should preferably extend the sample with silence so there is an even number of code words. If a sample has an odd number of code words, the last code word shall be discarded.
Because implementations of G.726 may order the code words differently within a byte, a codec that supports the opposite ordering must reorder the code words before storing or retrieving this content type.
Each byte holds two small numbers like two eggs in a box. The first egg always goes in the right-hand slot. If there is one egg left over at the end, it is thrown away.
§5IANA Registration Details
The registration template gives media type audio, sub-type 32KADPCM, no required and no optional parameters, and says binary or base64 is generally preferred. The published specification is ITU-T G.726 with little endian ordering. The listed applications are primarily voice messaging, the file extension is .726, the Macintosh file type code is APCM, and the intended usage is COMMON. The magic number entry is given as a question mark.
The authors are listed as the contacts and change controllers.
This is the form filled in to get the name officially written in the list of sound types.
§6Security Considerations
The RFC says there are no known security risks with sending or playing raw audio data. Audio data is typically interpreted only by an audio codec, and any unintended information introduced into the data stream will result in noise.
Sound data is only played by a sound program, so stray bits just make crackling noise instead of causing harm.