RFC2422 - Toll Quality Voice - 32 kbit/s ADPCM MIME Sub-type Registration

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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 RFC1911.

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 Specifcation 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
Glenn.Parsons@Nortel.ca

Gregory M. Vaudreuil
GregV@Lucent.Com

Intended usage: COMMON

Author/Change controller:

Glenn W. Parsons & Gregory M. Vaudreuil

5. Authors' Addresses

Glenn W. Parsons
Northern Telecom
P.O. Box 3511, Station C
Ottawa, ON K1Y 4H7
Canada

Phone: +1-613-763-7582
Fax: +1-613-763-4461
EMail: Glenn.Parsons@Nortel.ca

Gregory M. Vaudreuil
Lucent Technologies
17080 Dallas Parkway
Dallas, TX 75248-1905
United States

Phone/Fax: +1-972-733-2722
EMail:GregV@Lucent.Com

6. 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).

[MIME4] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures",
RFC2048, November 1996.

[VPIM1] Vaudreuil, G., "Voice Profile for Internet Mail", RFC1911,
February 1996.

[VPIM2] Vaudreuil, G., and G. Parsons, "Voice Profile for Internet
Mail - version 2", RFC2421, September 1998.

[X420] ITU-T Recommendation X.420 (1996) - ISO/IEC 10021-7:1996,
Message handling systems: Interpersonal messaging.

[REQ] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

7. Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to
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