RFC 3802

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

📅 published June 2004, from ietf

RFC 3802: Toll Quality Voice, 32 kbit/s ADPCM MIME Sub-type Registration 🎀

Bestie, this is the spec for registering the MIME type audio/32KADPCM, aka 32 kbit/s ADPCM audio that's ITU-T G.726 certified and made for toll-quality voice 🎧💖

Title block facts:

  • Authors: G. Vaudreuil (Lucent Technologies) and G. Parsons (Nortel Networks) 👯
  • Category: Standards Track 🛤️
  • Date: June 2004 📅
  • Obsoletes: RFC 2422 🔄
  • Updates: nothing listed, so this one doesn't touch any other RFC 🙅‍♀️

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).

Abstract 🌸

This doc registers the MIME sub-type audio/32KADPCM, which is Adaptive Differential Pulse Code Modulation for toll quality audio. The encoding is defined by the ITU-T in Recommendation G.726. 🎶

1. Introduction 🦋

Okay so this doc registers the MIME sub-type audio/32KADPCM for toll quality audio, and the encoding is defined by the ITU-T in Recommendation G.726 💅 It obsoletes an earlier sub-type registration that lived in RFC 1911, and it 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]. These are the official requirement-level words, so treat them seriously 🫶

2. ITU-T Definition 🧚

Recommendation G.726 [G726] defines the characteristics that are recommended for converting a 64 kbit/s A-law or m-law pulse code modulation (PCM) channel, running 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 🍓

G.726 was prepared by Study Group 15 of the Telecommunications Standardization Sector of the International Telecommunication Union (ITU-T). It 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 converting 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 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 holds 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 🎀

One more thing this doc 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. It's 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. Also, only the little endian bit ordering is valid. For proper usage, check the VPIM Specification 🧁

4. IANA Registration 📝

Here's the registration template, copied word for word (the MUST and shall in the ordering rules are part of it, so they stay exactly as written):

   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. That's it, no drama 🙌

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

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