RFC 2422

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

📅 published September 1998, from ietf

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

Network Working Group Request for Comments: 2422 🎀 Obsoletes: 1911 (it's the old one, bestie 💀) Category: Standards Track Authors: G. Vaudreuil (Lucent Technologies) & G. Parsons (Northern Telecom) 👑 Date: September 1998

Status of this Memo

okay so this doc is an Internet standards track protocol 🎀 It's open for discussion and suggestions for improvements, so speak up! For where it stands officially, check the current "Internet Official Protocol Standards" (STD 1). And yes, distribution is unlimited 🦋

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

Overview

This doc registers the MIME sub-type audio/32KADPCM for toll quality audio 🎧 The encoding comes from the ITU-T's Recommendation G.726 ✨

1. Abstract 🌸

Okay this is the core of it: this doc registers the MIME sub-type audio/32KADPCM for toll quality audio, and that encoding is defined by the ITU-T in Recommendation G.726. It also refines an earlier sub-type registration, the one in RFC 1911 💅

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]. So those capital words are the ones with real weight, so read them like they matter 🫶

2. ITU-T Definition 🧚

Recommendation G.726 [G726] is the one that defines the characteristics that are recommended for converting 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 runs on the PCM bit stream with an adaptive differential pulse code modulation (ADPCM) transcoding technique. This Recommendation also obsoletes G.721, which only covered 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 got approved under the ITU's Resolution No. 2 procedure on the 14 of December 1990 🌈

3. MIME Definition 💖

3.1 audio/32KADPCM

Here's the main event: CCITT Recommendation G.726 [G726] describes the algorithm that's recommended for converting a 64 kbit/s A-law or u-law PCM channel to and from a 32 kbit/s channel. (It's the same algorithm as the deprecated G.721, btw 🙈) The conversion happens on the PCM stream with an Adaptive Differential Pulse Code Modulation (ADPCM) transcoding technique ✨

The audio/32KADPCM sub-type is meant to hold binary audio data encoded in 32 kbit/s ADPCM, exactly as ITU-T Recommendation G.726 defines it. 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 spells out for clarity: the four bit code words use little endian ordering. That default ordering comes from ITU-T Recommendation X.420 [X420] for the matching X.400 body part, and you'll see the details again in the IANA Registration below 💅

3.2 VPIM Usage

The audio/32KADPCM sub-type is a primary component of the VPIM specification [VPIM]. In that 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, go check the VPIM Specifcation 🌸

4. IANA Registration 👑

This is the registration form, and it's copied exactly so nothing gets lost in translation 💅

   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

Now for the Little Endian Ordering bit, which is the heart of the whole thing 🔥

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) goes 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) goes 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's preferred that the voice sample gets extended with silence so the encoded value has an even number of code words. However, if the voice sample has an odd number of code words, then the last code word shall be discarded 🙈

Here's the picture of that mapping:

              +--+--+--+--+--+--+--+--+
              |B3|B2|B1|B0|A3|A2|A1|A0|
              +--+--+--+--+--+--+--+--+
      MSB ->  | 7| 6| 5| 4| 3| 2| 1| 0|  <- LSB
              +--+--+--+--+--+--+--+--+

              32K ADPCM / Octet Mapping

And the rest of the registration form, also copied verbatim:

   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", RFC 2048, November 1996.

[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.

[REQ] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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