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).
AbstractThis 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. 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].
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. 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.
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
[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. [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. RFC 2422 have been made to this document.
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