Network Working Group G. Vaudreuil Request for Comments: 3802 Lucent Technologies Obsoletes: 2422 G. Parsons Category: Standards Track Nortel Networks June 2004
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.
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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.
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].
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.
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.
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.
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
Vaudreuil, et al. Standards Track [Page 3]
RFC 3802 32 kbit/s ADPCM June 2004
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.
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.
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