Toll Quality Voice - 32 kbit/s ADPCM MIME Sub-type Registration
obsoleted by
- rfc-3802 — Toll Quality Voice - 32 kbit/s Adaptive Differential Pulse Code Modulation (ADPCM) MIME Sub-type Registration
obsoletes
- rfc-1911 — Voice Profile for Internet Mail
Extracted elements (10)
The choice of little endian ordering for the 4-bit code words is derived from ITU-T Recommendation X.420, which defines the equivalent ordering for the X.400 body part carrying this audio encoding. This alignment with X.400 facilitates interoperability with existing voice messaging infrastructure.
In VPIM usage (RFC 2421), only little endian bit ordering is valid for audio/32KADPCM. Implementations using this sub-type in a VPIM context must adhere to the VPIM specification for proper usage.
The four-bit code word ordering within a byte may differ between existing implementations of G.726 codecs. Since audio/32KADPCM permits only little endian ordering, codecs that support the opposite ordering must reorder code words before storing to or retrieving from this content type.
It is preferred that the voice sample be extended with silence so the encoded value comprises an even number of code words. If the sample comprises an odd number of code words, the last code word shall be discarded.
No header information shall be included as part of the audio/32KADPCM audio data.
The 4-bit code words of G.726 encoding MUST be packed into octets using the little endian ordering described in the IANA registration, with the first code word in the least significant four bits of each octet.
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; content transfer encoding is typically binary or base64.
This document registers the MIME media type audio/32KADPCM for binary audio data encoded in 32 kbit/s ADPCM as defined by ITU-T Recommendation G.726. No required or optional parameters are defined; intended usage is COMMON.
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, and unintended information introduced into the data stream will result in noise rather than a security vulnerability.
G.726 4-bit code words are packed into octets in little endian order: the first code word (A) occupies the four least significant bits of an octet (A0 in bit 0), and the second code word (B) occupies the four most significant bits (B3 in bit 7). Subsequent pairs follow the same pattern in successive octets.