Toll Quality Voice - 32 kbit/s Adaptive Differential Pulse Code Modulation (ADPCM) MIME Sub-type Registration
obsoletes
- rfc-2422 — Toll Quality Voice - 32 kbit/s ADPCM MIME Sub-type Registration
Extracted elements (10)
The choice of little endian ordering for 4-bit code words is adopted from ITU-T Recommendation X.420, which defines the equivalent ordering for the X.400 body part. This alignment ensures consistency with the existing X.400 voice messaging ecosystem that VPIM targets.
In VPIM usage, only the little endian bit ordering is valid for audio/32KADPCM. The Content-Description and Content-Disposition headers are used to describe the audio body contents.
The four-bit code word ordering within a byte may differ between existing implementations of G.726 codecs. Since audio/32KADPCM only permits little endian ordering, codecs that support the opposite (big endian) ordering must reorder the 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 voice 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 data in the audio/32KADPCM content type.
The 4-bit code words of the G.726 encoding MUST be packed into octets using little endian ordering, with the first code word of each pair 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.
IANA registration of MIME media type audio/32KADPCM for 32 kbit/s G.726 ADPCM audio with no required or optional parameters, little endian bit ordering, and file extension .726.
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 exploitable behavior.
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 each octet (A0 in bit 0), and the second code word (B) occupies the four most significant bits (B3 in bit 7). Subsequent code word pairs are packed the same way into successive octets.