draft Audio/L16 February 99
The Audio/L16 MIME content type
Feb 5 1999
James Salsman
james@bovik.org
Harald Tveit Alvestrand
UNINETT
Harald.T.Alvestrand@uninett.no
Status of this Memo
This draft document is being circulated for comment.
Please send comments to the authors.
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The file name of this version is draft-alvestrand-audio-l16-01.txt
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draft Audio/L16 February 99
1. Introduction
This document defines the audio/L16 MIME type, a reasonable
quality audio format for use in Internet applications.
Possible application areas include E-mail, Web served content,
file upload in Web forms, and more.
2. The need for the Audio/L16 MIME type
The set of IETF standard MIME types for audio is small; it
consists of only the audio/basic and audio/32kadpcm types, which
have a sampling rate of 8000 Kbits/second.
Rates below 11025 may obscure consonant information, even for
single-voice speech. Common compressions, such as LPC, are known
to be microphone-dependant and lossy. Thus far all IETF MIME
Audio types either default to 8000 samples per second or use LPC.
In order for advanced speech recognition and related educational
applications to make use of internet transports (such as RFC 1867
file uploading) which use MIME typing, higher standards are
required.
This type repairs that lack by registering a very simple MIME type
that allows higher rate, linear-encoded audio with multiple
channels.
This is an IESG approved MIME type, and its definition is
therefore published as an RFC.
Please note that there are many other Audio types described in RFC
1890 [1] which IANA may wish to formally register; this one, of
all of them, seems to be most immediately needed. This document
may also serve as a template for further registrations of these
audio types.
3. The definition of Audio/L16
Audio/L16 is based on the well know audio format "L16" described
in RFC 1890 section 4.4.8 for use with RTP transport.
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L16 denotes uncompressed audio data, using 16-bit signed
representation in twos-complement notation and network byte order.
(From section 4.4.8 of RFC 1890)
It may be parametrized by varying the sample rate and the number
of channels; the parameters are given on the MIME type header.
In order to promote interoperability, only a few rate values are
standardized here. Other values may NOT be used except by
bilateral agreement.
If multiple audio channels are used, channels are numbered left-
to- right, starting at one. Samples are put into the data stream
from each channel in succession; information from lower-numbered
channels precedes that from higher-numbered channels.
For more than two channels, the convention followed by the AIFF-C
audio interchange format should be followed [1], using the
following notation:
l left
r right
c center
S surround
F front
R rear
channels description channel
1 2 3 4 5 6
___________________________________________________________
2 stereo l r
3 l r c
4 quadrophonic Fl Fr Rl Rr
4 l c r S
5 Fl Fr Fc Sl Sr
6 l lc c r rc S
(From RFC 1890 section 4.1)
4. IANA registration form for Audio/L16
MIME media type name : Audio
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MIME subtype name : L16
Required parameters
rate: number of samples per second -- Permissible values for
rate are 8000, 11025, 16000, 22050, 24000, 32000, 44100, and
48000 samples per second.
Optional parameters
channels: how many audio streams are interleaved -- defaults
to 1; stereo would be 2, etc. Interleaving takes place
between individual two-byte samples.
Encoding considerations
Audio data is binary data, and must be encoded for non-binary
transport; the Base64 encoding is suitable for Email. Note
that audio data does not compress easily using lossless
compression.
Security considerations
Audio data is believed to offer no security risks.
Interoperability considerations
This type is compatible with the encoding used in the WAV
(Microsoft Windows RIFF) and Apple AIFF union types, and with
the public domain "sox" and "rateconv" programs.
Published specification
<<< Replace with RFC number at publication >>>
Applications which use this media
The public domain "sox" and "rateconv" programs accept this
type.
1. Magic number(s) : None
2. File extension(s) : WAV L16
3. Macintosh file type code : AIFF
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Person to contact for further information
1. Name : James Salsman
2. E-mail : jps-L16@bovik.org
Intended usage
Common
It is expected that many audio and speech applications will
use this type. Already the most popular platforms provide
this type with the rate=11025 parameter referred to as "radio
quality speech."
Author/Change controller
James Salsman
5. Security considerations
The audio data is believed to offer no security risks.
Note that RFC 1890 permits an application to choose to play a
single channel from a multichannel tranmission; an attacker who
knows that two different users will pick different channels could
concievably construct some confusing messages; this, however, is
ridiculous.
This type is perfect for hiding data using steganography.
6. References
[1] Audio-Video Transport Working Group, H. Schulzrinne: RTP
Profile for Audio and Video Conferences with Minimal Control,
RFC 1890, January 1996
7. Author's address
James Salsman
575 S. Rengstorff Avenue
Mountain View, CA 94040-1982 US
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draft Audio/L16 February 99
James@bovik.org
Harald Tveit Alvestrand
UNINETT
N-7034 TRONDHEIM
NORWAY
+47 73 59 70 94
Harald.T.Alvestrand@uninett.no
8. Copyright statement
The full ISOC copyright statement should be inserted here by the
RFC editor on publication.
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