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rfc:rfc2586

Network Working Group J. Salsman Request for Comments: 2586 H. Alvestrand Category: Informational UNINETT

                                                          May 1999
                  The Audio/L16 MIME content type

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

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 samples/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.

Salsman Informational [Page 1] RFC 2586 The Audio/L16 MIME content type May 1999

 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.  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)

Salsman Informational [Page 2] RFC 2586 The Audio/L16 MIME content type May 1999

4. IANA registration form for Audio/L16

 MIME media type name : Audio
 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
      RFC 2586
 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
 Person to contact for further information
      1. Name : James Salsman
      2. E-mail : jps-L16@bovik.org
 Intended usage
      Common

Salsman Informational [Page 3] RFC 2586 The Audio/L16 MIME content type May 1999

      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]  Schulzrinne, H., "RTP Profile for Audio and Video Conferences
      with Minimal Control", RFC 1890, January 1996.

7. Authors' Addresses

 James Salsman
 575 S. Rengstorff Avenue
 Mountain View, CA  94040-1982 US
 EMail: James@bovik.org
 Harald Tveit Alvestrand
 UNINETT
 N-7034 TRONDHEIM
 NORWAY
 Phone: +47 73 59 70 94
 EMail: Harald.T.Alvestrand@uninett.no

Salsman Informational [Page 4] RFC 2586 The Audio/L16 MIME content type May 1999

8. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Salsman Informational [Page 5]

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