INTERNET-DRAFT Laile L. Di Silvestro (Microsoft)
Expires in 6 months Greg Baribault (Microsoft)
Microsoft Corporation
June 20, 1999
Waveform Audio File Format
MIME Sub-type Registration
<draft-ema-vpim-wav-00.txt>
Status of this memo:
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document describes the registration of the MIME sub-type
audio/wav for Waveform Audio File Format. This audio file format
is based on RIFF and is defined by Microsoft in the Platform SDK.
1. Introduction
This document describes the registration of the MIME sub-type
audio/wav for the encapsulation of toll-quality audio in the
Waveform Audio File Format. This audio file format is based on
Resource Interchange File Format (RIFF), and is defined by Microsoft
in the Platform SDK.
The MIME subtype "wav" is being defined primarily for use in
multimedia and voice messaging standards. the Voice Profile for
Internet Messaging, version 3 [VPIM3] working draft specifies that
all VPIM version 3 compliant implementations MAY generate
audio/wav bodyparts and MUST receive audio/wav bodyparts. The VPIM
version 3 specification further states that all compliant
implementations MUST support receipt of wav-encapsulated 32KADPCM
(g.726 ADPCM), BASIC (g.711 mu-law), and MS-GSM (Microsoft g.610
GSM) encoded audio.
Because the Waveform Audio File format is not well-defined and has
not undergone a process of standardization, this document briefly
defines the format that will be supported by VPIM version 3. For
more detailed information, refer to the specification.
This document does not obsolete the informational draft RFC 2361
[WAVE] which describes audio/vnd.wav. Whereas RFC 2361 describes
a mechanism for indicating a codec registered in the wav or avi
vendor tree registries, this document proposes a standard for
specifying wav-encapsulated audio content in a MIME stream.
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].
2. WAV Definition
Waveform Audio File Format is a file format for the storing of
audio data in data chunks according to the Resource Interchange File
Format (RIFF). Although the Waveform format is described in detail
in xxxxxxx, lack of standardization and a proliferation of
interpretations and enhancements make the format difficult to
implement and support in an interoperable fashion. This document
seeks to rectify the situation by defining the Waveform Audio
File Format features that MUST be inplemented and supported for
conformance with the proposed VPIM version 3 standard.
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2.1 Data Organization
Data MUST be stored in 8-bit bytes in little-endian order.
Multi-byte values MUST be stored with the low-order bytes first,
and the bits left-justified:
(lsb = least-significant bit, msb = most-significant bit)
7 6 5 4 3 2 1 0
+-----------------------+
char: | msb lsb |
+-----------------------+
7 6 5 4 3 2 1 0 15 14 13 12 11 10 9 8
+-----------------------+-----------------------+
short: | msb byte 0 | byte 1 lsb |
+-----------------------+-----------------------+
2.2 File Format
The Waveform Audio File Format follows the Resource Interchange File
Format (RIFF) standard in which all data is organized into 'chunks'
and 'sub-chunks.' Each chunk MUST comprise a 4-byte chunk ID, a
4-byte length field specifying the size of the data, and the chunk
data.
To be compliant with this proposed standard, wav-formatted audio
data MUST include the following chunks:
RIFF header chunk: ID = 'RIFF'
Format chunk: ID = 'fmt '
Sound data chunk: ID = 'data'
Fact chunk: ID = 'fact'
The chunks MAY appear in any order except that the Format chunk
MUST be placed before the Sound data chunk (but not necessarily
contiguous to the Sound data chunk). Any additional chunks
MUST be expected and MAY be ignored.
2.2.1 The RIFF Header Chunk
The RIFF header corresponds to the outermost chunk. In an audio/wav
file, it MUST adhere to the following format:
OFFSET LENGTH VALUE DESCRIPTION
0 4 bytes 'RIFF' The file format ID.
4 4 bytes Length of the file minus (-) 8 bytes.
8 4 bytes 'WAVE' The data format ID.
2.2.2 The Format Chunk
The Format chunk specifies the characteristics of the audio data
necessary to decompress it and play it. Each audio/wav file MUST
include one and only one Format chunk. This chunk MUST include
the following fields:
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OFFSET LENGTH VALUE DESCRIPTION
12 4 bytes 'fmt ' The chunk ID.
16 4 bytes 32 Length of the chunk excluding the 8
bytes for the ID and length.
20 4 bytes The codec ID.
24 4 bytes The number of channels.
28 8 bytes Samples per second.
36 8 bytes Average bytes per second.
44 4 bytes Block alignment.
48 4 bytes Bits per sample.
Codec ID: The codec ID indicates what codec was used to compress
the audio data. Three codecs are supported by the proposed VPIM
version 3 standard, and one of them SHOULD be specified in the
Codec ID field. The Codec ID field MAY indicate a codec other
that the three listed below only in situations where it is certain
that the recipient has the corresponding capabilities.
CODEC ID
g.711 mu-law 0x0007
g.610 MS-GSM 0x0031
g.726 32kADPCM 0x0064
Number of Channels: To preserve network bandwidth and minimize
memory requirements, the Format chunk SHOULD specify and the Data
chunk SHOULD provide only one channel (mono) unless it is certain
that the recipient supports multi-channel playback.
CHANNELS VALUE
one (mono) 1
Samples per Second: This field indicates the rate at which the
audio is to be played (once uncompressed), expressed in sample
frames per second. The following table specified the samples per
second that correspond to each VPIM version 3 codec:
CODEC RATE (samples per second)
g.711 mu-law 8000
g.610 MS-GSM 8000
g.726 32kADPCM 8000
Average Bytes per Second: This field specifies the number of
bytes that play per second. It provides an indication of
the buffer size needed to store the audio in order to avoid
latency. It SHOULD be calculated according to the following
formula: samples/second * block alignment (rounded up to nearest
whole number).
CODEC RATE (average bytes per second)
g.711 mu-law 8000
g.610 MS-GSM 1625
g.726 32kADPCM 4000
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Block Alignment: This field indicates the size of a sample frame
in bytes. It SHOULD be calculated according to the following
formula: number of channels * (bits per sample / 8)
CODEC SIZE
g.711 mu-law 1
g.610 MS-GSM 65
g.726 32kADPCM 2
since there are 4 bits per sample, the frames will not
align on one byte. It is customary to add silence bits
(oxF) to the end of the sample to make the frame end on
a byte boundary.
Bits per Sample: This field specifies the bit resolution of a
sample point.
CODEC BITS (bits per sample)
g.711 mu-law 8
g.610 MS-GSM 0
data immediately followed by: 0x40 0x01
g.726 32kADPCM 4
2.2.3 The Data Chunk
The Data chunk contains the compressed audio data. This chunk
MUST be preceded (though not immediately) by the Format chunk.
The Data chunk MUST adhere to the following format:
OFFSET LENGTH VALUE DESCRIPTION
52 4 bytes 'data' The chunk ID.
56 4 bytes Length of the data
(chunk size minus (-) 8 bytes.
60 The compressed audio.
2.2.4 The Fact Chunk
All audio/wav files MUST include a Fact chunk as they contain
compressed data. The Fact chunk MUSt contain one field indicating
the size (in sample points) of the audio data after decompression.
The Fact chunk MUST adhere to the following format:
OFFSET LENGTH VALUE DESCRIPTION
4 bytes 'fact' The chunk ID.
4 bytes 8 Chunk size minus (-) 8 bytes.
8 bytes Sample length.
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3. MIME Definition
3.1 audio/wav
[Specification] describes a file format for the encapsulation of
raw and compressed audio data. This Waveform Audio File Format (WAVE)
is based on the Resource Interchange File Format specification
developed by Microsoft and IBM in 1991. The WAVE format organizes
audio data and the information needed to decompress and play it in
chunks.
The MIME sub-type audio/WAV is defined to hold binary audio data
encoded in 32 kbit/s ADPCM (g.726), mu-law (g.711), or MS-GSM
(g.610), and encapsulated in the WAVE format. The content transfer
encoding is typically either binary or base64.
3.2 VPIM Usage
The audio/wav sub-type is a component of the proposed VPIM version
3 specification [VPIM3]. In this context, the Content-Description
headers is used to succinctly describe the contents of the audio
body.
All VPIM Version 3 systems MUST be capable of receiving audio
encapsulated in a WAVE file format. Sending systems MAY choose to
send raw audio data or encapsulate it in the WAVE file format. All
audio data MUST be compressed in one of the VPIM v3 codecs and
encapsulated according to the guidelines provided in the section
2.0 of this document.
Refer to the VPIM Specifcation for proper usage.
3.3 Relation to RFC 2361
RFC 2361, "WAVE and AVI Codec Registries," is an informational
draft describing IANA namespaces for codecs registered in
Microsoft's WAVE and AVI registries. Such codecs may be described
in the following format: audio/vnd.wave; codec = [codec ID].
This format is not suited to the description of a wave file as
defined in this document, as it does not indicate the format
standard that audio/wav must adhere to for interoperability
between messaging systems. On desktop-oriented messaging systems,
audio/wav (rather than audio/vnd.wave) is the defacto standard.
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4. IANA Registration
To: ietf-types@iana.org
Subject: Registration of MIME media type audio/wav
MIME media type name: audio
MIME subtype name: wav
Required parameters: none
Optional parameters: codec = [codec id]
Encoding considerations:
Binary or Base-64 generally preferred
Security considerations:
There are no known security risks with the sending or
playing of audio data. Wav-encapsulated audio data is
typically interpreted only by a codec supported by a
wav audio player. Unintended information introduced into
the data stream will result in noise.
Interoperability considerations:
Published specification:
None
Applications which use this media type:
Multimedia and voice messaging applications
Additional information:
Magic number(s): ?
File extension(s): .wav
Macintosh File Type Code(s): WAVE
Person & email address to contact for further information:
Laile L. Di Silvestro
lailed@microsoft.com
Greg Baribault
gregbari@microsoft.com
Intended usage: COMMON
Author/Change controller:
Laile L. Di Silvestro
Greg Baribault
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5. Authors' Addresses
Laile L. Di Silvestro
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
lailed@microsoft.com
Greg Baribault
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
gregbari@microsoft.com
6. References
[G726] CCITT Recommendation G.726 (1990), General Aspects of Digital
Transmission Systems, Terminal Equipment - 40, 32, 24,16
kbit/s Adaptive Differential Pulse Code Modulation (ADPCM).
[MIME4] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures",
RFC 2048, November 1996.
[VPIM1] Vaudreuil, G., "Voice Profile for Internet Mail", RFC 1911,
February 1996.
[VPIM2] Vaudreuil, G., and G. Parsons, "Voice Profile for Internet
Mail - version 2", RFC 2421, September 1998.
[VPIM3] Vaudreuil, Greg, "Voice Profile for Internet Mail, Version
2", Work In Progress, <draft-ema-VPIMv3-00.txt>,
December 1998.
[REQ] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[WAVE] Fleischman, E., "WAVE and AVI Codec Registries", RFC 2361,
June 1998.
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7. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
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