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Revision as of 22:51, 6 June 2013
AIFF
Audio 1-32 bit samples (Mac,AppleII,Synthia Pro)
provided by Steve Milne and Matt Deatherage, Apple Computer, Inc.
AIFF: Audio Interchange File Format File
----------------------------------------
The Audio Interchange File Format (Audio IFF) provides a standard for storing
sampled sounds. The format is quite flexible, allowing the storage of
monaural or multichannel sampled sounds at a variety of sample rates and
sample widths.
Audio IFF conforms to the "`EA IFF 85' Standard for Interchange Format Files"
developed by Electronic Arts.
Audio IFF is primarily an interchange format, although application designers
should find it flexible enough to use as a data storage format as well. If an
application does choose to use a different storage format, it should be able
to convert to and from the format defined in this document. This ability to
convert will facilitate the sharing of sound data between applications.
Audio IFF is the result of several meetings held with music developers over a
period of ten months during 1987 and 1988. Apple Computer greatly appreciates
the comments and cooperation provided by all developers who helped define this
standard.
Another "EA IFF 85" sound storage format is "`8SVX' IFF 8-bit Sampled Voice",
by Electronic Arts. "8SVX," which handles eight-bit monaural samples, is
intended mainly for storing sound for playback on personal computers. Audio
IFF is intended for use with a larger variety of computers, sampled sound
instruments, sound software applications, and high fidelity recording devices.
Data Types
A C-like language will be used to describe the data structures in this document
The data types used are listed below.
char: 8 bits signed. A char can contain more than just ASCII
characters. It can contain any number from -128 to 127
(inclusive).
unsigned char: 8 bits signed. Contains any number from 0 to 255 (inclusive).
short: 16 bits signed. Contains any number from -32,768 to 32,767
(inclusive).
unsigned short: 16 bits unsigned. Contains any number from 0 to 65,535
(inclusive).
long: 32 bits signed. Contains any number from -2,147,483,648
to 2,147,483,647 (inclusive).
unsigned long: 32 bits unsigned. Contains any number from 0 to
4,294,967,295 (inclusive).
extended: 80 bit IEEE Standard 754 floating point number (Standard
Apple Numeric Environment [SANE] data type Extended)
pstring: Pascal-style string, a one-byte count followed by text
bytes. The total number of bytes in this data type should
be even. A pad byte can be added to the end of the text to
accomplish this. This pad byte is not reflected in the
count.
ID: 32 bits, the concatenation of four printable ASCII characters
in the range " " (space, 0x20) through "~" (tilde, 0x7E).
Leading spaces are not allowed in the ID but trailing spaces
are OK. Control characters are forbidden.
Constants
Decimal values are referred to as a string of digits, for example 123, 0, 100
are all decimal numbers. Hexadecimal values are preceded by a 0x - e.g., 0x0A,
0x1, 0x64.
Data Organization
All data is stored in Motorola 68000 format. The bytes of multiple-byte
values are stored with the high-order bytes first. Data is organized as
follows:
7 6 5 4 3 2 1 0
+-----------------------+
char: | msb lsb |
+-----------------------+
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
+-----------------------+-----------------------+
char: | msb byte 0 | byte 1 lsb |
+-----------------------+-----------------------+
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
+-----------------------+-----------------------+
char: | msb byte 0 | byte 1 |
+-----------------------+-----------------------+
char: | byte 2 | byte 3 lsb |
+-----------------------+-----------------------+
Figure 1: IFF data storage formats
Referring to Audio IFF
The official name for this standard is Audio Interchange File Format. If an
application program needs to present the name of this format to a user, such
as in a "Save As..." dialog box, the name can be abbreviated to Audio IFF.
Referring to Audio IFF files by a four-letter abbreviation (i.e., "AIFF") in
user-level documentation or program-generated messages should be avoided.
File Structure
The "`EA IFF 85' Standard for Interchange Format Files" defines an overall
structure for storing data in files. Audio IFF conforms to those portions
of "EA IFF 85" that are germane to Audio IFF. For a more complete discussion
of "EA IFF 85", please refer to the document "`EAIFF 85', Standard for
Interchange Format Files."
An "EA IFF 85" file is made up of a number of chunks of data. Chunks are the
building blocks of "EA IFF 85" files. A chunk consists of some header
information followed by data:
+--------------------+
| ckID |\
+--------------------+ } header info
| ckSize |/
+--------------------+
| |
| |
| data |
| |
| |
+--------------------+
Figure 2: IFF Chunk structure
A chunk can be represented using our C-like language in the following manner:
typedef struct {
ID ckID; /* chunk ID */
long ckSize; /* chunk Size */
char ckData[]; /* data */
} Chunk;
The ckID describes the format of the data portion of a chunk. A program can
determine how to interpret the chunk data by examining ckID.
The ckSize is the size of the data portion of the chunk, in bytes. It does
not include the 8 bytes used by ckID and ckSize.
The ckData contains the data stored in the chunk. The format of this data is
determined by ckID. If the data is an odd number of bytes in length, a zero pad
byte must be added at the end. The pad byte is not included in ckSize.
Note that an array with no size specification (e.g., char ckData[];) indicates a
variable-sized array in our C-like language. This differs from standard C.
An Audio IFF file is a collection of a number of different types of chunks.
There is a Common Chunk which contains important parameters describing the
sampled sound, such as its length and sample rate. There is a Sound Data
Chunk which contains the actual audio samples. There are several other
optional chunks which define markers, list instrument parameters, store
application-specific information, etc. All of these chunks are described in
detail in later sections of this document.
The chunks in an Audio IFF file are grouped together in a container chunk.
"EA IFF 85" Standard for Interchange Format Files defines a number of
container chunks, but the one used by Audio IFF is called a FORM. A FORM has
the following format:
typedef struct {
ID ckID;
long ckSize;
ID formType;
char chunks[];
}
The ckID is always 'FORM'. This indicates that this is a FORM chunk.
The ckSize contains the size of data portion of the 'FORM' chunk. Note that
the data portion has been broken into two parts, formType and chunks[].
The formType field describes what's in the 'FORM' chunk. For Audio IFF files,
formType is always 'AIFF'. This indicates that the chunks within the FORM
pertain to sampled sound. A FORM chunk of formType 'AIFF' is called a FORM
AIFF.
The chunks field are the chunks contained within the FORM. These chunks are
called local chunks. A FORM AIFF along with its local chunks make up an
Audio IFF file.
Here is an example of a simple Audio IFF file. It consists of a file containing
single FORM AIFF which contains two local chunks, a Common Chunk and a Sound
Data Chunk.
__________________________
| FORM AIFF Chunk |
| ckID = 'FORM' |
| formType = 'AIFF' |
| __________________ |
| | Common Chunk | |
| | ckID = 'COMM' | |
| |__________________| |
| __________________ |
| | Sound Data Chunk | |
| | ckID = 'SSND' | |
| |__________________| |
|__________________________|
Figure 3: Simple Audio IFF File
There are no restrictions on the ordering of local chunks within a FORM AIFF.
A more detailed example of an Audio IFF file can be found in Appendix A. Please
refer to this example as often as necessary while reading the remainder of this
document.
Storage of AIFF on Apple and Other Platforms
On a Macintosh, the FORM AIFF, is stored in the data fork of an Audio IFF file.
The Macintosh file type of an Audio IFF file is 'AIFF'. This is the same as
the formType of the FORM AIFF. Macintosh applications should not store any
information in Audio IFF file's resource fork, as this information may not be
preserved by all applications. Applications can use the Application Specific
Chunk, defined later in this document, to store extra information specific to
their application.
Audio IFF files may be identified in other Apple file systems as well. On a
Macintosh under MFS or HFS, the FORM AIFF is stored in the data fork of a file
with file type "AIFF." This is the same as the formType of the FORM AIFF.
On an operating system such as MS-DOS or UNIX, where it is customary to use a
file name extension, it is recommended that Audio IFF file names use ".AIF"
for the extension.
On an Apple II, FORM AIFF is stored in a file with file type $D8 and auxiliary
type $0000. Versions 1.2 and earlier of the Audio IFF standard used file type
$CB and auxiliary type $0000. This is incorrect; the assignment listed in
this document is the correct assignment.
On the Apple IIGS stereo data is stored with right data on even channels and
left data on odd channels. Some portions of AIFF do not follow this
convention. Even where it does follow the convention, AIFF usually uses
channel two for right data instead of channel zero as most Apple IIGS
standards do. Be prepared to interpret data accordingly.
Local Chunk Types
The formats of the different local chunk types found within a FORM AIFF are
described in the following sections, as are their ckIDs.
There are two types of chunks: required and optional. The Common Chunk is
required. The Sound Data chunk is required if the sampled sound has a length
greater than zero. All other chunks are optional. All applications that use
FORM AIFF must be able to read the required chunks and can choose to
selectively ignore the optional chunks. A program that copies a FORM AIFF
should copy all the chunks in the FORM AIFF, even those it chooses not to
interpret.
The Common Chunk
The Common Chunk describes fundamental parameters of the sampled sound.
#define CommonID 'COMM' /* ckID for Common Chunk */
typedef struct {
ID ckID;
long ckSize;
short numChannels;
unsigned long numSampleFrames;
short sampleSize;
extended sampleRate;
} CommonChunk;
The ckID is always 'COMM'. The ckSize is the size of the data portion of the
chunk, in bytes. It does not include the 8 bytes used by ckID and ckSize.
For the Common Chunk, ckSize is always 18.
The numChannels field contains the number of audio channels for the sound.
A value of 1 means monophonic sound, 2 means stereo, and 4 means four channel
sound, etc. Any number of audio channels may be represented. For
multichannel sounds, single sample points from each channel are interleaved.
A set of interleaved sample points is called a sample frame.
The actual sound samples are stored in another chunk, the Sound Data Chunk,
which will be described shortly.
Single sample points from each channel are interleaved such that each
sample frame is a sample point from the same moment in time for each channel
available.
The numSampleFrames field contains the number of sample frames. This is not
necessarily the same as the number of bytes nor the number of samplepoints in
the Sound Data Chunk. The total number of sample points in the file is
numSampleFrames times numChannels.
The sampleSize is the number of bits in each sample point. It can be any
number from 1 to 32. The format of a sample point will be described in the
next section.
The sampleRate field is the sample rate at which the sound is to be played
back in sample frames per second.
One, and only one, Common Chunk is required in every FORM AIFF.
Sound Data Chunk
The Sound Data Chunk contains the actual sample frames.
#define SoundDataID 'SSND' /* ckID for Sound Data Chunk */
typedef struct {
ID ckID;
long ckSize;
unsigned long offset;
unsigned long blockSize;
unsigned char SoundData [];
} SoundDataChunk;
The ckID is always 'SSND'. The ckSize is the size of the data portion of the
chunk, in bytes. It does not include the 8 bytes used by ckID and ckSize.
The offset field determines where the first sample frame in the soundData
starts. The offset is in bytes. Most applications won't use offset and
should set it to zero. Use for a non-zero offset is explained in the
Block-Aligning Sound Data section below.
The blockSize is used in conjunction with offset for block-aligning sound
data. It contains the size in bytes of the blocks that sound data is aligned
to. As with offset, most applications won't use blockSize and should set it
to zero. More information on blockSize is in the Block-Aligning Sound Data
section below.
The soundData field contains the sample frames that make up the sound. The
number of sample frames in the soundData is determined by the numSampleFrames
field in the Common Chunk. Sample points and sample frames are explained in
detail in the next section.
The Sound Data Chunk is required unless the numSampleFrames field in the
Common Chunk is zero. A maximum of one Sound Data Chunk may appear in a FORM
AIFF.
Sample Points and Sample Frames
A large part of interpreting Audio IFF files revolves around the two concepts
of sample points and sample frames.
A sample point is a value representing a sample of a sound at a given point in
time. Each sample point is stored as a linear, 2's-complement value which may
be from 1 to 32 bits wide, as determined by sampleSize in the Common Chunk.
Sample points are stored in an integral number of contiguous bytes. One- to
eight-bit wide sample points are stored in one byte, 9- to 16-bit wide sample
points are stored in two bytes, 17- to 24-bit wide sample points are stored
in three bytes, and 25- to 32-bit wide sample points are stored in four bytes
(most significant byte first). When the width of a sample point is not a
multiple of eight bits, the sample point data is left justified, with the
remaining bits zeroed. An example case is illustrated in Figure 4. A 12-bit
sample point, binary 101000010111, is stored left justified in two bytes.
The remaining bits are set to zero.
___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___
| | | | | | | | | | | | | | | | |
| 1 0 1 0 0 0 0 1 | 0 1 1 1 0 0 0 0 |
|___|___|___|___|___|___|___|___|___|___|___|___|___|___|___|___|
<---------------------------------------------> <------------->
12 bit sample point is left justified rightmost
4 bits are
zero padded
Figure 4: 12-Bit Sample Point
For multichannel sounds, single sample points from each channel are
interleaved. A set of interleaved sample points is called a sample frame.
Single sample points from each channel are interleaved such that each
sample frame is a sample point from the same moment in time for each channel
available. This is illustrated in Figure 5 for the stereo (two channel) case.
sample sample sample
frame 0 frame 1 frame N
_____ _____ _____ _____ _____ _____
| ch1 | ch2 | ch1 | ch2 | . . . | ch1 | ch2 |
|_____|_____|_____|_____| |_____|_____|
_____
| | = one sample point
|_____|
Figure 5: Sample Frames for Multichannel Sound
For monophonic sound, a sample frame is a single sample point. For
multichannel sounds, you should follow the conventions in Figure 6.
channel
1 2 3 4 5 6
_________ _________ _________ _________ _________ _________
| left | right | | | | |
stereo | | | | | | |
|_________|_________|_________|_________|_________|_________|
| left | right | center | | | |
3 channel | | | | | | |
|_________|_________|_________|_________|_________|_________|
| front | front | rear | rear | | |
quad | left | right | left | right | | |
|_________|_________|_________|_________|_________|_________|
| left | center | right | surround| | |
4 channel | | | | | | |
|_________|_________|_________|_________|_________|_________|
| left | left | center | right | right |surround |
6 channel | | center | | | center | |
|_________|_________|_________|_________|_________|_________|
Figure 6: Sample Frame Conventions for Multichannel Sound
Sample frames are stored contiguously in order of increasing time. The sample
points within a sample frame are packed together; there are no unused bytes
between them. Likewise, the sample frames are packed together with no pad
bytes.
Block-Aligning Sound Data
There may be some applications that, to ensure real time recording and
playback of audio, wish to align sampled sound data with fixed-size blocks.
This alignment can be accomplished with the offset and blockSize parameters of
the Sound Data Chunk, as shown in Figure 7.
____________ __________________________________ ____________
|\\ unused \\| sample frames |\\ unused \\|
|____________|__________________________________|____________|
<-- offset --><- numSampleFrames sample frames ->
| blockSize | | | |
|<- bytes ->| | | |
|_______________|_______________|_______________|_______________|
block N-1 block N block N+1 block N+2
Figure 7: Block-Aligned Sound Data
In Figure 7, the first sample frame starts at the beginning of block N. This
is accomplished by skipping the first offset bytes of the soundData. Note
too, that the soundData bytes can extend beyond valid sample frames, allowing
the soundData bytes to end on a block boundary as well.
The blockSize specifies the size in bytes of the block to which you would
align the sound data. A blockSize of zero indicates that the sound data does
not need to be block-aligned. Applications that don't care about block
alignment should set the blockSize and offset to zero when creating Audio IFF
files. Applications that write block-aligned sound data should set blockSize
to the appropriate block size. Applications that modify an existing Audio IFF
file should try to preserve alignment of the sound data, although this is not
required. If an application does not preserve alignment, it should set the
blockSize and offset to zero. If an application needs to realign sound data
to a different sized block, it should update blockSize and offset accordingly.
The Marker Chunk
The Marker Chunk contains markers that point to positions in the sound data.
Markers can be used for whatever purposes an application desires. The
Instrument Chunk, defined later in this Note, uses markers to mark loop
beginning and end points.
Markers
A marker has the following format.
typedef short MarkerId;
typedef struct {
MarkerID id;
unsigned long position;
pstring markerName;
} Marker;
The id is a number that uniquely identifies that marker within a FORM AIFF.
The id can be any positive non-zero integer, as long as no other marker
within the same FORM AIFF has the same id.
The marker's position in the sound data is determined by the position field.
Markers conceptually fall between two sample frames. A marker that falls
before the first sample frame in the sound data is at position zero, while a
marker that falls between the first and second sample frame in the sound data
is at position 1. Note that the units for position are sample frames, not
bytes nor sample points.
Sample Frames
___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___
| | | | | | | | | | | | |
|___|___|___|___|___|___|___|___|___|___|___|___|
^ ^ ^
position 0 position 5 position 12
Figure 8: Sample Frame Marker Positions
The markerName field is a Pascal-style text string containing the name of the
mark.
Note: Some "EA IFF 85" files store strings a C-strings (text bytes followed by
a null terminating character) instead of Pascal-style strings. Audio IFF uses
pstrings because they are more efficiently skipped over when scanning through
chunks. Using pstrings, a program can skip over a string by adding the string
count to the address of the first character. C strings require that each
character in the string be examined for the null terminator.
Marker Chunk Format
The format for the data within a Marker Chunk is shown below.
#define MarkerID 'MARK' /* ckID for Marker Chunk */
typedef struct {
ID ckID;
long ckSize;
unsigned short numMarkers;
Marker Markers [];
} MarkerChunk;
The ckID is always 'MARK'. The ckSize is the size of the data portion of the
chunk in bytes. It does not include the 8 bytes used by ckID and ckSize.
The numMarkers field is the number of markers in the Marker Chunk. If
numMarkers is non-zero, it is followed by the markers themselves. Because
all fields in a marker are an even number of bytes, the length of any marker
will always be even. Thus, markers are packed together with no unused bytes
between them. The markers need not be ordered in any particular manner.
The Marker Chunk is optional. No more than one Marker Chunk can appear in a
FORM AIFF.
The Instrument Chunk
The Instrument Chunk defines basic parameters that an instrument, such as a
sample, could use to play the sound data.
Looping
Sound data can be looped, allowing a portion of the sound to be repeated in
order to lengthen the sound. The structure below describes a loop.
typedef struct {
short PlayMode;
MarkerId beginLoop;
MarkerId endLoop;
} Loop;
A loop is marked with two points, a begin position and an end position. There
are two ways to play a loop, forward looping and forward/backward looping.
In the case of forward looping, playback begins at the beginning of the sound,
continues past the begin position and continues to the end position, at which
point playback starts again at the begin position. The segment between the
begin and end positions, called the loop segment, is played repeatedly until
interrupted by a user action, such as the release of a key on a sampling
instrument.
___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___
sample frames | | | |<--- loop segment ---->| | | |
|___|___|___|___|___|___|___|___|___|___|___|___|
^ ^
begin position end position
Figure 9: Sample Frame Looping
With forward/backward looping, the loop segment is first played from the begin
position to the end position, and then played backwards from the end position
to the begin position. This flip-flop pattern is repeated over and over again
until interrupted.
The playMode specifies which type of looping is to be performed:
#define NoLooping 0
#define ForwardLooping 1
#define ForwardBackwardLooping 2
If NoLooping is specified, then the loop points are ignored during playback.
The beginLoop is a marker id that marks the begin position of the loop segment.
The endLoop marks the end position of a loop. The begin position must be
less than the end position. If this is not the case, then the loop segment
has zero or negative length and no looping takes place.
The Instrument Chunk Format
The format of the data within an Instrument Chunk is described below.
#define InstrumentID 'INST' /*ckID for Instruments Chunk */
typedef struct {
ID ckID;
long ckSize;
char baseNote;
char detune;
char lowNote;
char highNote;
char lowvelocity;
char highvelocity;
short gain;
Loop sustainLoop;
Loop releaseLoop;
} InstrumentChunk;
The ckID is always 'INST'. ckSize is the size of the data portion of the
chunk, in bytes. For the Instrument Chunk, ckSize is always 20.
The baseNote is the note at which the instrument plays back the sound data
without pitch modification. Units are MIDI (MIDI is an acronym for Musical
Instrument Digital Interface) note numbers, and are in the range 0 through
127. Middle C is 60.
The detune field determines how much the instrument should alter the pitch of
the sound when it is played back. Units are in cents (1/100 of a semitone)
and range from -50 to +50. Negative numbers mean that the pitch of the sound
should be lowered, while positive numbers mean that it should be raised.
The lowNote and highNote fields specify the suggested range on a keyboard for
playback of the sound data. The sound data should be played if the instrument
is requested to play a note between the low and high, inclusive. The base
note does not have to be within this range. Units for lowNote and highNote
are MIDI note values.
The lowVelocity and highVelocity fields specify the suggested range of
velocities for playback of the sound data. The sound data should be played
if the note-on velocity is between low and high velocity, inclusive. Units
are MIDI velocity values, 1 (lowest velocity) through 127 (highest velocity).
The gain is the amount by which to change the gain of the sound when it is
played. Units are decibels. For example, 0db means no change, 6db means
double the value of each sample point, while -6db means halve the value of
each sample point.
The sustainLoop field specifies a loop that is to be played when an instrument
is sustaining a sound.
The releaseLoop field specifies a loop that is to be played when an instrument
is in the release phase of playing back a sound. The release phase usually
occurs after a key on an instrument is released.
The Instrument Chunk is optional. No more than one Instrument Chunk can
appear in a FORM AIFF.
ASIF Note: The Apple IIGS Sampled Instrument Format also defines a
chunk with ID of "INST," which is not the same as the Audio
IFF Instrument Chunk. A good way to tell the two chunks
apart in generic IFF-style readers is by the ckSize fields.
The Audio IFF Instrument Chunk's ckSize field is always 20,
whereas the Apple IIGS Sampled Instrument Format Instrument
Chunk's ckSize field, for structural reasons, can never be
20.
The MIDI Data Chunk
The MIDI Data Chunk can be used to store MIDI data. Please refer to Musical
Instrument Digital Interface Specification 1.0, available from the
International MIDI Association, for more details on MIDI.
The primary purpose of this chunk is to store MIDI System Exclusive messages,
although other types of MIDI data can be stored in the block as well. As more
instruments come to market, they will likely have parameters that have not
been included in the Audio IFF specification. The MIDI System Exclusive
messages for these instruments may contain many parameters that are not
included in the Instrument Chunk. For example, a new sampling instrument may
have more than the two loops defined in the Instrument Chunk. These loops
will likely be represented in the MIDI System Exclusive message for the new
machine. This MIDI System Exclusive message can be stored in the MIDI Data
Chunk.
#define MIDIDataID 'MIDI' /* ckID for MIDI Data Chunk */
typedef struct {
ID ckID;
long ckSize;
unsigned char MIDIdata[];
} MIDIDataChunk;
The ckID is always 'MIDI'. ckSize of the data portion of the chunk, in bytes.
It does not include the 8 bytes used by ckID and ckSize.
The MIDIData field contains a stream of MIDI data.
The MIDI Data Chunk is optional. Any number of MIDI Data Chunks may exist in
a FORM AIFF. If MIDI System Exclusive messages for several instruments are to
be stored in a FORM AIFF, it is better to use one MIDI Data Chunk per
instrument than one big MIDI Data Chunk for all of the instruments.
The Audio Recording Chunk
The Audio Recording Chunk contains information pertinent to audio recording
devices.
#define AudioRecordingID 'AESD' /* ckID for Audio Recording */
/* Chunk. */
typedef struct {
ID ckID
long ckSize;
unsigned char AESChannelStatusData[24];
} AudioRecordingChunk;
The ckID is always 'AESD'. The ckSize is the size of the data portion of the
chunk, in bytes For the Audio Recording Chunk, ckSize is always 24.
The 24 bytes of AESCChannelStatusData are specified in the "AES Recommended
Practice for Digital Audio Engineering - Serial Transmission Format for Linearly
Represented Digital Audio Data", transmission of digital audio between audio
devices. This information is duplicated in the Audio Recording Chunk for
convenience. Of general interest would be bits 2, 3, and 4 of byte 0, which
describe recording emphasis.
The Audio Recording Chunk is optional. No more than one Audio Recording Chunk
may appear in a FORM AIFF.
The Application Specific Chunk
The Application Specific Chunk can be used for any purposes whatsoever by
developers and application authors. For example, an application that edits
sounds might want to use this chunk to store editor state parameters such as
magnification levels, last cursor position, etc.
#define ApplicationSpecificID 'APPL' /* ckID for Application */
/* Specific Chunk. */
typedef struct {
ID ckID;
long ckSize;
OSType applicationSignature;
char data[];
} ApplicationSpecificChunk;
The ckID is always 'APPL'. The ckSize is the size of the data portion of the
chunk, in bytes. It does not include the 8 bytes used by ckID and ckSize.
The applicationSignature identifies a particular application. For Macintosh
applications, this will be the application's four character signature.
The OSType field is used by applications which run on platforms from Apple
Computer, Inc. For the Apple II, the OStype field should be set to 'pdos'.
For the Macintosh, this field should be set to the four character signature
as registered with Apple Technical Support.
The data field is the data specific to the application.
The Application Specific Chunk is optional. Any number of Application
Specific Chunks may exist in a single FORM AIFF.
The Comments Chunk
The Comments Chunk is used to store comments in the FORM AIFF. "EA IFF 85"
has an Annotation Chunk (used in ASIF) that can be used for comments, but the
Comments Chunk has two features not found in the "EA IFF 85" chunk. They are
a time-stamp for the comment and a link to a marker.
Comment
A comment consists of a time stamp, marker id, and a text count followed by
text.
typedef struct {
unsigned long timeStamp;
MarkerID marker;
unsigned short count;
char text;
} Comment;
The timeStamp indicates when the comment was created. On the Amiga, units
are the number of seconds since January 1, 1978. On the Macintosh, units are
the number of seconds since January 1, 1904.
A comment can be linked to a marker. This allows applications to store long
descriptions of markers as a comment. If the comment is referring to a marker,
then the marker field is the ID of that marker. Otherwise, marker is zero,
indicating that this comment is not linked to a marker.
The count is the length of the text that makes up the comment. This is a 16-bit
quantity, allowing much longer comments than would be available with a pstring.
The text field contains the comment itself.
The Comments Chunk is optional. No more than one Comments Chunk may appear in
a single FORM AIFF.
Comments Chunk Format
#define CommentID 'COMT' /* ckID for Comments Chunk */
typedef struct {
ID ckID;
long ckSize;
unsigned short numComments;
Comment comments[];
}CommentsChunk;
The ckID is always 'COMT'. The ckSize is the size of the data portion of
the chunk, in bytes. It does not include the 8 bytes used by ckID and ckSize.
The numComments field contains the number of comments in the Comments Chunk.
This is followed by the comments themselves. Comments are always even
numbers of bytes in length, so there is no padding between comments in
the Comments Chunk.
The Comments Chunk is optional. No more than one Comments Chunk may appear
in a single FORM AIFF.
The Text Chunks, Name, Author, Copyright, Annotation
These four chunks are included in the definition of every "EA IFF 85" file.
All are text chunks; their data portion consists solely of text. Each of
these chunks is optional.
#define NameID 'NAME' /* ckID for Name Chunk */
#define NameID 'AUTH' /* ckID for Author Chunk */
#define NameID '(c) ' /* ckID for Copyright Chunk */
#define NameID 'ANNO' /* ckID for Annotation Chunk */
typedef struct {
ID ckID;
long ckSize;
char text[];
}TextChunk;
The ckID is either 'NAME', 'AUTH', '(c) ', or 'ANNO' depending on whether the
chunk is a Name Chunk, Author Chunk, Copyright Chunk, or Annotation Chunk,
respectively. For the Copyright Chunk, the 'c' is lowercase and there is a
space (0x20) after the close parenthesis.
The ckSize is the size of the data portion of the chunk, in this case the text.
The text field contains pure ASCII characters. it is not a pstring or a C
string. The number of characters in text is determined by ckSize. The
contents of text depend on the chunk, as described below:
Name Chunk. The text contains the name of the sampled sound. The Name Chunk
is optional. No more than one Name Chunk may exist within a FORM AIFF.
Author Chunk. The text contains one or more author names. An author in this
case is the creator of a sampled sound. The Author Chunk is optional. No
more than one Author Chunk may exist within a FORM AIFF.
Copyright Chunk. The Copyright Chunk contains a copyright notice for the
sound. The text field contains a date followed by the name of the copyright
owner. The chunk ID '(c) ' serves as the copyright character. For example,
a Copyright Chunk containing the text "2012 Hyperion Entertainment" means
"(c) 2012 Hyperion Entertainment" The Copyright Chunk is optional. No more
than one Copyright Chunk may exist within a FORM AIFF.
Annotation Chunk. The text contains a comment. Use of this chunk is
discouraged within a FORM AIFF. The more powerful Comments Chunk should be
used instead. The Annotation Chunk is optional. Many Annotation Chunks may
exist within a FORM AIFF.
Chunk Precedence
Several of the local chunks for FORM AIFF may contain duplicate information.
For example, the Instrument Chunk defines loop points and MIDI System
Exclusive data in the MIDI Data Chunk may also define loop points. What
happens if these loop points are different? How is an application supposed to
loop the sound? Such conflicts are resolved by defining a precedence for
chunks. This precedence is illustrated in Figure 10.
Common Chunk Highest Precedence
|
Sound Data Chunk
|
Marker Chunk
|
Instrument Chunk
|
Comment Chunk
|
Name Chunk
|
Author Chunk
|
Copyright Chunk
|
Annotation Chunk
|
Audio Recording Chunk
|
MIDI Data Chunk
|
Application Specific Chunk Lowest Precedence
Figure 10: Chunk Precedence
The Common Chunk has the highest precedence, while the Application Specific
Chunk has the lowest. Information in the Common Chunk always takes precedence
over conflicting information in any other chunk. The Application Specific
Chunk always loses in conflicts with other chunks. By looking at the chunk
hierarchy, for example, one sees that the loop points in the Instrument Chunk
take precedence over conflicting loop points found in the MIDI Data Chunk.
It is the responsibility of applications that write data into the lower
precedence chunks to make sure that the higher precedence chunks are updated
accordingly.
Figure 11 illustrates an example of a FORM AIFF. An Audio IFF file is simple
a file containing a single FORM AIFF. The FORM AIFF is stored in the data
fork of Macintosh file systems that can handle resource forks.
_____________________________________________________________________
| FORM AIFF |
| _____________ |
| ckID |_ 'FORM' ____| |
| ckSize |_ 176516 ____| |
| _____________ formType |_ 'AIFF' ____| __________________________ |
| | Common ckID |_ 'COMM' ____| | |
| | Chunk ckSize |_ 18 ________| | |
| | numChannels |_ 2 ___|_____ | |
| | numSampleFrames |_ 88200 _____| | |
| | sampleSize |_ 16 __|_______________________________ | |
| |___________ sampleRate |_ 44100.00 ____________________________| | |
| | Marker ckID |_ 'MARK' _____| | |
| | Chunk ckSize |_ 34 _________| | |
| | numMarkers |_ 2 ___| | |
| | id |_ 1 ___|_______ | |
| | position |_ 44100 ___ ___|___ ___ ___ ___ ___ ___ | |
| | markerName | 8 |'b'|'e'|'g'|' '|'l'|'o'|'o'|'p'| 0 | | |
| | id |_ 2 ___|_______ | |
| | position |_ 88200 _______|___ ___ ___ ___ ___ ___ | |
| |___________ markerName | 8 |'e'|'n'|'d'|' '|'l'|'o'|'o'|'p'| 0 | | |
| | Instrument ckID |_ 'INST' ______| | |
| | Chunk ckSize |_ 20 __________| | |
| | baseNote | 60| | |
| | detune | -3| | |
| | lowNote | 57| | |
| | highNote | 63| | |
| | lowVelocity | 1 | | |
| | highVelocity |127|__ | |
| | gain |_ 6 __| | |
| | sustainLoop.playMode |_ 1 __| | |
| | sustainLoop.beginLoop |_ 1 __| | |
| | sustainLoop.endLoop |_ 2 __| | |
| | releaseLoop.playMode |_ 0 __| | |
| | releaseLoop.beginLoop |_ - __| | |
| |__ releaseLoop.endLoop |_ - __|__________________________________| |
| | Sound ckID |_ 'SSND' ______| | |
| | Data ckSize |_ 176408 ______| | |
| | Chunk offset |_ 0 ___________| | |
| | blockSize |_ 0 ___________| _______ _______ | |
| | soundData |_ch 1 _|_ch 2 _| . . . |_ch 1 _|_ch 2 _| | |
| | first sample frame 88200th sample frame | |
| |_________________________________________________________________| |
|_____________________________________________________________________|
Figure 11: Sample FORM AIFF
Further Reference
_____________________________________________________________________________
o "Inside Macintosh", Volume II, Apple Computer, Inc.
o "Apple Numerics Manual", Second Edition, Apple Computer, Inc.
o "File Type Note: File Type $D8, Auxiliary Type $0002, Apple IIGS
Sampled Instrument Format", Apple Computer, Inc.
o "Audio Interchange File Format v1.3", APDA
o "AES Recommended Practice for Digital Audio Engineering--Serial
Transmission Format for Linearly Represented Digital Audio Data",
Audio Engineering Society, 60 East 42nd Street, New York, NY 10165
o "MIDI: Musical Instrument Digital Interface, Specification 1.0", the
International MIDI Association.
o "`EA IFF 85' Standard for Interchange Format Files", Electronic Arts
o "`8SVX' IFF 8-bit Sampled Voice", Electronic Arts
