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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 "1991 Commodore-Amiga, Inc." means "(c) 1991 Commodore-Amiga, Inc." 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