aifc¶

Stuff to parse AIFF-C and AIFF files.

Unless explicitly stated otherwise, the description below is true both for AIFF-C files and AIFF files.

An AIFF-C file has the following structure.

FORM
<size>
	AIFC
	<chunks> .

An AIFF file has the string “AIFF” instead of “AIFC”.

A chunk consists of an identifier (4 bytes) followed by a size (4 bytes, big endian order), followed by the data. The size field does not include the size of the 8 byte header.

The following chunk types are recognized.

FVER

<version number of AIFF-C defining document> (AIFF-C only).

MARK

<# of markers> (2 bytes) list of markers:

<marker ID> (2 bytes, must be > 0) <position> (4 bytes) <marker name> (“pstring”)

COMM

<# of channels> (2 bytes) <# of sound frames> (4 bytes) <size of the samples> (2 bytes) <sampling frequency> (10 bytes, IEEE 80-bit extended

floating point)

in AIFF-C files only: <compression type> (4 bytes) <human-readable version of compression type> (“pstring”)

SSND

<offset> (4 bytes, not used by this program) <blocksize> (4 bytes, not used by this program) <sound data>

A pstring consists of 1 byte length, a string of characters, and 0 or 1 byte pad to make the total length even.

Usage.

Reading AIFF files:

f = aifc.open(file, ‘r’)

where file is either the name of a file or an open file pointer. The open file pointer must have methods read(), seek(), and close(). In some types of audio files, if the setpos() method is not used, the seek() method is not necessary.

This returns an instance of a class with the following public methods:

getnchannels() – returns number of audio channels (1 for

mono, 2 for stereo)

getsampwidth() – returns sample width in bytes getframerate() – returns sampling frequency getnframes() – returns number of audio frames getcomptype() – returns compression type (‘NONE’ for AIFF files) getcompname() – returns human-readable version of

compression type (‘not compressed’ for AIFF files)

getparams() – returns a tuple consisting of all of the

above in the above order

getmarkers() – get the list of marks in the audio file or None

if there are no marks

getmark(id) – get mark with the specified id (raises an error

if the mark does not exist)

readframes(n) – returns at most n frames of audio rewind() – rewind to the beginning of the audio stream setpos(pos) – seek to the specified position tell() – return the current position close() – close the instance (make it unusable)

The position returned by tell(), the position given to setpos() and the position of marks are all compatible and have nothing to do with the actual position in the file. The close() method is called automatically when the class instance is destroyed.

Writing AIFF files:

f = aifc.open(file, ‘w’)

where file is either the name of a file or an open file pointer. The open file pointer must have methods write(), tell(), seek(), and close().

This returns an instance of a class with the following public methods:

aiff() – create an AIFF file (AIFF-C default) aifc() – create an AIFF-C file setnchannels(n) – set the number of channels setsampwidth(n) – set the sample width setframerate(n) – set the frame rate setnframes(n) – set the number of frames setcomptype(type, name)

– set the compression type and the

human-readable compression type

setparams(tuple)

– set all parameters at once

setmark(id, pos, name)

– add specified mark to the list of marks

tell() – return current position in output file (useful

in combination with setmark())

writeframesraw(data)

– write audio frames without pathing up the

file header

writeframes(data)

– write audio frames and patch up the file header

close() – patch up the file header and close the

output file

You should set the parameters before the first writeframesraw or writeframes. The total number of frames does not need to be set, but when it is set to the correct value, the header does not have to be patched up. It is best to first set all parameters, perhaps possibly the compression type, and then write audio frames using writeframesraw. When all frames have been written, either call writeframes(‘’) or close() to patch up the sizes in the header. Marks can be added anytime. If there are any marks, you must call close() after all frames have been written. The close() method is called automatically when the class instance is destroyed.

When a file is opened with the extension ‘.aiff’, an AIFF file is written, otherwise an AIFF-C file is written. This default can be changed by calling aiff() or aifc() before the first writeframes or writeframesraw.