21.3. "aifc" — Read and write AIFF and AIFC files
*************************************************

**Source code:** Lib/aifc.py

======================================================================

This module provides support for reading and writing AIFF and AIFF-C
files. AIFF is Audio Interchange File Format, a format for storing
digital audio samples in a file.  AIFF-C is a newer version of the
format that includes the ability to compress the audio data.

Note: Some operations may only work under IRIX; these will raise
  "ImportError" when attempting to import the "cl" module, which is
  only available on IRIX.

Audio files have a number of parameters that describe the audio data.
The sampling rate or frame rate is the number of times per second the
sound is sampled.  The number of channels indicate if the audio is
mono, stereo, or quadro.  Each frame consists of one sample per
channel.  The sample size is the size in bytes of each sample.  Thus a
frame consists of *nchannels***samplesize* bytes, and a second’s worth
of audio consists of *nchannels***samplesize***framerate* bytes.

For example, CD quality audio has a sample size of two bytes (16
bits), uses two channels (stereo) and has a frame rate of 44,100
frames/second.  This gives a frame size of 4 bytes (2*2), and a
second’s worth occupies 2*2*44100 bytes (176,400 bytes).

Module "aifc" defines the following function:

aifc.open(file[, mode])

   Open an AIFF or AIFF-C file and return an object instance with
   methods that are described below.  The argument *file* is either a
   string naming a file or a file object.  *mode* must be "'r'" or
   "'rb'" when the file must be opened for reading, or "'w'"  or
   "'wb'" when the file must be opened for writing.  If omitted,
   "file.mode" is used if it exists, otherwise "'rb'" is used.  When
   used for writing, the file object should be seekable, unless you
   know ahead of time how many samples you are going to write in total
   and use "writeframesraw()" and "setnframes()".

Objects returned by "open()" when a file is opened for reading have
the following methods:

aifc.getnchannels()

   Return the number of audio channels (1 for mono, 2 for stereo).

aifc.getsampwidth()

   Return the size in bytes of individual samples.

aifc.getframerate()

   Return the sampling rate (number of audio frames per second).

aifc.getnframes()

   Return the number of audio frames in the file.

aifc.getcomptype()

   Return a four-character string describing the type of compression
   used in the audio file.  For AIFF files, the returned value is
   "'NONE'".

aifc.getcompname()

   Return a human-readable description of the type of compression used
   in the audio file.  For AIFF files, the returned value is "'not
   compressed'".

aifc.getparams()

   Return a tuple consisting of all of the above values in the above
   order.

aifc.getmarkers()

   Return a list of markers in the audio file.  A marker consists of a
   tuple of three elements.  The first is the mark ID (an integer),
   the second is the mark position in frames from the beginning of the
   data (an integer), the third is the name of the mark (a string).

aifc.getmark(id)

   Return the tuple as described in "getmarkers()" for the mark with
   the given *id*.

aifc.readframes(nframes)

   Read and return the next *nframes* frames from the audio file.  The
   returned data is a string containing for each frame the
   uncompressed samples of all channels.

aifc.rewind()

   Rewind the read pointer.  The next "readframes()" will start from
   the beginning.

aifc.setpos(pos)

   Seek to the specified frame number.

aifc.tell()

   Return the current frame number.

aifc.close()

   Close the AIFF file.  After calling this method, the object can no
   longer be used.

Objects returned by "open()" when a file is opened for writing have
all the above methods, except for "readframes()" and "setpos()".  In
addition the following methods exist.  The "get*()" methods can only
be called after the corresponding "set*()" methods have been called.
Before the first "writeframes()" or "writeframesraw()", all parameters
except for the number of frames must be filled in.

aifc.aiff()

   Create an AIFF file.  The default is that an AIFF-C file is
   created, unless the name of the file ends in "'.aiff'" in which
   case the default is an AIFF file.

aifc.aifc()

   Create an AIFF-C file.  The default is that an AIFF-C file is
   created, unless the name of the file ends in "'.aiff'" in which
   case the default is an AIFF file.

aifc.setnchannels(nchannels)

   Specify the number of channels in the audio file.

aifc.setsampwidth(width)

   Specify the size in bytes of audio samples.

aifc.setframerate(rate)

   Specify the sampling frequency in frames per second.

aifc.setnframes(nframes)

   Specify the number of frames that are to be written to the audio
   file. If this parameter is not set, or not set correctly, the file
   needs to support seeking.

aifc.setcomptype(type, name)

   Specify the compression type.  If not specified, the audio data
   will not be compressed.  In AIFF files, compression is not
   possible.  The name parameter should be a human-readable
   description of the compression type, the type parameter should be a
   four-character string.  Currently the following compression types
   are supported: NONE, ULAW, ALAW, G722.

aifc.setparams(nchannels, sampwidth, framerate, comptype, compname)

   Set all the above parameters at once.  The argument is a tuple
   consisting of the various parameters.  This means that it is
   possible to use the result of a "getparams()" call as argument to
   "setparams()".

aifc.setmark(id, pos, name)

   Add a mark with the given id (larger than 0), and the given name at
   the given position.  This method can be called at any time before
   "close()".

aifc.tell()

   Return the current write position in the output file.  Useful in
   combination with "setmark()".

aifc.writeframes(data)

   Write data to the output file.  This method can only be called
   after the audio file parameters have been set.

aifc.writeframesraw(data)

   Like "writeframes()", except that the header of the audio file is
   not updated.

aifc.close()

   Close the AIFF file.  The header of the file is updated to reflect
   the actual size of the audio data. After calling this method, the
   object can no longer be used.
