Audio Processing
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Octave provides a few functions for dealing with audio data.  An audio `sample' is a single output value from an A/D converter, i.e., a small integer number (usually 8 or 16 bits), and audio data is just a series of such samples.  It can be characterized by three parameters:  the sampling rate (measured in samples per second or Hz, e.g. 8000 or 44100), the number of bits per sample (e.g. 8 or 16), and the number of channels (1 for mono, 2 for stereo, etc.).

There are many different formats for representing such data.  Currently, only the two most popular, linear encoding and mu-law encoding, are supported by Octave.  There is an excellent FAQ on audio formats by Guido van Rossum <guido@cwi.nl> which can be found at any FAQ ftp site, in particular in the directory /pub/usenet/news.answers/audio-fmts of the archive site rtfm.mit.edu.

Octave simply treats audio data as vectors of samples (non-mono data are not supported yet).  It is assumed that audio files using linear encoding have one of the extensions lin or raw, and that files holding data in mu-law encoding end in au, mu, or snd.

 -- Function File: lin2mu (X, N)
     Converts audio data from linear to mu-law.  Mu-law values use 8-bit unsigned integers.  Linear values use N-bit signed integers or floating point values in the range -1<=X<=1 if N is 0. If N is not specified it defaults to 0, 8 or 16 depending on the range values in X.

mu2lin, loadaudio, saveaudio, playaudio, setaudio, and record

 -- Function File: mu2lin (X, BPS)
     Converts audio data from linear to mu-law.  Mu-law values are 8-bit unsigned integers.  Linear values use N-bit signed integers or floating point values in the range -1<=y<=1 if N is 0.  If N is not specified it defaults to 8.

lin2mu, loadaudio, saveaudio, playaudio, setaudio, and record

 -- Function File: loadaudio (NAME, EXT, BPS)
     Loads audio data from the file NAME.EXT into the vector X.

     The extension EXT determines how the data in the audio file is interpreted;  the extensions lin (default) and raw correspond to linear, the extensions au, mu, or snd to mu-law encoding.

     The argument BPS can be either 8 (default) or 16, and specifies the number of bits per sample used in the audio file.

lin2mu, mu2lin, saveaudio, playaudio, setaudio, and record

 -- Function File: saveaudio (NAME, X, EXT, BPS)
     Saves a vector X of audio data to the file NAME.EXT.  The optional parameters EXT and BPS determine the encoding and the number of bits per sample used in the audio file (see loadaudio);  defaults are lin and 8, respectively.

lin2mu, mu2lin, loadaudio, playaudio, setaudio, and record

The following functions for audio I/O require special A/D hardware and operating system support.  It is assumed that audio data in linear encoding can be played and recorded by reading from and writing to /dev/dsp, and that similarly /dev/audio is used for mu-law encoding.  These file names are system-dependent.  Improvements so that these functions will work without modification on a wide variety of hardware are welcome.

 -- Function File: playaudio (NAME, EXT)
 -- Function File: playaudio (X)
     Plays the audio file NAME.EXT or the audio data stored in the vector X.

lin2mu, mu2lin, loadaudio, saveaudio, setaudio, and record

 -- Function File: record (SEC, SAMPLING_RATE)
     Records SEC seconds of audio input into the vector X.  The default value for SAMPLING_RATE is 8000 samples per second, or 8kHz.  The program waits until the user types RET and then immediately starts to record.

lin2mu, mu2lin, loadaudio, saveaudio, playaudio, and setaudio

 -- Function File: setaudio ([W_TYPE [, VALUE]])
     Execute the shell command mixer [W_TYPE [, VALUE]]