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# Audio Processing

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)
  If the vector x represents mono audio data in 8- or 16-bit linear encoding, `lin2mu (x)` is the corresponding mu-law encoding.

Function File: mu2lin (x, bps)
  If the vector x represents mono audio data in mu-law encoding, `mu2lin` converts it to linear encoding. The optional argument bps specifies whether the input data uses 8 bit per sample (default) or 16 bit.

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.

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.

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.

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.

Function File: setaudio (type)
Function File: setaudio (type, value)
  Set or display various properties of your mixer hardware.

  For example, if `vol` corresponds to the volume property, you can set it to 50 (percent) by `setaudio ("vol", 50)`.

  This is an simple experimental program to control the audio hardware settings. It assumes that there is a `mixer` program which can be used as `mixer type value`, and simply executes `system ("mixer type value")`. Future releases might get rid of this assumption by using the `fcntl` interface.

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