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'.

@anchor{doc-lin2mu}

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.

@anchor{doc-mu2lin}

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.

@anchor{doc-loadaudio}

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.

@anchor{doc-saveaudio}

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.

@anchor{doc-playaudio}

Function File: playaudio (name, ext)

Function File: playaudio (x)

Plays the audio file `name.ext' or the audio data stored in the vector x.

@anchor{doc-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.

@anchor{doc-setaudio}

@deftypefn{Function File}: setaudio ([w_type [, value]])

executes the shell command `mixer [w_type [, value]]'

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