32. 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, 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.

See also: mu2lin, loadaudio, saveaudio, playaudio, setaudio, 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.

See also: lin2mu, loadaudio, saveaudio, playaudio, setaudio, 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.

See also: lin2mu, mu2lin, saveaudio, playaudio, setaudio, 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.

See also: lin2mu, mu2lin, loadaudio, playaudio, setaudio, 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.

See also: lin2mu, mu2lin, loadaudio, saveaudio, setaudio, 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.

See also: lin2mu, mu2lin, loadaudio, saveaudio, playaudio, setaudio.

Function File: setaudio ([w_type [, value]]): Execute the shell command ‘mixer [w_type [, value]]’

Function File: y = wavread (filename)

Load the RIFF/WAVE sound file filename, and return the samples in vector y. If the file contains multichannel data, then y is a matrix with the channels represented as columns.

Function File: [y, Fs, bits] = wavread (filename)

Additionally return the sample rate (fs) in Hz and the number of bits per sample (bits).

Function File: […] = wavread (filename, n)

Read only the first n samples from each channel.

Function File: […] = wavread (filename,[n1 n2])

Read only samples n1 through n2 from each channel.

Function File: [samples, channels] = wavread (filename, "size")

Return the number of samples (n) and channels (ch) instead of the audio data.

See also: wavwrite.

Function File: wavwrite (y, filename)

Function File: wavwrite (y, fs, filename)

Function File: wavwrite (y, fs, bits, filename)

Write y to the canonical RIFF/WAVE sound file filename with sample rate fs and bits per sample bits. The default sample rate is 8000 Hz with 16-bits per sample. Each column of the data represents a separate channel.

See also: wavread.

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