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31 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. 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’.

31.1 Audio Conversion Functions

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 \leq x \leq 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 \leq y \leq 1 if n is 0. If n is not specified it defaults to 8.

See also lin2mu, loadaudio, saveaudio, playaudio, setaudio, record

31.2 Loading and Saving Audio Files

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