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33 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)

Convert 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 of values in x.

See also: mu2lin, loadaudio, saveaudio.

Function File: mu2lin (x, n)

Convert audio data from mu-law to linear. 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 0.

See also: lin2mu, loadaudio, saveaudio.

Function File: loadaudio (name, ext, bps)

Load 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)

Save 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)

Play 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)

Record 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 ()
Function File: setaudio (w_type)
Function File: setaudio (w_type, value)

Execute the shell command ‘mixer’, possibly with optional arguments w_type and value.

Function File: y = wavread (filename)
Function File: [y, Fs, bps] = wavread (filename)
Function File: […] = wavread (filename, n)
Function File: […] = wavread (filename, [n1 n2])
Function File: [samples, channels] = wavread (filename, "size")

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.

[y, Fs, bps] = wavread (filename)

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

[…] = wavread (filename, n)

Read only the first n samples from each channel.

wavread (filename, [n1 n2])

Read only samples n1 through n2 from each channel.

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

Return the number of samples (n) and number of 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, bps, filename)

Write y to the canonical RIFF/WAVE sound file filename with sample rate Fs and bits per sample bps. The default sample rate is 8000 Hz with 16-bits per sample. Each column of the data represents a separate channel. If y is either a row vector or a column vector, it is written as a single channel.

See also: wavread.


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