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<title>Audio Processing - Untitled</title>
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<h2 class="chapter">32 Audio Processing</h2>
<p>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.).
<p>There are many different formats for representing such data. Currently,
only the two most popular, <em>linear encoding</em> and <em>mu-law
encoding</em>, 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
<samp><span class="file">/pub/usenet/news.answers/audio-fmts</span></samp> of the archive site
<code>rtfm.mit.edu</code>.
<p>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 <samp><span class="file">lin</span></samp> or <samp><span class="file">raw</span></samp>, and that
files holding data in mu-law encoding end in <samp><span class="file">au</span></samp>, <samp><span class="file">mu</span></samp>, or
<samp><span class="file">snd</span></samp>.
<!-- ./audio/lin2mu.m -->
<p><a name="doc_002dlin2mu"></a>
<div class="defun">
— Function File: <b>lin2mu</b> (<var>x, n</var>)<var><a name="index-lin2mu-2238"></a></var><br>
<blockquote><p>Converts audio data from linear to mu-law. Mu-law values use 8-bit
unsigned integers. Linear values use <var>n</var>-bit signed integers or
floating point values in the range -1<=<var>x</var><=1 if <var>n</var> is 0.
If <var>n</var> is not specified it defaults to 0, 8 or 16 depending on
the range values in <var>x</var>.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dmu2lin.html#doc_002dmu2lin">mu2lin</a>, <a href="doc_002dloadaudio.html#doc_002dloadaudio">loadaudio</a>, <a href="doc_002dsaveaudio.html#doc_002dsaveaudio">saveaudio</a>, <a href="doc_002dplayaudio.html#doc_002dplayaudio">playaudio</a>, <a href="doc_002dsetaudio.html#doc_002dsetaudio">setaudio</a>, <a href="doc_002drecord.html#doc_002drecord">record</a>.
</p></blockquote></div>
<!-- ./audio/mu2lin.m -->
<p><a name="doc_002dmu2lin"></a>
<div class="defun">
— Function File: <b>mu2lin</b> (<var>x, bps</var>)<var><a name="index-mu2lin-2239"></a></var><br>
<blockquote><p>Converts audio data from linear to mu-law. Mu-law values are 8-bit
unsigned integers. Linear values use <var>n</var>-bit signed integers
or floating point values in the range -1<=y<=1 if <var>n</var> is 0. If
<var>n</var> is not specified it defaults to 8.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dlin2mu.html#doc_002dlin2mu">lin2mu</a>, <a href="doc_002dloadaudio.html#doc_002dloadaudio">loadaudio</a>, <a href="doc_002dsaveaudio.html#doc_002dsaveaudio">saveaudio</a>, <a href="doc_002dplayaudio.html#doc_002dplayaudio">playaudio</a>, <a href="doc_002dsetaudio.html#doc_002dsetaudio">setaudio</a>, <a href="doc_002drecord.html#doc_002drecord">record</a>.
</p></blockquote></div>
<!-- ./audio/loadaudio.m -->
<p><a name="doc_002dloadaudio"></a>
<div class="defun">
— Function File: <b>loadaudio</b> (<var>name, ext, bps</var>)<var><a name="index-loadaudio-2240"></a></var><br>
<blockquote><p>Loads audio data from the file <samp><var>name</var><span class="file">.</span><var>ext</var></samp> into the
vector <var>x</var>.
<p>The extension <var>ext</var> determines how the data in the audio file is
interpreted; the extensions <samp><span class="file">lin</span></samp> (default) and <samp><span class="file">raw</span></samp>
correspond to linear, the extensions <samp><span class="file">au</span></samp>, <samp><span class="file">mu</span></samp>, or <samp><span class="file">snd</span></samp>
to mu-law encoding.
<p>The argument <var>bps</var> can be either 8 (default) or 16, and specifies
the number of bits per sample used in the audio file.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dlin2mu.html#doc_002dlin2mu">lin2mu</a>, <a href="doc_002dmu2lin.html#doc_002dmu2lin">mu2lin</a>, <a href="doc_002dsaveaudio.html#doc_002dsaveaudio">saveaudio</a>, <a href="doc_002dplayaudio.html#doc_002dplayaudio">playaudio</a>, <a href="doc_002dsetaudio.html#doc_002dsetaudio">setaudio</a>, <a href="doc_002drecord.html#doc_002drecord">record</a>.
</p></blockquote></div>
<!-- ./audio/saveaudio.m -->
<p><a name="doc_002dsaveaudio"></a>
<div class="defun">
— Function File: <b>saveaudio</b> (<var>name, x, ext, bps</var>)<var><a name="index-saveaudio-2241"></a></var><br>
<blockquote><p>Saves a vector <var>x</var> of audio data to the file
<samp><var>name</var><span class="file">.</span><var>ext</var></samp>. The optional parameters <var>ext</var> and
<var>bps</var> determine the encoding and the number of bits per sample used
in the audio file (see <code>loadaudio</code>); defaults are <samp><span class="file">lin</span></samp> and
8, respectively.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dlin2mu.html#doc_002dlin2mu">lin2mu</a>, <a href="doc_002dmu2lin.html#doc_002dmu2lin">mu2lin</a>, <a href="doc_002dloadaudio.html#doc_002dloadaudio">loadaudio</a>, <a href="doc_002dplayaudio.html#doc_002dplayaudio">playaudio</a>, <a href="doc_002dsetaudio.html#doc_002dsetaudio">setaudio</a>, <a href="doc_002drecord.html#doc_002drecord">record</a>.
</p></blockquote></div>
<p>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
<samp><span class="file">/dev/dsp</span></samp>, and that similarly <samp><span class="file">/dev/audio</span></samp> 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.
<!-- ./audio/playaudio.m -->
<p><a name="doc_002dplayaudio"></a>
<div class="defun">
— Function File: <b>playaudio</b> (<var>name, ext</var>)<var><a name="index-playaudio-2242"></a></var><br>
— Function File: <b>playaudio</b> (<var>x</var>)<var><a name="index-playaudio-2243"></a></var><br>
<blockquote><p>Plays the audio file <samp><var>name</var><span class="file">.</span><var>ext</var></samp> or the audio data
stored in the vector <var>x</var>.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dlin2mu.html#doc_002dlin2mu">lin2mu</a>, <a href="doc_002dmu2lin.html#doc_002dmu2lin">mu2lin</a>, <a href="doc_002dloadaudio.html#doc_002dloadaudio">loadaudio</a>, <a href="doc_002dsaveaudio.html#doc_002dsaveaudio">saveaudio</a>, <a href="doc_002dsetaudio.html#doc_002dsetaudio">setaudio</a>, <a href="doc_002drecord.html#doc_002drecord">record</a>.
</p></blockquote></div>
<!-- ./audio/record.m -->
<p><a name="doc_002drecord"></a>
<div class="defun">
— Function File: <b>record</b> (<var>sec, sampling_rate</var>)<var><a name="index-record-2244"></a></var><br>
<blockquote><p>Records <var>sec</var> seconds of audio input into the vector <var>x</var>. The
default value for <var>sampling_rate</var> is 8000 samples per second, or
8kHz. The program waits until the user types <RET> and then
immediately starts to record.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dlin2mu.html#doc_002dlin2mu">lin2mu</a>, <a href="doc_002dmu2lin.html#doc_002dmu2lin">mu2lin</a>, <a href="doc_002dloadaudio.html#doc_002dloadaudio">loadaudio</a>, <a href="doc_002dsaveaudio.html#doc_002dsaveaudio">saveaudio</a>, <a href="doc_002dplayaudio.html#doc_002dplayaudio">playaudio</a>, <a href="doc_002dsetaudio.html#doc_002dsetaudio">setaudio</a>.
</p></blockquote></div>
<!-- ./audio/setaudio.m -->
<p><a name="doc_002dsetaudio"></a>
<div class="defun">
— Function File: <b>setaudio</b> ([<var>w_type </var>[<var>, value</var>]])<var><a name="index-setaudio-2245"></a></var><br>
<blockquote><p>Execute the shell command ‘<samp><span class="samp">mixer [</span><var>w_type</var><span class="samp"> [, </span><var>value</var><span class="samp">]]</span></samp>’
</p></blockquote></div>
<!-- ./audio/wavread.m -->
<p><a name="doc_002dwavread"></a>
<div class="defun">
— Function File: <var>y</var> = <b>wavread</b> (<var>filename</var>)<var><a name="index-wavread-2246"></a></var><br>
<blockquote><p>Load the RIFF/WAVE sound file <var>filename</var>, and return the samples
in vector <var>y</var>. If the file contains multichannel data, then
<var>y</var> is a matrix with the channels represented as columns.
— Function File: [<var>y</var>, <var>Fs</var>, <var>bits</var>] = <b>wavread</b> (<var>filename</var>)<var><a name="index-wavread-2247"></a></var><br>
<blockquote><p>Additionally return the sample rate (<var>fs</var>) in Hz and the number of bits
per sample (<var>bits</var>).
— Function File: [<small class="dots">...</small>] = <b>wavread</b> (<var>filename, n</var>)<var><a name="index-wavread-2248"></a></var><br>
<blockquote><p>Read only the first <var>n</var> samples from each channel.
— Function File: [<small class="dots">...</small>] = <b>wavread</b> (<var>filename,</var>[<var>n1 n2</var>])<var><a name="index-wavread-2249"></a></var><br>
<blockquote><p>Read only samples <var>n1</var> through <var>n2</var> from each channel.
— Function File: [<var>samples</var>, <var>channels</var>] = <b>wavread</b> (<var>filename, "size"</var>)<var><a name="index-wavread-2250"></a></var><br>
<blockquote><p>Return the number of samples (<var>n</var>) and channels (<var>ch</var>)
instead of the audio data.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dwavwrite.html#doc_002dwavwrite">wavwrite</a>.
</p></blockquote></div>
<!-- ./audio/wavwrite.m -->
<p><a name="doc_002dwavwrite"></a>
<div class="defun">
— Function File: <b>wavwrite</b> (<var>y, filename</var>)<var><a name="index-wavwrite-2251"></a></var><br>
— Function File: <b>wavwrite</b> (<var>y, fs, filename</var>)<var><a name="index-wavwrite-2252"></a></var><br>
— Function File: <b>wavwrite</b> (<var>y, fs, bits, filename</var>)<var><a name="index-wavwrite-2253"></a></var><br>
<blockquote><p>Write <var>y</var> to the canonical RIFF/WAVE sound file <var>filename</var>
with sample rate <var>fs</var> and bits per sample <var>bits</var>. The
default sample rate is 8000 Hz with 16-bits per sample. Each column
of the data represents a separate channel.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dwavread.html#doc_002dwavread">wavread</a>.
</p></blockquote></div>
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