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<a name="Audio-Processing"></a>
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<p>
Next: <a href="Object-Oriented-Programming.html#Object-Oriented-Programming" accesskey="n" rel="next">Object Oriented Programming</a>, Previous: <a href="Image-Processing.html#Image-Processing" accesskey="p" rel="prev">Image Processing</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Audio-Processing-1"></a>
<h2 class="chapter">33 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>
<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 <a href="mailto:guido@cwi.nl">guido@cwi.nl</a> which can be found at any
FAQ ftp site, in particular in the directory
<samp>/pub/usenet/news.answers/audio-fmts</samp> of the archive site
<code>rtfm.mit.edu</code>.
</p>
<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>lin</samp> or <samp>raw</samp>, and that
files holding data in mu-law encoding end in <samp>au</samp>, <samp>mu</samp>, or
<samp>snd</samp>.
</p>
<a name="XREFlin2mu"></a><dl>
<dt><a name="index-lin2mu"></a>Function File: <em></em> <strong>lin2mu</strong> <em>(<var>x</var>, <var>n</var>)</em></dt>
<dd><p>Convert 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.
</p>
<p>If <var>n</var> is not specified it defaults to 0, 8, or 16 depending on
the range of values in <var>x</var>.
</p>
<p><strong>See also:</strong> <a href="#XREFmu2lin">mu2lin</a>, <a href="#XREFloadaudio">loadaudio</a>, <a href="#XREFsaveaudio">saveaudio</a>.
</p></dd></dl>
<a name="XREFmu2lin"></a><dl>
<dt><a name="index-mu2lin"></a>Function File: <em></em> <strong>mu2lin</strong> <em>(<var>x</var>, <var>n</var>)</em></dt>
<dd><p>Convert audio data from mu-law to linear. 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.
</p>
<p>If <var>n</var> is not specified it defaults to 0.
</p>
<p><strong>See also:</strong> <a href="#XREFlin2mu">lin2mu</a>, <a href="#XREFloadaudio">loadaudio</a>, <a href="#XREFsaveaudio">saveaudio</a>.
</p></dd></dl>
<a name="XREFloadaudio"></a><dl>
<dt><a name="index-loadaudio"></a>Function File: <em></em> <strong>loadaudio</strong> <em>(<var>name</var>, <var>ext</var>, <var>bps</var>)</em></dt>
<dd><p>Load audio data from the file <samp><var>name</var>.<var>ext</var></samp> into the
vector <var>x</var>.
</p>
<p>The extension <var>ext</var> determines how the data in the audio file is
interpreted; the extensions <samp>lin</samp> (default) and <samp>raw</samp>
correspond to linear, the extensions <samp>au</samp>, <samp>mu</samp>, or <samp>snd</samp>
to mu-law encoding.
</p>
<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.
</p>
<p><strong>See also:</strong> <a href="#XREFlin2mu">lin2mu</a>, <a href="#XREFmu2lin">mu2lin</a>, <a href="#XREFsaveaudio">saveaudio</a>, <a href="#XREFplayaudio">playaudio</a>, <a href="#XREFsetaudio">setaudio</a>, <a href="#XREFrecord">record</a>.
</p></dd></dl>
<a name="XREFsaveaudio"></a><dl>
<dt><a name="index-saveaudio"></a>Function File: <em></em> <strong>saveaudio</strong> <em>(<var>name</var>, <var>x</var>, <var>ext</var>, <var>bps</var>)</em></dt>
<dd><p>Save a vector <var>x</var> of audio data to the file
<samp><var>name</var>.<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>lin</samp> and
8, respectively.
</p>
<p><strong>See also:</strong> <a href="#XREFlin2mu">lin2mu</a>, <a href="#XREFmu2lin">mu2lin</a>, <a href="#XREFloadaudio">loadaudio</a>, <a href="#XREFplayaudio">playaudio</a>, <a href="#XREFsetaudio">setaudio</a>, <a href="#XREFrecord">record</a>.
</p></dd></dl>
<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>/dev/dsp</samp>, and that similarly <samp>/dev/audio</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.
</p>
<a name="XREFplayaudio"></a><dl>
<dt><a name="index-playaudio"></a>Function File: <em></em> <strong>playaudio</strong> <em>(<var>name</var>, <var>ext</var>)</em></dt>
<dt><a name="index-playaudio-1"></a>Function File: <em></em> <strong>playaudio</strong> <em>(<var>x</var>)</em></dt>
<dd><p>Play the audio file <samp><var>name</var>.<var>ext</var></samp> or the audio data
stored in the vector <var>x</var>.
</p>
<p><strong>See also:</strong> <a href="#XREFlin2mu">lin2mu</a>, <a href="#XREFmu2lin">mu2lin</a>, <a href="#XREFloadaudio">loadaudio</a>, <a href="#XREFsaveaudio">saveaudio</a>, <a href="#XREFsetaudio">setaudio</a>, <a href="#XREFrecord">record</a>.
</p></dd></dl>
<a name="XREFrecord"></a><dl>
<dt><a name="index-record"></a>Function File: <em></em> <strong>record</strong> <em>(<var>sec</var>, <var>sampling_rate</var>)</em></dt>
<dd><p>Record <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 <tt class="key">RET</tt> and then
immediately starts to record.
</p>
<p><strong>See also:</strong> <a href="#XREFlin2mu">lin2mu</a>, <a href="#XREFmu2lin">mu2lin</a>, <a href="#XREFloadaudio">loadaudio</a>, <a href="#XREFsaveaudio">saveaudio</a>, <a href="#XREFplayaudio">playaudio</a>, <a href="#XREFsetaudio">setaudio</a>.
</p></dd></dl>
<a name="XREFsetaudio"></a><dl>
<dt><a name="index-setaudio"></a>Function File: <em></em> <strong>setaudio</strong> <em>()</em></dt>
<dt><a name="index-setaudio-1"></a>Function File: <em></em> <strong>setaudio</strong> <em>(<var>w_type</var>)</em></dt>
<dt><a name="index-setaudio-2"></a>Function File: <em></em> <strong>setaudio</strong> <em>(<var>w_type</var>, <var>value</var>)</em></dt>
<dd><p>Execute the shell command ‘<samp>mixer</samp>’, possibly with optional
arguments <var>w_type</var> and <var>value</var>.
</p></dd></dl>
<a name="XREFwavread"></a><dl>
<dt><a name="index-wavread"></a>Function File: <em><var>y</var> =</em> <strong>wavread</strong> <em>(<var>filename</var>)</em></dt>
<dt><a name="index-wavread-1"></a>Function File: <em>[<var>y</var>, <var>Fs</var>, <var>bps</var>] =</em> <strong>wavread</strong> <em>(<var>filename</var>)</em></dt>
<dt><a name="index-wavread-2"></a>Function File: <em>[…] =</em> <strong>wavread</strong> <em>(<var>filename</var>, <var>n</var>)</em></dt>
<dt><a name="index-wavread-3"></a>Function File: <em>[…] =</em> <strong>wavread</strong> <em>(<var>filename</var>, [<var>n1</var> <var>n2</var>])</em></dt>
<dt><a name="index-wavread-4"></a>Function File: <em>[<var>samples</var>, <var>channels</var>] =</em> <strong>wavread</strong> <em>(<var>filename</var>, "size")</em></dt>
<dd>
<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.
</p>
<p><code>[<var>y</var>, <var>Fs</var>, <var>bps</var>] = wavread (<var>filename</var>)</code>
</p>
<p>Additionally return the sample rate (<var>fs</var>) in Hz and the number of bits
per sample (<var>bps</var>).
</p>
<p><code>[…] = wavread (<var>filename</var>, <var>n</var>)</code>
</p>
<p>Read only the first <var>n</var> samples from each channel.
</p>
<p><code>wavread (<var>filename</var>, [<var>n1</var> <var>n2</var>])</code>
</p>
<p>Read only samples <var>n1</var> through <var>n2</var> from each channel.
</p>
<p><code>[<var>samples</var>, <var>channels</var>] = wavread (<var>filename</var>, "size")</code>
</p>
<p>Return the number of samples (<var>n</var>) and number of channels (<var>ch</var>)
instead of the audio data.
</p>
<p><strong>See also:</strong> <a href="#XREFwavwrite">wavwrite</a>.
</p></dd></dl>
<a name="XREFwavwrite"></a><dl>
<dt><a name="index-wavwrite"></a>Function File: <em></em> <strong>wavwrite</strong> <em>(<var>y</var>, <var>filename</var>)</em></dt>
<dt><a name="index-wavwrite-1"></a>Function File: <em></em> <strong>wavwrite</strong> <em>(<var>y</var>, <var>Fs</var>, <var>filename</var>)</em></dt>
<dt><a name="index-wavwrite-2"></a>Function File: <em></em> <strong>wavwrite</strong> <em>(<var>y</var>, <var>Fs</var>, <var>bps</var>, <var>filename</var>)</em></dt>
<dd><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>bps</var>. The
default sample rate is 8000 Hz with 16-bits per sample. Each column
of the data represents a separate channel. If <var>y</var> is either a
row vector or a column vector, it is written as a single channel.
</p>
<p><strong>See also:</strong> <a href="#XREFwavread">wavread</a>.
</p></dd></dl>
<hr>
<div class="header">
<p>
Next: <a href="Object-Oriented-Programming.html#Object-Oriented-Programming" accesskey="n" rel="next">Object Oriented Programming</a>, Previous: <a href="Image-Processing.html#Image-Processing" accesskey="p" rel="prev">Image Processing</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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