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<a name="Audio-Data-Processing"></a>
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Previous: <a href="Audio-Recorder.html#Audio-Recorder" accesskey="p" rel="prev">Audio Recorder</a>, Up: <a href="Audio-Processing.html#Audio-Processing" accesskey="u" rel="up">Audio Processing</a> &nbsp; [<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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<a name="Audio-Data-Processing-1"></a>
<h3 class="section">33.5 Audio Data Processing</h3>

<p>Octave provides a few functions for dealing with audio data.  An audio
&lsquo;sample&rsquo; 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><em><var>y</var> =</em> <strong>lin2mu</strong> <em>(<var>x</var>, <var>n</var>)</em></dt>
<dd><p>Convert audio data from linear to mu-law.
</p>
<p>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 &le; <var>x</var>
&le; 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>.
</p></dd></dl>


<a name="XREFmu2lin"></a><dl>
<dt><a name="index-mu2lin"></a><em><var>y</var> =</em> <strong>mu2lin</strong> <em>(<var>x</var>, <var>n</var>)</em></dt>
<dd><p>Convert audio data from mu-law to linear.
</p>
<p>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 &le; <var>y</var>
&le; 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>.
</p></dd></dl>


<a name="XREFrecord"></a><dl>
<dt><a name="index-record-2"></a><em></em> <strong>record</strong> <em>(<var>sec</var>)</em></dt>
<dt><a name="index-record-3"></a><em></em> <strong>record</strong> <em>(<var>sec</var>, <var>fs</var>)</em></dt>
<dd><p>Record <var>sec</var> seconds of audio from the system&rsquo;s default audio input at
a sampling rate of 8000 samples per second.
</p>
<p>If the optional argument <var>fs</var> is given, it specifies the sampling rate
for recording.
</p>
<p>For more control over audio recording, use the <code>audiorecorder</code> class.
</p>
<p><strong>See also:</strong> <a href="#XREFsound">sound</a>, <a href="#XREFsoundsc">soundsc</a>.
</p></dd></dl>


<a name="XREFsound"></a><dl>
<dt><a name="index-sound"></a><em></em> <strong>sound</strong> <em>(<var>y</var>)</em></dt>
<dt><a name="index-sound-1"></a><em></em> <strong>sound</strong> <em>(<var>y</var>, <var>fs</var>)</em></dt>
<dt><a name="index-sound-2"></a><em></em> <strong>sound</strong> <em>(<var>y</var>, <var>fs</var>, <var>nbits</var>)</em></dt>
<dd><p>Play audio data <var>y</var> at sample rate <var>fs</var> to the default audio
device.
</p>
<p>The audio signal <var>y</var> can be a vector or a two-column array, representing
mono or stereo audio, respectively.
</p>
<p>If <var>fs</var> is not given, a default sample rate of 8000 samples per second
is used.
</p>
<p>The optional argument <var>nbits</var> specifies the bit depth to play to the
audio device and defaults to 8 bits.
</p>
<p>For more control over audio playback, use the <code>audioplayer</code> class.
</p>
<p><strong>See also:</strong> <a href="#XREFsoundsc">soundsc</a>, <a href="#XREFrecord">record</a>.
</p></dd></dl>


<a name="XREFsoundsc"></a><dl>
<dt><a name="index-soundsc"></a><em></em> <strong>soundsc</strong> <em>(<var>y</var>)</em></dt>
<dt><a name="index-soundsc-1"></a><em></em> <strong>soundsc</strong> <em>(<var>y</var>, <var>fs</var>)</em></dt>
<dt><a name="index-soundsc-2"></a><em></em> <strong>soundsc</strong> <em>(<var>y</var>, <var>fs</var>, <var>nbits</var>)</em></dt>
<dt><a name="index-soundsc-3"></a><em></em> <strong>soundsc</strong> <em>(&hellip;, [<var>ymin</var>, <var>ymax</var>])</em></dt>
<dd><p>Scale the audio data <var>y</var> and play it at sample rate <var>fs</var> to the
default audio device.
</p>
<p>The audio signal <var>y</var> can be a vector or a two-column array, representing
mono or stereo audio, respectively.
</p>
<p>If <var>fs</var> is not given, a default sample rate of 8000 samples per second
is used.
</p>
<p>The optional argument <var>nbits</var> specifies the bit depth to play to the
audio device and defaults to 8 bits.
</p>
<p>By default, <var>y</var> is automatically normalized to the range [-1, 1].  If
the range [<var>ymin</var>, <var>ymax</var>] is given, then elements of <var>y</var>
that fall within the range <var>ymin</var> &le; <var>y</var> &le; <var>ymax</var>
are scaled to the range [-1, 1] instead.
</p>
<p>For more control over audio playback, use the <code>audioplayer</code> class.
</p>
<p><strong>See also:</strong> <a href="#XREFsound">sound</a>, <a href="#XREFrecord">record</a>.
</p></dd></dl>




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