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Daniele Nicolodi <nicolodi@science.unitn.it> |
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Wed, 07 Dec 2011 17:29:47 +0100 (2011-12-07) |
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<h1 class="title"><a name="f3-12899" id="f3-12899"></a>Downsampling data</h1>
<hr>
<p>
<h2>Description</h2>
<p>
Downsampling is the process of reducing the sampling rate of a signal. <a href="matlab:doc('ao/downsample')"><tt>Downsample</tt></a>
reduces the sampling rate of the input AOs by an integer factor by picking up one out of N samples.
Note that no anti-aliasing filter is applied to the original data. Moreover, a <tt>offset</tt> can be
specified, i.e., the sample at which the output data starts ---see examples below.
</p>
<h2>Syntax</h2>
<div class="fragment"><pre>
<br>
b = downsample(a, pl)
</pre>
</div>
<h2>Parameters</h2>
<p> The following parameters can be set in this method:
<ul>
<li> <tt>factor</tt> - decimation factor [by default is 1: no downsampling] (must be an integer) </li>
<li> <tt>offset</tt> - sample offset for decimation </li>
</ul>
</p>
<p>
<h2>Examples</h2>
</p>
<p>
1. Downsampling a sequence of random data at original sampling rate of 10 Hz by a factor of 4 (<tt>fsout</tt> = 2.5 Hz) and no <tt>offset</tt>.
</p>
<div class="fragment"><pre>
<br>
<span class="comment">% create an AO of random data with fs = 10 Hz</span>
pl = plist(<span class="string">'tsfcn'</span>, <span class="string">'randn(size(t))'</span>,<span class="string">'fs'</span>,10,<span class="string">'nsecs'</span>,10,<span class="string">'yunits'</span>,<span class="string">'m'</span>);
x = ao(pl)
pl_down = plist(<span class="string">'factor'</span>, 4); <span class="comment">% add the decimation factor</span>
y = downsample(x, pl_down); <span class="comment">% downsample the input AO, x</span>
iplot(x, y)
</pre>
</div>
<img src="images/downsample1.png" alt="Downsample" border="0">
<p>
2. Downsampling a sequence of random data at original sampling rate of 10 Hz by a factor of 4 (<tt>fsout</tt> = 2.5 Hz) and <tt>offset</tt> = 10.
</p>
<div class="fragment"><pre>
<br>
<span class="comment">% create an AO of random data with fs = 10 Hz</span>
pl = plist(<span class="string">'tsfcn'</span>, <span class="string">'randn(size(t))'</span>,<span class="string">'fs'</span>,10,<span class="string">'nsecs'</span>,10,<span class="string">'yunits'</span>,<span class="string">'m'</span>);
x = ao(pl)
pl_downoff = plist(<span class="string">'factor'</span>, 4,<span class="string">'offset'</span>,10); <span class="comment">% add the decimation factor and the offset parameter</span>
y = downsample(x, pl_downoff); <span class="comment">% downsample the input AO, x</span>
iplot(x, y)
</pre>
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<img src="images/downsample2.png" alt="Downsample" border="0">
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