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Daniele Nicolodi <nicolodi@science.unitn.it> |
| date |
Mon, 05 Dec 2011 16:20:06 +0100 (2011-12-05) |
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<h1 class="title"><a name="f3-12899" id="f3-12899"></a>Downsampling a time-series AO</h1>
<hr>
<p>
Downsampling reduces the sampling rate of the input AOs by an integer factor, which can be very useful for example to reduce data load.
<p>The <tt>downsample</tt> method takes the following parameters:
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<th class="categorylist">Key</th>
<th class="categorylist">Description</th>
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<!-- Key 'factor' -->
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<p><tt>FACTOR</tt></p>
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<p>The decimation factor [by default is 1: no downsampling] (must be an integer)</p>
</td>
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<!-- Key 'offset' -->
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<p><tt>OFFSET</tt></p>
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<p>The sample offset for where the downsampling starts counting. By default, this value is
zero so it starts counting from the first sample.</p>
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</p>
<h2><a name="example1">Example 1</h2>
<p>
First we'll create a time-series of random white noise at 10Hz. To do that,
define a <tt>plist</tt> with the key/value pairs shown below, and pass these
to the <tt>ao</tt> constructor. If you're using the workbench, add an <tt>ao</tt>
constructor block to the canvas and select the parameter set "From Time-series Function" and
set the parameters as they are in the <tt>plist</tt> below.
</p>
<div class="fragment"><pre>
<span class="comment">% create an AO of random data with fs = 10 Hz;</span>
pl = plist(<span class="string">'name'</span>, <span class="string">'None'</span>, ...
<span class="string">'tsfcn'</span>, <span class="string">'randn(size(t))'</span>, <span class="keyword">...</span>
<span class="string">'fs'</span>, 10, <span class="keyword">...</span>
<span class="string">'yunits'</span>, <span class="string">'m'</span>, <span class="keyword">...</span>
<span class="string">'nsecs'</span>, 10);
x = ao(pl); <span class="comment">% create AO</span>
</pre>
</div>
<p>
Now we will downsample this data by a factor 5 to 2Hz. We use the method <tt>ao/downsample</tt> and set
the downsample factor in the <tt>plist</tt> as shown below.
</p>
<div class="fragment"><pre>
pl_down = plist(<span class="string">'factor'</span>, 5); <span class="comment">% add the decimation factor</span>
x_down = downsample(x, pl_down); <span class="comment">% downsample the input AO, x</span>
iplot(x, x_down) <span class="comment">% plot original,x, and decimated,x_down, AOs</span>
</pre>
</div>
<img src="images/ltpda_training_1/topic2/down1.png" alt="Downsample" border="3">
<br>
<br>
<h2><a name="example2">Example 2</h2>
<p>
The <tt>downsample</tt> method takes an 'offset' key which controls where the counting starts. The default is
to output every Nth sample starting with the first. The 'offset' key tells <tt>downsample</tt> to start the counting
from a sample other than the first. The example
below outputs every 4th sample starting from sample 10.
</p>
<div class="fragment"><pre>
pl_downoff = plist(<span class="string">'factor'</span>, 4,<span class="string">'offset'</span>,10); <span class="comment">% add decimation factor and offset parameter</span>
x_downoff = downsample(x, pl_downoff); <span class="comment">% downsample the input AO, x</span>
iplot(x, x_downoff) <span class="comment">% plot original,x, and decimated,x_downoff, AOs</span>
</pre>
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<img src="images/ltpda_training_1/topic2/down2.png" alt="Downsample" border="3">
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