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author Daniele Nicolodi <nicolodi@science.unitn.it>
date Mon, 05 Dec 2011 18:04:03 +0100
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  <title>Downsampling a time-series AO (LTPDA Toolbox)</title>
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      <td valign="baseline"><b>LTPDA Toolbox</b></td><td><a href="../helptoc.html">contents</a></td>

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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:
  <table cellspacing="0" class="body" cellpadding="2" border="0" width="80%">
    <colgroup>
      <col width="25%"/>
      <col width="75%"/>
    </colgroup>
    <thead>
      <tr valign="top">
        <th class="categorylist">Key</th>
        <th class="categorylist">Description</th>
      </tr>
    </thead>
    <tbody>
      <!-- Key 'factor' -->
      <tr valign="top">
        <td bgcolor="#f3f4f5">
          <p><tt>FACTOR</tt></p>
        </td>
        <td bgcolor="#f3f4f5">
          <p>The decimation factor [by default is 1: no downsampling] (must be an integer)</p>
        </td>
      </tr>
      <!-- Key 'offset' -->
      <tr valign="top">
        <td bgcolor="#f3f4f5">
          <p><tt>OFFSET</tt></p>
        </td>
        <td bgcolor="#f3f4f5">
          <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>
        </td>
      </tr>
    </tbody>
  </table>
</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>
</div>
<img src="images/ltpda_training_1/topic2/down2.png" alt="Downsample" border="3">



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      <td align="left" width="20"><a href="ltpda_training_topic_2.html"><img src=
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      <td align="right">Upsampling a time-series AO</td>

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