title: Envelope of signal in time domain
body: <p>I'm trying to get envelope of a signal.  In an old topic, I found some old code <code>abs(hilbert(signal))</code>. </p>
  
  <p>After filtering the acceleration signal in time domain with 1/3-octave-band and using the code from above my signal look like this:</p>
  
  <p>Red -->Hilbert Blue-->Signal</p>
  
  <p><a href="https://i.stack.imgur.com/JYaKk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/JYaKk.png" alt="result"></a><br>
  <sub>(source: <a href="http://1.ii.gl/gaNtcE9To.png" rel="nofollow noreferrer">1.ii.gl</a>)</sub>  </p>
  
  <p>I was expecting something more like this: </p>
  
  <p><a href="https://i.stack.imgur.com/wRCLZ.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wRCLZ.gif" alt="example"></a><br>
  <sub>(source: <a href="http://www.emt.tugraz.at/publications/diplomarbeiten/da_hoebenreich/img56.gif" rel="nofollow noreferrer">tugraz.at</a>)</sub>  </p>
  
  <p>It seems like my envelope doesn't quite fit. </p>
  
  <p>Here's my code for the 1/3-octave-band:</p>
  
  <pre><code>Fs = 5000;
  N= 4;     
  Fc1= 1425;  
  Fc2= 1795;  
  h= fdesign.bandpass('N,F3dB1,F3dB2', N, Fc1, Fc2, Fs);
  Hd= design(h, 'butter');
  </code></pre>
  
  <p>What am I doing wrong?</p>