title: Obspy spectrogram ValueError (noverlap must be less than n)
body: <p>I working with seismic data using obspy and I want to get a spectrogram for my data. When I apply the spectrogram function to the initial data everything works fine.</p>
  <pre><code>st.spectrogram(log=True, title='BW.RJOB ' + str(st[0].stats.starttime))
  </code></pre>
  <p>However when I slice and interpolate the data</p>
  <pre><code>tr = st[0]
  sr=4.9999
  tr2 = tr.slice(point - 20, point + 180)
  
  tr2.interpolate(sampling_rate=sr)  
  </code></pre>
  <p>*point is some arbitrary point in my data</p>
  <p>*initial sampling rate is 124.99</p>
  <p>and then perform the spectrogram</p>
  <pre><code>tr2.spectrogram(log=True, title='BW.RJOB ' + str(tr2.stats.starttime))
  </code></pre>
  <p>I get the following error:</p>
  <p>*** ValueError: noverlap must be less than n</p>
  <p>Why is this happening?</p>