title: butterworth-bandpass-filter with scipy
body: <p>I have a problem with my butterworth bandpass filter. I have an 1D array (eeg signal) recorded with 250Hz. The cutoff frequencies are 6 and 11 Hz.
  What I have now is this, which doesn´t work:</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  from pylab import *
  import scipy.io
  import scipy.signal
  import scipy.fftpack
  from scipy.signal import butter, lfilter
  def butter_bandpass(lowcut, highcut, fs, order=6):
      nyq = 0.5 * fs
      low = lowcut / nyq
      high = highcut / nyq
      b, a = butter(order, [low, high], btype='band')
      return b, a
  
  
  def butter_bandpass_filter(data, lowcut, highcut, fs, order=6):
      b, a = butter_bandpass(lowcut, highcut, fs, order=order)
      y = lfilter(b, a, data)
      return y
  
  
  if __name__ == "__main__":
      fs = 250.0
      lowcut = 6.0
      highcut = 11.0
      t = range(len(eeg))
      x = eeg[t]
      y = butter_bandpass_filter(x, lowcut, highcut, fs, order=6)
      plt.plot(t, y)
      plt.show() 
  </code></pre>
  
  <p>What´s wrong? </p>
  
  <p>Thanks
  Mike</p>