code: def _bandpass(self, fc_low=5, fc_high=20):
          
          self.fc_low = fc_low
          self.fc_high = fc_high
  
          b, a = signal.butter(2, [float(fc_low) * 2 / self.fs,
                                   float(fc_high) * 2 / self.fs], 'pass')
          self.sig_f = signal.filtfilt(b, a, self.sig[self.sampfrom:self.sampto],
                                       axis=0)
          # Save the passband gain (x2 due to double filtering)
          self.filter_gain = get_filter_gain(b, a, np.mean([fc_low, fc_high]),
                                             self.fs) * 2
docs: Apply a bandpass filter onto the signal, and save the filtered
          signal.