[SciPy-dev] problem with signal.butter

[John Hunter]

I am trying to design a bandpass butterworth filter and am getting an
error

    import scipy.signal as sig
    Fs = 400.
    Nyq = Fs/2.
    # low pass stop freq, high pass corner freq, etc...
    lpsf = 5.
    lpcf = 7.
    hpcf = 12.
    hpsf = 18.
    Nyq = Fs/2.
    wp = [lpcf/Nyq, hpcf/Nyq]
    ws = [lpsf/Nyq, hpsf/Nyq]
    gpass = 3.
    gstop = 15.
    ord, Wn = sig.buttord(wp, ws, gpass, gstop)
    mybutt = sig.butter(ord, Wn, btype='bandpass') # pun intended


With scipy version 0.3.2 I get the following the traceback

Traceback (most recent call last):
  File "<console>", line 1, in ?
  File "/usr/lib/python2.4/site-packages/scipy/signal/filter_design.py", line 482, in butter
    return iirfilter(N, Wn, btype=btype, analog=analog, output=output, ftype='butter')
  File "/usr/lib/python2.4/site-packages/scipy/signal/filter_design.py", line 456, in iirfilter
    b, a = lp2bp(b,a,wo=wo,bw=bw)
  File "/usr/lib/python2.4/site-packages/scipy/signal/filter_design.py", line 253, in lp2bp
    aprime[Dp-j] = val
TypeError: can't convert complex to float; use abs(z)


Any ideas?

Thanks,
JDH