Band Pass Filter Valueerror: Digital Filter Critical Frequencies Must Be 0 < Wn < 1

I am trying to use bandpass filter on an ecg signals, this is the code: from scipy.signal import butter def bandpass_filter(self, data, lowcut, highcut, signal_freq, filter_order)

Solution 1:

It may be because of the input of parameter fs. It has to be larger than any of 2 * Wn. As in scipy/signal/filter_design.py source code:

if fs isnotNone:
    if analog:
        raise ValueError("fs cannot be specified for an analog filter")
    Wn = 2*Wn/fs

And later:

if not analog:
    if numpy.any(Wn <= 0) or numpy.any(Wn >= 1):
        raise ValueError("Digital filter critical frequencies must be 0 < Wn < 1")

Solution 2:

scipy.signal.butter(N, Wn, btype='low', analog=False, output='ba')

Wn : array_like

A scalar or length-2 sequence giving the critical frequencies. For a Butterworth filter, this is the point at which the gain drops to 1/sqrt(2) that of the passband (the “-3 dB point”). For digital filters, Wn is normalized from 0 to 1, where 1 is the Nyquist frequency, pi radians/sample. (Wn is thus in half-cycles / sample.) For analog filters, Wn is an angular frequency (e.g. rad/s).

Your exception is here where low/high is not [0,1]

b, a = butter(filter_order, [low, high], btype='band', analog=False)

for filter_type == 1, Wn is float other than nparray.

https://github.com/scipy/scipy/blob/v0.19.1/scipy/signal/filter_design.py#L2226-L2297

>>>N, Wn = signal.buttord([20, 50], [14, 60], 3, 40, True)>>>b, a = signal.butter(N, Wn, 'band', True)