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pythonscipy
Shruthi kr
Shruthi kr

Reputation: 3

Error using scipy.signal.lfilter() in Python3.7.0 : <built-in function _linear_filter> returned NULL without setting an error

I have a list of 256 data elements. I want to filter this data using elliptical filter.

import matplotlib.pyplot as plt
from scipy.signal import *
import numpy as np

def elliptical_bandpass():
    Fs=256
    lowcut=5
    highcut=30
    order=5
    Rp = 0.5;      # Passband Ripple (dB)
    Rs = 30;     #  Stopband Ripple (dB)
    nyq = Fs/2 #Nyquist frequency
    wp = lowcut / nyq
    ws = highcut / nyq

    c3=['221', '262', '333', '429', '522', '592', '630', '656', '668', '645', '581', '486', '395', '324', '265', '214', '172', '171', '214', '282', '353', '420', '498', '584', '650', '679', '661', '622', '571', '503', '415', '316', '240', '200', '185', '188', '204', '256', '344', '443', '527', '582', '627', '665', '676', '644', '567', '481', '404', '337', '271', '204', '168', '175', '218', '277', '340', '419', '513', '599', '653', '662', '649', '622', '578', '506', '407', '317', '252', '213', '188', '173', '194', '258', '352', '445', '517', '578', '632', '671', '672', '626', '561', '491', '422', '341', '254', '188', '165', '184', '224', '271', '337', '424', '522', '598', '638', '652', '653', '637', '585', '497', '397', '314', '258', '215', '180', '172', '202', '272', '352', '427', '502', '579', '649', '680', '664', '615', '555', '498', '424', '335', '251', '195', '180', '187', '212', '258', '338', '442', '533', '594', '628', '649', '661', '640', '579', '490', '402', '332', '266', '206', '164', '166', '216', '285', '357', '425', '501', '584', '644', '669', '655', '624', '580', '509', '414', '311', '236', '202', '190', '191', '207', '258', '345', '441', '521', '577', '626', '667', '676', '643', '567', '483', '407', '334', '261', '194', '162', '176', '222', '280', '342', '422', '517', '603', '654', '662', '650', '626', '579', '505', '404', '315', '252', '213', '187', '173', '196', '262', '352', '442', '513', '580', '642', '679', '674', '622', '553', '483', '413', '336', '254', '196', '177', '191', '221', '260', '328', '422', '524', '603', '640', '655', '656', '637', '583', '492', '397', '319', '263', '217', '176', '168', '204', '278', '361', '436', '509', '583', '645', '672', '656', '616', '565', '507', '425', '325', '238', '188', '179', '190', '213', '260', '338', '440']

    n, Wn = ellipord(wp, ws, Rp,Rs)
    print('Wn IS ----', Wn)
    b,a=ellip(order,Rp,Rs,[wp, ws], btype='band') #get filter coefficients
    print('b coeff from filter code -- ',b)
    print('a coeff from filter code -- ',a)
    c3_filtered=lfilter(b,a,c3)
    print('filtered data-',c3_filtered)
    print('len of filtered data', len(c3_filtered))
    w, h = freqz(b, a, worN=2000) #used to plot the frequency response
    plt.figure()
    plt.plot((Fs * 0.5 / np.pi) * w, abs(h), label="order = %d" % order)
    plt.xlabel('Frequency (Hz)')
    plt.ylabel('Gain')
    plt.grid(True)
    plt.legend(loc='best')
    plt.show()

elliptical_bandpass()

When I run this, I see filter design and coefficients to be correct, but I get an error using lfilter

File "C:\Users\gtec\AppData\Local\Programs\Python\Python37-32\lib\site-packages\scipy\signal\signaltools.py", line 1354, in lfilter return sigtools._linear_filter(b, a, x, axis) SystemError: returned NULL without setting an error

Previously I was using python2.7 and it executed without any errors. Now I am using Python3.7.0

Upvotes: 0

Views: 551

Answers (1)

Warren Weckesser
Warren Weckesser

Reputation: 114831

The problem is that c3 is a list of strings. lfilter expects a sequence of numerical values. It will not automatically convert strings to numbers, so you'll have to convert those strings to numbers in your code before calling lfilter.

Do something like

c3 = [float(t) for t in c3]

before passing c3 to lfilter.

Even better would be to look back at how you actually create c3 in your "real" code (assuming the code in the question is a simplified example). It would make sense to convert the strings to numbers at the point where c3 is created.

(The cryptic error message is a bug in lfilter; you should have gotten a nicer error message. :)

Upvotes: 1

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