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I am recording a 30 second 1000Hz Sine wave, saving it to a list, splitting the list into 5 second sublists and I would like to apply a notch filter to each sublist to check when the signal drops out for a few samples (I'm filtering out the 1000Hz frequencies). I am using python along with scipy for signal processing. I had some success using this method when I recorded the signal and applied a filter to the entire signal at once, but whenever I try to apply a filter consecutively to 5 second chunks of the signal, I seem to detect glitches that aren't there. Here is the code for my filter. 

   f0 = 1000.0  # Frequency to be removed from signal (Hz)
    Q = 30.0  # Quality factor
    w0 = f0/(fs/2.0)  # Normalized Frequency
    # Design notch filter
    b, a = signal.iirnotch(w0, Q)
    sos = signal.tf2sos(b,a)
    zi = signal.sosfilt_zi(sos)*lists[0][0]
    for list in lists: #each list is 5 seconds long
        Time=np.linspace(0, len(list)/fs, num=len(list))
        plot_graph(Time,list,fs)
        y_sos, zi = signal.sosfilt(sos, list, zi=zi*list[0])
        plot_graph(Time,y_sos,fs)

The graph below shows the first filtered 5 second chunk. enter image description here

The large peak just after one second is a genuine dropout but the 3 smaller peaks should not occur at all.

The graph below shows the next filtered 5 second chunk enter image description here

There are no glitches at all in this section, yet the small peaks still occur. I would also ideally like to get rid of the peak that begins at 0 and 5 seconds of each chunk. Can anyone tell me what is wrong with my filter/method?

Edit:

Changed my code to the following:

    f0 = 1000.0  # Frequency to be removed from signal (Hz)
    Q = 30.0  # Quality factor
    w0 = f0/(fs/2.0)  # Normalized Frequency
    # Design notch filter
    b, a = signal.iirnotch(w0, Q)
    zi = signal.lfilter_zi(b,a)*lists[0][0]
    for list in lists: #each list is 5 seconds long
        Time=np.linspace(0, len(list)/fs, num=len(list))
        plot_graph(Time,list,fs)
        y, zi = signal.lfilter(b, a, list, zi=zi)
        plot_graph(Time,y,fs)

I'm still obtaining the same problem of strange peaks occuring where they shouldn't although it seems to remove the peak that occurs at the beginning and the end of each chunk. The following graph shows the first filtered 5 seconds, the big peak after 1 second being a genuine dropout, the smaller ones are not sropouts. enter image description here

The following graph shows the next filtered 5 seconds where there are no dropouts and these peaks should not be occuring at all. enter image description here

Edit 2: Zoomed in miniature peak enter image description here

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  • $\begingroup$ iirnotch is always 2nd-order, so there's no point in converting it to 2nd-order sections sos form $\endgroup$
    – endolith
    Commented Aug 24, 2017 at 13:58

1 Answer 1

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y_sos, zi = signal.sosfilt(sos, list, zi=zi*list[0])

should just be 

y_sos, zi = signal.sosfilt(sos, list, zi=zi)

same as the question you asked yesterday

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  • $\begingroup$ I altered my code to reflect this, shown in the edit to my question above. I still have the same problem of strange peaks occuring where they shouldn't although it seems to remove the peak that occurs at the beginning and the end of each chunk. $\endgroup$
    – CJF
    Commented Aug 24, 2017 at 16:13
  • $\begingroup$ What does it look like when you zoom in on the glitches? $\endgroup$
    – endolith
    Commented Aug 24, 2017 at 16:24
  • $\begingroup$ Also what does it look like if your input is just ones(N)? A notch filter should leave DC alone and the output should just be ones, also. $\endgroup$
    – endolith
    Commented Aug 24, 2017 at 17:33
  • $\begingroup$ When I zoom in on where the glitches appear in the unfiltered signal, everything seems to be fine- There is no glitch. I tried the filter on an array of ones and It did not affect the output at all. I read in a 30 second wav file, split it into 5 second segments and applied the filter to each segment and everything worked perfectly so I think there may be something wrong with how I'm recording the audio. $\endgroup$
    – CJF
    Commented Aug 24, 2017 at 20:49
  • $\begingroup$ I mean zoom in on your plot after filtering $\endgroup$
    – endolith
    Commented Aug 24, 2017 at 22:34

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