filt-filt function Error: First denominator filter coefficient must be non-zero.

Hello all,

I've run into an error that I was hoping someone could help shed light on for me. I am trying to create a bandpass filter kernal using firls to apply with the filt filt command. My code is highlighted below:

PitchBL = detrend(PitchBL); %(Remove the mean from the signal)
Fs = 100; %(Sampling Freq, in Hz)
nyquist = Fs/2;
n = 255; %(Filter Order)
f = [0 0.5 0.75 1.75 2 nyquist]/nyquist; %(Filter Kernal Shape Vector)
a = [0 0 1 1 0 0]; %(Filter Attenuation Vector)
d = firls(n,f,a);%(Construct Filter Kernal)

fvtool(d,5,'OverlayedAnalysis','phase')

PitchBL = filtfilt(d,a,PitchBL);

In which the variable nyquist = Fs/2 = 100 Hz/, and where the variable PitchBL is my orginal 1-D time series data vector that I am applying the bandpass filter on. I would like the upper transition region to run from 0.5 Hz - 0.75 Hz, I would like to run the pass band frequency region from 0.75 Hz - 1.75 Hz, and lower transition region from 1.75 Hz - 2 Hz.

Unfortunalely, I keep running into the following error, that I do not understand how to solve:

Error using filter
First denominator filter coefficient must be non-zero.

Error in filtfilt>ffOneChan (line 261)
    [~,zo] = filter(b(:,ii),a(:,ii), xt, zi(:,ii)*xt(1)); % yc1 not needed

Error in filtfilt (line 104)
        y = ffOneChan(b,a,x,zi,nfact,L);
        
        

I have attached a copy of the PitchBL time series data vertor in case its needed. Thanks in advance for your help.

Answer 1

[Star Strider]

I’m not entirely certain what you want to do. However assigning ‘1’ to the numerator usually works for FIR filters.

Try this to see how your filter works:

Fs = 100; %(Sampling Freq, in Hz)
nyquist = Fs/2;
n = 255; %(Filter Order)
f = [0 0.5 0.75 1.75 2 nyquist]/nyquist; %(Filter Shape Vector, [0,0.5)
a = [0 0 1 1 0 0]; %(Filter Attenuation Vector)
d = firls(n,f,a);%(Construct Filter Kernal)

figure
freqz(d, 1, 2^16, Fs)

You obviously know what you’re doing with respect to filter design, so I’m certain you can take it from there!