Notch filtering from coefficients

Hi, I'm working on a notch filter to remove a peak in my spectrum. I designed the filter in different ways (fdesign.notch principally) and I get 6 coefficients. I tried filtering just with notchfilt(signal) but by plot seems nothing happens. Is this correct?

BTW how can I filter by using these coefficients? filtfilt accepts only 2..

Thanks

F0 = 2690;   % interference is at 
Fs = 44100; % sampling frequency is
notchspec = fdesign.notch('N,F0,Q',2,F0,100,Fs);
notchfilt = design(notchspec,'SystemObject',true);
% fvtool(notchfilt,'Color','white');

FOA_mag = notchfilt(Xa_mag);  % xa_mag is the magnitude from fft of the signal

% got these coefficients from struct:
notchfilt.SOSMatrix

ans =

    1.0000   -1.8549    1.0000    1.0000   -1.8514    0.9962

Answer 1

[Star Strider]

The function gives you the ‘b’ vector for the filter. The ‘a’ vector is 1, since this appears to be a FIR filter design.

To understand how the filter works, use freqz:

figure
freqz([1.0000   -1.8549    1.0000    1.0000   -1.8514    0.9962], 1, 2^16, 44100)

Use the first two arguments with filtfilt.

(I do not have the DSP System Toolbox, so I cannot comment further on your code.)

EDIT — (5 Jul 2020 at 00:18)

I would design the filter this way:

F0 = 2690;
Fs = 44100;
Fn = Fs/2;
fcuts = [2600 [-5 5]+F0 2790];
mags = [10 0 10];
devs = [0.1 0.05 0.1];
[n,Wn,beta,ftype] = kaiserord(fcuts,mags,devs,Fs);
b = fir1(n, Wn, 'stop', kaiser(n+1, beta), 'noscale');

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

.

[Giorgio Frignani]

Fantastic, it seems to work in both ways! I just didn't get coefficients meaning..

Thank you

[Star Strider]

As always, my pleasure!