# Help with Filtering an Interference Signal

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#### ~farah_r~

##### Newbie level 2 Hi All,

I have this one small audio file which is corrupted by an unknown interference signal which needs to be filtered out.

I loaded the file in matlab and found that its sampling frequency is 44.1kHz...I then plotted its FFT and found two peaks at + and - 21.99 kHz...So, I figured that I need to design a filter to remove the interference at this frequency.

I first designed a IIR notch filter with cut-off at 21.99k and filtered the sound signal through this filter using the filter command, but my sound file got worse instead of better.

I then designed a low pass filter, but the results were equally worse. I have no idea what I'm doing wrong.

Below is the code I used and the 3 filters I designed. I plotted the frequency response of all the three filters and found that my filters have the correct cut-off freq., pass band, and so on...so, there can be nothing wrong with the filters that I can think of ;(

Could someone please tell me what I'm doing wrong? The only thing I can think of now is that my conclusion that the interference signal is at 21.99 kHz is wrong? If that's the case, then how do I figure out what the interference band is from loading the file in matlab and plotting it's FFT? I'm really lost here ;(

Thanks,
Farah.
P.S. I did attach the corrupted audio file in the zipped extension added...it's a wav audio file, if it helps...

matlab code:

soundsc(y,fs)
Y = fft ; % FFT of the signal
f = -fs/2 : fs/(length(Y)-1) : fs/2; % frequency scale
figure
plot(f,abs(Y));

% NOTCH FILTER
wo=21.99e3/(fs/2); bw = 0.6; % with cut-off at 21.99 kHz
[b,a]=iirnotch(wo,bw);
zplane(b,a); % pole-zero plot
fvtool(b,a); % freq response of filter
filtered_audio = filter(b, a, y); % filtering the signal
norm_audio = filtered_audio/max(abs(filtered_audio));
norm = fft(norm_audio); % FFT of the filtered signal
f1 = -fs/2 : fs/(length(norm)-1) : fs/2; % frequency scale
figure
plot(f1,abs(norm));
soundsc(norm,fs)

% IIR Butterworth LPF
p=[-0.3360346+i*0.3737106, -0.3360346-i*0.3737106]; %poles of filter
z=-1+i*1e-50; % zeros of filter
[b,a] = zp2tf(z, p, 1); % converting poles and zeros to transfer function of filter
zplane(b,a);
fvtool(b,a);

% IIR Butterworth NOTCH FILTER
p=[-0.3360346+i*0.3737106, -0.3360346-i*0.3737106];%poles of filter
z=-1+i*1e-50;% zeros of filter
[b,a] = zp2tf(z, p, 1);% converting poles and zeros to transfer function of filter
zplane(b,a);
fvtool(b,a);

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