# Time-shift after fft & filter?

7 vues (au cours des 30 derniers jours)
Shaula Garibbo le 12 Juin 2020
Commenté : Shaula Garibbo le 12 Juin 2020
I tried to implement a simple LP filter on an audio file, but I'm slightly confused by the apparent shifting of the signal in the time domain. What have I overlooked / misunderstood? Thanks.
filename = 'Raw_Wav_20180425_000226.wav';
subplot(1,2,1)
plot(t,y)
xlabel('Time (s)')
ylabel('Amplitude')
title('Raw')
fmax1 = 40; %end of pass band
fmax2 = 60; %end of transition band
nt = length(y);
t = [0:nt-1]/fs;
y = y-mean(y);
dF = fs/nt;
f = -fs/2:dF:fs/2-dF;
yft = fftshift(fft(y));
I = find(abs(f)>fmax2);
yft(I) = 0;
I = find( abs(f)>fmax1 & abs(f)<=fmax2 );
yft(I) = yft(I) .* hanning(length(I));
y = ifftshift(ifft(yft));
subplot(1,2,2)
plot(t,y)
xlabel('Time (s)')
ylabel('Amplitude')
title('Post-filter')
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### Réponse acceptée

Sugar Daddy le 12 Juin 2020
As you have taken fft first and then fftshift, so if you want to do inverse of this, you need inverse of fftshift first as it is performed at the end. so the order of inverse must be
1. ifftshift
2. fft
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Shaula Garibbo le 12 Juin 2020
Thank you very much!

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