Why damping amplitude the amplitude shifting position of y azis

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I damped the amplitude signal using by multiplied the freq with 0.5 after fft result, and returning the signal to time domain using ifft, but the returning signal that i get is shifting in Y axis as shown below. Should I multiplied y to some number to ifft result. Why it is happened? Is it effect of fft? But it does't happend if i use sinewave starting with 0 in y axis.
%DAMPLING AMPLITUDE USING REAL DATA
SMR=load ("sig_fft.txt");
SMR_data=SMR(:,2);
srateSMR=1/100;
SMR_Damp=real(ifft(fft(SMR_data)*0.5));
tSMR=(1:length(SMR_data));
figure(2)
plot(tSMR,SMR_data,'r',tSMR,SMR_Damp,'b');

Accepted Answer

Star Strider
Star Strider on 24 Aug 2023
It would help to have the file to demonstrate with it, however that may not be absolutely necessary.
The signal in the file (red curve) has an obvious constnt offset (D-C offset) value that looks to be about -1.2. Muttiplying the fft of that signal by 0.5 produces a result (blue curve) that is offset by about -0.6. The sine curve used to test it has a 0 offset, so there is no similar shift.
  4 Comments
nirwana
nirwana on 24 Aug 2023
but by write it mean that the damping amplitude is zero, what i want is damping amp stil perform but plot the signal with overlap (in the same start of y axis)
Star Strider
Star Strider on 24 Aug 2023
I am not certain that I understand.
If you want to decrease the signal amplitude without affecting the D-C offset, do something like this —
Fs = 10000;
Tlen = 1000;
t = linspace(0, Tlen*Fs, Tlen*Fs+1).'/Fs;
SMR_data = sin(2*pi*0.015*t) .* sin(2*pi*0.001*t+pi)-1.2;
SMR_Damp = (real(ifft(fft(SMR_data)))-mean(SMR_data))*0.5 + mean(SMR_data);
figure
plot(t, SMR_data,'r', t, SMR_Damp,'b')
legend('SMR\_data','SMR\_Damp', 'Location','best')
The procedure is to subtract the mean (D-C offset), do the multiplication, then add the mean value back.
.

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