Why damping amplitude the amplitude shifting position of y azis

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nirwana 2023-8-24

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评论： Star Strider 2023-8-24

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');

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Answer 1

Star Strider 2023-8-24

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https://ww2.mathworks.cn/matlabcentral/answers/2012322-why-damping-amplitude-the-amplitude-shifting-position-of-y-azis#answer_1293082

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.

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nirwana 2023-8-24

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)