CODEWITHSUNDEEP
signal-processingfrequency
sal
sal

Reputation: 11

change in value of one frequency bin affect FFT and IFFT values of non changing bins

I have a 3001x577 matrix. I want to apply a operation to the first 120 samples. I have applied to the first 120 samples which accounts to 20 Hz of frequency. The sampling rate is 2 msec. So I have Fnyq =250hz. Now I have taken out the first 120 samples. I noticed that after applying the filter and replacing it with the older 120 samples, the values of bins greater than 120 has changed after I applied an IFFT . And this is evident on my final result. I got the desired filter result but it ends up changing values of samples which i want untouched.

Can someone explain why change in value of few frequency bins affect the ifft or fft of non changing bins. I am using matlab. And how can i prevent it?

Upvotes: 0

Views: 273

Answers (1)

Mouse On Mars
Mouse On Mars

Reputation: 1132

You took part of the spectrum (the first 120 samples), changed this part somehow and transformed the outcome back into the time domain by using an IFFT. It is to be expected that the signal has changed beyond the 120 samples since you manipulated frequency components which will alter all samples in the time domain. Think of it this way: You changed the amplitude (and phase) of 120 sinuses and then expect that the outcome to be limited to a certain time extent. Maybe you can post a new question where you describe what you actually want to achieve instead of the experiment you perform to get the job done.

Upvotes: 1

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