My data is sampled at $f_s$, and I'm interested in analyzing certain frequencies $0<f_1<\ldots<f_N<\frac{f_s}{4}$. I then take $T$ samples of data, $\{x_1,\ldots\,x_T\}$ where $T\gg \frac{1}{f_s}$, FFT it after padding to the next power of 2 after $T$ and then pick the FFT bins closest to my $f_i$ of interest to proceed with the next step.

My question is: Is it necessary to (or is there any advantage if I) bandpass filter (BPF) my data between $f_1$ and $f_N$? I figured that since there was no downsampling involved, there won't be any aliasing and I needn't BPF it. Besides, a well designed filter should not modify the frequencies of interest, so it makes no difference. Am I missing something or is filtering a necessary step before FFT?

Note: I won't be inverting it back to the time domain. I'm just interested in the specific bins (but several that FFT is much faster than Goertzel).


1 Answer 1


Filtering is not necessary as it would only effect those frequency bins outside of the passband.

However, depending on how small your passband is, you could potentially save yourelf a lot of FFT processing by low-pass filtering and then downsampling before your perform your FFT. For example, if you downsample by 4, the resulting FFT could be 4 times shorter and still have the same frequency resolution.


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