# Zeroing FFT bins without taking inverse FFT

I know the question has been asked about why it's a bad idea to zero bins in an FFT, but those all seemed to be taking the IFFT afterwards. What if I want to do all my analysis in the frequency domain? Then would it be alright to zero the bins with frequencies i don't care about, and then perform my analysis?

And to add to that, if I only care about a certain range of frequencies, can I perform the DFT instead of an FFT, but were I'm only calculating the bins that I care about (I believe this should be the same as zeroing out all the other frequencies).

For example, lets say I have a signal sampled at 16000Hz. and the frequency range I care about is between 2000-4000Hz. if I'm taking a 64 point FFT/DFT, then I only care about bins 8-16. Since my signal is real, that should save some time on the DFT (real number multiplied by complex number as opposed to two complex numbers multiplied), and then I just do 9 DFT calculations and forget about the other 55. I think this should be less memory need and should perform the calculation quicker. I think this is right, but I'm probably missing something. Thanks for your time!

• I believe you are correct on both accounts. (Can someone please explain the downvotes?) I just wouldn't call this "zeroing", since you just ignore the values, rather than explicitly setting them to 0 and then doing some processing on them. One of the usual issues still applies, though: You need to apply an appropriate window function first, to avoid lots of different frequency content leaking into your 9 bins. – Sebastian Reichelt Mar 2 '16 at 21:34
• I was wondering about the downvotes also. As for the windowing, that would be in the time domain, correct? such as a bandpass filter around 2000-4000Hz? I'm planing on doing a circular convolution on my 64-point FFT signal, which would include all of the zeroed out/not calculated bins so this is my main reason for asking about the zeroing of the bins. – gerrgheiser Mar 3 '16 at 3:32