filtering before/after decimating

Asked Jul 03 '13 at 09:11

Active Jul 03 '13 at 09:42

Viewed 367 times

I deal with a signal of sampling frequency of 48000 Hz.

I do not need this much and doing decimation in order to get to 8000 Hz, so, decimation of 6-th order.

Additionally, I know I need to filter out 50 Hz and its harmonics. I am going to do it with the help of FFT-iFFT procedure, as I really do not know how to design a FIR filter with all its zeros and poles...

My question is:

Shall I filter out harmonics before decimating, or I can do it later?

Because if I need to filter out harmonics before, its gonna be insane to filter out 24000/50=480 of harmonics, and after decimating the computational memory decreases times (4000/50=80)!

edited Jul 03 '13 at 09:42

Radim Köhler

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asked Jul 03 '13 at 09:11

Elen Che

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You have to filter *before* decimating otherwise you will get aliassing. – Paul R Jul 03 '13 at 09:36
yeap, this is what I thought. Thank you! – Elen Che Jul 03 '13 at 12:46

filtering before/after decimating

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