how to use FFT to filter a 50Hz noise from a data array?

Question

Suppose I have a data array from an ECG like this one

http://joachim.behar.perso.neuf.fr/Joachim/ECG_tuto_1/ECG_filters_basics/ecg_sample.txt

and I need to filter a 50Hz noise out of it, using an algorithm like that

http://joachim.behar.perso.neuf.fr/Joachim/ECG_tuto_1/ECG_filters_basics/notch_filter.m

and the result should be like that ( red - filtered, blue - unfiltered ) seen in Figure 2:

enter image description here

What would be the best way of doing it in Java/android? Would this library help https://sites.google.com/site/piotrwendykier/software/jtransforms ?

Thanks! )

I'm not an expert in Android. Is there any problem doing FFT->filtering 50hz out->FFT^-1? — Acorbe, Oct 20 '12 at 16:51
multiplication in the frequency domain is convolution in the time domain. Convolution might be faster than FFT. — John Dvorak, Oct 20 '12 at 16:54
I'm sorry for my bad knowledge of the subject, but that's FFT and what's Convolution and how can one use it? ) — Roger Travis, Oct 20 '12 at 16:57
I found an FFT library here https://sites.google.com/site/piotrwendykier/software/jtransforms , what do you think, would it fit? — Roger Travis, Oct 20 '12 at 17:17
@RogerTravis Did you succeed in finding a solution for your question? — ivkremer, May 12 '14 at 00:14

score 2 · Answer 1 · edited May 23 '17 at 12:21

2

In the article ECG elementary filtering, a notch filter is applied in the (mis-numbered) section on Removing power line noise; the result is displayed in Figure 2. This Java ImageJ plugin, Notch_Filters, "implements the most common Notch filters in the frequency domain." While the example is not directly portable to android, it might allow you to select a particular implementation to re-implement.

Note that the article expands on the limitations of the approach.

Addendum: The notch filter relies on an FFT, so you'll need both.

edited May 23 '17 at 12:21

Community

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answered Oct 20 '12 at 17:44

trashgod

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Great suggestion, thanks. Just how would it be getter to implement that notch filter into a common android activity? :) – Roger Travis Oct 20 '12 at 18:01
I'm not sure. What activity? EKG? Telephony? What noise? – trashgod Oct 20 '12 at 18:14
a simple android activity, that has an array from this file http://joachim.behar.perso.neuf.fr/Joachim/ECG_tuto_1/ECG_filters_basics/ecg_sample.txt and needs to filter off the 50hz noise. – Roger Travis Oct 20 '12 at 18:30
Sorry, I'm not sure about an android UI. I'd definitely prototype on the desktop; [tag:jfreechart] is my [favorite](http://stackoverflow.com/a/5048863/230513), and android [ports](http://stackoverflow.com/q/4788576/230513) are available. – trashgod Oct 20 '12 at 22:02

how to use FFT to filter a 50Hz noise from a data array?

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