ECG spectrum wrong?
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Stephan Lallinger on 27 Oct 2019
Edited: Stephan Lallinger on 4 Nov 2019
I have a question about an EKG spectrum.
Someone gave me this ECG spectrum for an exercise.
I'm not sure but the spectrum looks weird.
You clearly see a 50 Hz tone and its harmonics.
The whole thing looks like a modulation.
I would say there was no anti aliasing filter used.
Or does an ECG spectrum with 50 Hz + harmonics look like that?
Thanks for the help.
Daniel M on 27 Oct 2019
I'd say it looks fairly normal once you notch filter the line noise. But there is other preprocessing to do, like a low pass filter. You would probably get a lot out of this article <https://www.ncbi.nlm.nih.gov/m/pubmed/20851409/>
More Answers (3)
Daniel M on 29 Oct 2019
I'm not sure how you're doing your notch filters, but try this:
Fs = 625; % or whatever your sampling frequency is.
Fn = Fs/2; % Nyquist frequency
numHarmonics = 4; % Let's do 50, 100, 150, 200, 250
lineFreq = 50; % Hz
for fq = ((0:numHarmonics)+1) * lineFreq
Fl = fq + [-1, 1]; % notch around Fl. Could try [-2, 2] if too tight
[z,p,k] = butter(6, Fl/Fn, 'stop');
sos = zp2sos(z,p,k);
data = filtfilt(sos, 1, data); % assumes data is [time x ... dimensions]
% overwrites data, and filters sequentially for each notch
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