currently i'am trying to get rid off power line noise (50/60 Hz) from an ecg.
- Bandwidth of the ecg 0.05 - 150 (achieving this with a butterworth second order bandpass)
- sampling rate 500 Hz
What filters did i try to remove 50/60 Hz noise?
- Butterworth bandstop 6th order bandwidth (+/-0.5 Hz up to +/2 Hz)
- Notch filter placeing zeros at z = e^(+/-j*2*pi*f_notch/f_sample)
- Notch filter with additional poles with same angular as zeros at distance to unit circle with r= 0.5 up to 0.7
What did happen?
- Butterworth filter causes ringing in the ecg signal
- Notch filter with only zeros causes ringing in the ecg signal
- Notch filter with additional poles causes an "overshoot" in the QRS-complex
What is the problem? (I think)
Actually my ecg signal contains information arround 50 Hz due to the phase response (and group delay) of the applied filters, i get this kind of distoration. I mean you clearly remove information from the signal it makes absolutely sense it gets "destroyed".
My question (finally)
Is there are a "better" way to remove 50/60 Hz noise? Secondly i was thinking to take the ringing effect and try to smooth it out (maybe savitzky golay filter?)