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Math Help - bandstop filter working - just dont understand something

  1. #1
    edm
    edm is offline
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    bandstop filter working - just dont understand something

    Hi,

    i created a bandstop filter to remove noise from a noisey ecg 70 - 90Hz. see psd:



    The filter works well - when i look at ecg back in the time domain its very clear, but i am just confused as to why there is a dip between 75-85hz?

    fs = 200;
    Wn = [65*2/fs,90*2/fs];
    deg2 = 6;%filter deg
    [B,A] = butter(deg2,Wn,'stop');

    filteredSignal = filter(B,A,filteredSignal1);
    thanks for your help.

    Ed
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by edm View Post
    Hi,

    i created a bandstop filter to remove noise from a noisey ecg 70 - 90Hz. see psd:



    The filter works well - when i look at ecg back in the time domain its very clear, but i am just confused as to why there is a dip between 75-85hz?

    thanks for your help.

    Ed
    Because it is a band stop filter, and 75-85 Hz is the core of the stop band (an ideal bandstop filter with stop band 70-90 Hz would have a hole where the filtered signal spectrum is zero (-infty dB), as it is you have ~ -60 dB gain in the stop band).

    CB
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