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Incorrect bidirectional Butterworth filtering of analog channels in Mokka/BTK

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  • Incorrect bidirectional Butterworth filtering of analog channels in Mokka/BTK

    Hello,

    A very technical issue:

    Apparently, Mokka has options to (zero-lag, bidirectionally) filter the analog data in a C3D using a Butterworth filter. As Mokka is based on BTK, I was interested in how this could be done efficiently in BTK and checked the code (see below)

    I noticed that Mokka nicely changes the order and cutoff frequency when zero-lag filtering is selected (using the formula's in Winter's book). However, I noticed that Mokka uses the order of the filter instead of the number of passes when calculating the scaling/compensation factor.

    Is this correct? When looking at Winter's book, the cutoff frequency correction should only be based on the number of passes... Unless I am mistaking, this would mean that zero-lag filtering in Mokka only works for 2nd order BW filters.

    Regards,

    Ben van Basten
    Motek

    PS. I posted this on the BTK forum as well, but from my experience, this forum is very inactive, so I would like to consult the bm-l community as well.


    Code:
    FilterAnalog.cpp:
    
    const double hfs = data->acquisition()->analogFrequency() / 2.0;
    int n = dialog.orderSpinBox->value();
    bool zeroLagFilter = (dialog.zeroLagComboBox->currentIndex() == 0);
    
    
    if (dialog.typeComboBox->currentIndex() == 0)
    {
         double wn = dialog.lowerCutOffSpinBox->value() / hfs;
         if (zeroLagFilter) btkEigen::adjustZeroLagButterworth(n, wn);
         btkEigen::butter(&b, &a, n, wn, btkEigen::LowPass);
    }
    
    IIRFilterDesign.h:
    
    void adjustZeroLagButterworth(int& n, double& wn)
    {
        const double c = 1.0 / std::pow(std::pow(2,1.0/static_cast<double>(n))-1.0, 0.25); //BEN: n should be 2 (number of passes)
        wn *= c;
            n /= 2;
    };
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