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Optimal cutoff frequency for data smoothing

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  • Optimal cutoff frequency for data smoothing

    I have been analysing the signal from a rotary encoder which provides
    displacement time data in an attempt to determine the optimal cut-off
    frequency for filtering the data prior to differentiation to provide
    velocity and acceleration data. I have been smoothing the data using
    a Butterworth 4th Order digital filter with cutoff frequencies ranging from
    1 to 60Hz and subsequently calculating the residual as the mean square
    difference between the filtered and raw data. Having plotted the residual
    against cutoff frequency I have been attempting to determine the optimal
    cutoff frequency by projecting the linear part of the resulting curve to
    the vertical axis and then back to the curve to determine the cutoff. The
    process is outlined on pages 41-43 of David Winter's book
    "Biomechanics and Motor Control of Human Movement, 2nd Edition".

    My problem is that the plot at the higher cutoff frequencies is not linear
    but curvelinear and I an unable to determine over what range of cutoff
    frequencies should I project my line from. The calculated optimal cutoff
    frequency is affected to a great extent by what range I define as the
    linear part of the curve.

    Can anyone provide advice on how I might determine my optimal
    cutoff frequency? Is there a source of a more detailed explanation of the
    method for determining optimal cutoff frequency?

    Any assistance is greatly appreciated. As always I will post a summary of
    the replies to the List.

    Robert Newton
    Robert Newton Phone Int+ 1 814
    865 7107
    Center for Sports Medicine Fax Int+ 1 814 865 7077
    The Pennsylvania State University Email RUN1@PSU.EDU
    117 Ann Building
    University Park, PA 16802
    United States of America