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Re: Active Electrode Advice Needed

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  • Re: Active Electrode Advice Needed

    >Dr Gordon Chalmers - re low frequency artefact around heel strike

    It still seems that there is a low frequency artefact in the signal. In the
    simplest sense it could be movement artefact yet you have noted is unlikely
    from observation of the leads etc - it could be skin movement (or the
    muscle under the skin).
    I'm not clear on how you are collecting & processing the data.- here are
    some things to consider.
    If you have access to the raw data then you may wish to consider the power
    spectrum to see if it is truely a low frequency problem - then it may be
    advantageous to increase your high pass cut off filter frequency - Lower
    your low pass to 1/3rd Sampling Freq.

    Another point that may not be raised by others and could be worth an
    outside guess...
    Some digital processing packages offer the ability to increase the order of
    the filter - this has the advantage of making the cut-off frequency profile
    much sharper. The disadvantage is that they become unstable and develop a
    ripple effect in the processed data. This is easily seen if you run a
    square pulse through the processing program and is exaggerated with "high
    impulse" data.
    The fact that the ripple is just before heal strike is a little more
    difficult to explain - however the digital processes pass the data in both
    directions so it is possible to have it before and after the actual

    Finally, if you are creating a linear envelop (leaky integrators) before
    the AD conversion the raw data may be clipping (too high a gain) which is
    then mis-interpreted as (intermittent - corresponding with expected high
    amplitudes) low frequency noise of large amplitude wiping out a signal with
    was previously seen to be normal.

    Hope this helps

    Dr Garry T Allison Lecturer in Functional Rehabilitation,
    School of Physiotherapy,
    Curtin University of Technology, Tel. +61 8 9266 3648
    Selby Street, Shenton Park, Fax. +61 8 9266 3636
    Western Australia 6008
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