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Garry Allison
05-27-1998, 11:12 AM
>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
impulse.

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
cheers
Garry.

------------------------------------------------------------------------
Dr Garry T Allison Lecturer in Functional Rehabilitation,
School of Physiotherapy, email:iallison@info.curtin.edu.au
Curtin University of Technology, Tel. +61 8 9266 3648
Selby Street, Shenton Park, Fax. +61 8 9266 3636
Western Australia 6008
http://www.curtin.edu.au/curtin/dept/physio/pt/staff/allison/
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