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View Full Version : Woltring Filter cutt-off-frequency <--> VAL/P-value

Rettig, Oliver
09-14-2006, 07:29 PM
Dear all,

we implement an algorithm for gait event detection.

For smoothing and derivating the data we want to use the WOLTRING
filter. We have found Tony Reina's Matlab MEX interface for the GCVSPL
and SPLDER packages based on Dwight Meglan's C Code on the web address
http://www.isbweb.org/software/sigproc.html.

We have the problem to find the correct VAL value for the WOLTRING
GCVSPL package.
We found three approaches:

1. From dan@indy1.motorlab.nsi.eduSun

VAL = sampling_freq/(2*PI*cut_off_freq)**(2*order)

2. from Ton van den Bogert

w0factor = (sqrt(2.0)-1)**(0.5/m)

val = 1.0/dt*(2*pi*f0/w0factor)**(-2*m)

3. Tony Reina

VAL = (EMG-sampling_freqency/1000.0) /
pow(2*PI*EMG_CUTOFF_FREQUENCY/1000.0/pow( (sqrt(2.0)-1),0.5/M),2.0*M)

(including in EMG example)

Our questions are:

Do anybody know what is the correct equation for calculating the mode
value VAL from given cut-off-frequeny?

And why the factor 1000 appears in the equation from Tony Reina?

Many thanks

Kai Kraft
Oliver Rettig