Re: Butterworth Filter

Helmut Sennewald wrote:
----- Original Message ----- From: <dhruveenews@xxxxxxxxx>
Newsgroups: sci.electronics.design
Sent: Thursday, June 23, 2005 12:20 AM
Subject: Butterworth Filter



Hello,
I am working on data acquisition.  I have collected some EMG data and
now according to the research papers, the EMG data has to be filtered
using 2nd order dual pass butterworth filter with the cut-off frequency
of 40 Hz.  What is 'dual pass'?  Can anyone elaborate on it.

thank you 


Hello Dan,

I bet it means run the signal twice through this filter.
So it's finally a 4th degree filtering.
The trick is to run it backward through the filter in the
second pass. This removes nonlinear phase and delay.
It can be only done with with digital signal processing.

http://www.abdn.ac.uk/~psy359/dept/Papers/obsavoid.pdf
"The raw X-, Y- and Z-coordinates
of each IRED were digitally filtered by a dual pass through a
2nd-order Butterworth filter with a cut-off frequency of 20 Hz
(equivalent to a 4th-order filter with no phase lag and a cut-off
of ~16 Hz)."

Best regards,
Helmut 

Hi Helmut,

can you clarify "run it backward" please?

do you mean take N samples, n = 0...(N-1)

and feed through the filter, giving y0...yN-1

Then starting with the last output YN-1 and working backwards to the first output Y0, feed them through the same filter to give yy0...yyN-1 ?

So the second pass through the filter is in negative time, hence the phase lags cancel.

Cheers
Terry
.

Quantcast