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From: Florian on 19 Oct 2009 10:34
Hi,

I want to create a 4th order zero-phase digital butterworth filter with a cutoff frequency of 10Hz. Sofar I was using:

Fs=1000; % sample frequency
Fc=10; %cutoff frequency
[B,A]=butter(4,Fc/(Fs/2));
signal_filt=filtfilt(B,A,signal);

Now, I have two questions:
1. I just read in the help function that the "filter order is double the order of the filter specified by B and A".
Does that mean that I am currently using a 8th order zero-phase digital butterworth filter with a cutoff of 10Hz??

2. Talking about the cutoff frequency. Filtering also in the reverse direction, does this process decrease the defined cutoff fc (10Hz)?

Your help for clarification is much appreciated.
Thanks
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