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From: Javier Lopez-Calderon on 27 Apr 2010 17:03
"Florian " <sppgwff(a)brunel.ac.uk> wrote in message <hbhtcr$b1$1(a)fred.mathworks.com>...
> 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??

Yes. And also your cutoff (10 Hz) is at -6db (half-amplitude), not -3db as using filter.m

>
> 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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