filtfilt function

Florian — Mon, 19 Oct 2009 14:34:03 -0400

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

Re: filtfilt function

Javier Lopez-Calderon — Tue, 27 Apr 2010 21:03:04 -0400

"Florian " <sppgwff@brunel.ac.uk> wrote in message <hbhtcr$b1$1@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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filtfilt function

Re: filtfilt function