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From: Paul on 21 Jun 2010 10:58
Dear all,

I can't seem to get my simple filter working.

First I created a simple bandpass filter using fdatool.
Response filter=bandfilter
Design method=IIR butterworth
the specified order =8
the Fs =250Hz
Fc1=1Hz
Fc2=40Hz
If I design this filter i see that this gives a perfect flat magitude response between 2.4 and 28 Hz, which is what I want to achieve.

The m-code for this design is:
Fs = 250; % Sampling Frequency
N = 8; % Order
Fc1 = 1; % First Cutoff Frequency
Fc2 = 40; % Second Cutoff Frequency
h = fdesign.bandpass('N,Fc1,Fc2', N, Fc1, Fc2, Fs);
Hd = design(h, 'butter');

Now to filter my input signal, I use:
output_filt=filter(Hd,x_input); %with dim(x_input)=1*2500
but this gives:
"Empty transfer function."
->How do I solve this?

Second since this is a simple butterworth filter I could also use the buttercommand:
[B,A]=butter(8,[1 40]/500);
Now when I check the magnitude and frequency response with:
freqz(B,A)
I get a completly different response!
->Why do I not get the sample response as with fdatool?

with kind regards,
Paul
From: Paul on 21 Jun 2010 11:23
Dear all,

I've found a roundabout solution for the first question:
if I use the Hd design, I can use the function sos2tf:
[B,A]=sos2tf(Hd.sosMatrix,Hd.Scalevalues);
to extract the A and B coefficients.
then I use filtfilt to filter my input:
output=filtfilt(A,B,input);

I still don't know why a butterworth design doesn't give the same respons if you design it with fdatool or with the butter function.




"Paul " <paul.verkempynck(a)student.kuleuven.be> wrote in message <hvnum2$98r$1(a)fred.mathworks.com>...
> Dear all,
>
> I can't seem to get my simple filter working.
>
> First I created a simple bandpass filter using fdatool.
> Response filter=bandfilter
> Design method=IIR butterworth
> the specified order =8
> the Fs =250Hz
> Fc1=1Hz
> Fc2=40Hz
> If I design this filter i see that this gives a perfect flat magitude response between 2.4 and 28 Hz, which is what I want to achieve.
>
> The m-code for this design is:
> Fs = 250; % Sampling Frequency
> N = 8; % Order
> Fc1 = 1; % First Cutoff Frequency
> Fc2 = 40; % Second Cutoff Frequency
> h = fdesign.bandpass('N,Fc1,Fc2', N, Fc1, Fc2, Fs);
> Hd = design(h, 'butter');
>
> Now to filter my input signal, I use:
> output_filt=filter(Hd,x_input); %with dim(x_input)=1*2500
> but this gives:
> "Empty transfer function."
> ->How do I solve this?
>
> Second since this is a simple butterworth filter I could also use the buttercommand:
> [B,A]=butter(8,[1 40]/500);
> Now when I check the magnitude and frequency response with:
> freqz(B,A)
> I get a completly different response!
> ->Why do I not get the sample response as with fdatool?
>
> with kind regards,
> Paul
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