# Re: problems implementing 8th order bessel-filter

Started by December 23, 2007
Thanks so much, it's finally working.

cheers,
Lukas

Dear Dwisuu,
>
>I think I know what's is going wrong. You are using the coefficients in the
>s-domain in a command that you should feed with coefficients in the
>z-domain. You are trying to make an analog filter using the command
>"besself" and matlab is returning coefficients in the s-domain, when you use
>the matlab command "filtfilt" matlab thinks that you are entering with
>z-coefficients. All you have to do is to use a method for analog-to-digital
>filter conversion. A good method is the bilinear.
>
>Below follows some code to explain.
>
>%****************************************************************
>n = 5
>w0 = 2*pi*2500; %The frequency with constant group delay
> %(it isn't cut-off frequency);
>
>%Calculating the filter in the "s-domain"
>[b,a]= besself(n,w0);
>
>%Plotting the bode of the filter.
>transfer = tf(b,a);
>bode(transfer);
>
>%If you want to use the matlab command filter or filtfilt you have
>%to use a method for analog-to-digital filter conversion.
>
>%Defining the sampling frequency. fs > 2*max_frequency.
>fs = 4*2500;
>
>%Maybe you have to use the prewarp fp, when using this command.
>[num,den] = bilinear(b,a,fs);
>
>%Filtering the signal X.
>y = filter(num,den,X);
>
>%*******************************************************************
>
>Best regards,
>Andr� Tahim
>2007/12/9, d...@gmail.com :
>>
>> hi, I'm trying to low-pass filter my input signal with an 8th order
>> bessel filter, cut-off at 2500Hz.
>>
>> my code goes like that:
>>
>> [b,a] = besself(8,1250/5000);
>> filteredSignal = filtfilt(b,1,inputSignal);
>>
>> Problem: the filtered vector doesn't show ANY difference in shape compared
>> to the original input signal, except that it has a much lower magnitude.
>> If I substitute besself with butter it does show a nice filtering effect,
>> but I need a bessel filter.
>> I tried it with higher orders and lower cut-off freqs, but no effect
>> whatsoever.
>>
>> what am I missing here?
>>
>>--
>Andr� Tahim
>a...@gmail.com
>MSN: c...@hotmail.com
>(48) 9132-3007