How to use use a butterworth narrow passband filter signal on work window

I designed a narrow passband butterworth filter which passband are 20-29Hz by fdatool.But,when i save it as a M-file,and use it to filter a sound at work window,the result is incredible,its figure is a straigh line.

Sampling frequence is 48000Hz;Stop band:10-40;Order is minimum;Sound is Chirp;

clc;clear;
BPF=load('20-29.mat');
[b,a]=sos2tf(BPF.SOS,BPF.G);
Load chirp;fs=8000;
f1=filter(b1,a1,y);
N=length(y);
t=(0:N-1)/fs;
subplot 211
plot(t,y);title('original signal')
subplot 212
plot(t,f1);title('filter signal')

Answer 1

[Star Strider]

This is likely impossible. No filter that I am aware of will be able to define such a narrow passband at such a high sampling rate. I cannot even get highpass and lowpass elliptical filters (that I would then cascade) to work with those design constraints.

[zhe wang]

yeh,thank you,i use resample,and get a good result.

[Star Strider]

My pleasure.

Did you resample to a lower frequency?

I do not usually recommend resampling, unless it is to provide uniform sampling times so that digital signal processing techniques will work correctly. (That is the reason I did not suggest it here.) Also, I do not know what you are doing and what your design or data analysis constraints are.

It would be easier to implement this sort of filter in hardware (using a Bessel design), and filter your signal before you sampled it. That way, you could avoid resampling it, and possibly losing details in your signal (assuming that you resampled it to a lower frequency). .

[zhe wang]

yeh,and i find the results lost some details.And i find i can use fdesign.bandpass to design my filter thought the fs is so large.

I design filters with software, and just study sinal&system about 6 mouths,so there are many difficulties for me.

thank you!Have a nice day.

[Star Strider]

My pleasure!