# How to find out roll-off from magnitude response of filter in matlab

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632541 on 23 Sep 2021
Commented: 632541 on 23 Sep 2021
How to find out roll-off from magnitude response of filter in matlab?
This is from doc frequency response of matlab site
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### Answers (1)

Chunru on 23 Sep 2021
fs = 2000;
[b,a] = butter(9,400/(fs/2));
[h, f] = freqz(b,a,16384,fs);
h = 20*log10(abs(h));
[~, i3db] = min(abs(h - 10*log10(1/2)));
f3db = f(i3db);
plot(f, h);
xline(f3db)
grid on
ylim([-60 0])
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632541 on 23 Sep 2021
Please have a look at it
fs= 50e3;
cutOff= 5;
order = 3;
t=0:1/fs:1-(1/fs);
Sig=sin(2*pi*200*t)+sin(2*pi*150*t);
noise=2*rand(size(t))+sin(2*pi*1*t)+sin(2*pi*5*t)+sin(2*pi*3*t)+sin(2*pi*7*t)+sin(2*pi*11*t)+sin(2*pi*0.5*t)+sin(2*pi*0.2*t);
noisySig = Sig+noise;
wn= 2*cutOff/fs;
[B, A] = butter(order,wn,'high');
freqz(B,A); %%Freq response of filter
filteredData=filtfilt(B,A,noisySig);

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