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

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

This is from doc frequency response of matlab site

https://in.mathworks.com/help/signal/ug/frequency-response.html

Answer 1

[Chunru]

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])

[632541]

Hi Chunru ,

Thank you for your response

What is the rolloff is it 20dB/decade or someother in your design

[632541]

How do I calculate roll-off rate from the plot

[Chunru]

How do you define roll-off rate?

[632541]

Slope of the line in transition band.

dB/decade or dB/ octave.

Is it not the one

[Chunru]

In programming, you need to define the quantity that is computable. Where is the "line" from which point to which point? Do you want to compute the slope at certain point or certain type of average slope over certain range? If there is no definition given, it is up to everyone to give a different version.

[632541]

From which point to which point not decided. In the general design it is from pass-band frequency to stopband frequency. As I can only cutoff stopband and pass band frequencise are not confirmed .

Can you please tell for your design

[Chunru]

Shouldn't you come out your design first?

[632541]

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);