Need help to determine stopband

I got a frequency of 6 - 16 Hz to pass through. but how do i determine what is my stopband?

xd = double(myRecording); % Convert to ‘double’

Fs = 8000; % Sampling Frequency

Fn = Fs/2; % Nyquist Frequency

Fpb = [6 16]/Fn; % Passband

Fsb = []/Fn; % Stopband

Answer 1

[Star Strider]

If you’re designing a bandpass filter, everything within [6 16] Hz (normalised by ‘Fn’ as you correctly compute) is your passband. Everything else is your stopband by definition, so you don’t need to define it specifically. If you need to define passband and stopband ripple (a requirment of buttord and related functions), choose 1 and 10 dB respectively.

[Daemian]

Yes, is for buttord. And what do you mean 1 and 10dB respectively?

[Star Strider]

The 1 and 10 dB are the passband and stopband ripple, respectively. This is more important in the Chebyshev design than for Butterworth filters, but buttord wants them. Choose whatever values meet your needs. Note that the butter function simply wants the filter order and (for a bandpass filter) passband frequencies. It doesn’t need any other information unless you want to design a highpass or bandstop filter, then it needs a third argument specifying filter type.

I didn’t realise you’re using the buttord function. It does require you to specify the passband and stopband, as well as the ripple for both, so I usually specify the stopbands to be 0.8 and 1.25 times the passband frequencies. I always convert the filter to second-order-section representation for stability.

For your filter, this becomes:

Fs = 8000; % Sampling Frequency
Fn = Fs/2; % Nyquist Frequency
Fpb = [6 16]/Fn; % Passband
Fsb = Fpb.*[0.8  1/0.8]; % Stopband
Rp =  1;
Rs = 10;
[n,Wn] = buttord(Fpb,Fsb,Rp,Rs);
[b,a] = butter(n,Wn);
[sos,g] = tf2sos(b,a);

figure(1)
freqz(sos,1024,Fs)

The freqz call lets you see what your filter looks like. Experiment with the stopband characteristics to get the result you want. You may want to widen them a bit.

[Daemian]

Sure thanks a lot :)

[Star Strider]

My pleasure!