Chebyshev Type 2 filter not working as expected

I have some EEG data and I designed a 20th order bandpass chebyshev type 2 filter with a stopband of 10hz and passband of 4hz. I filtered my signal with this filter however the response I obtained does not seem to be making sense. Can anyone help/identify what I am doing wrong?

EEGdata = load('EEGdata.csv');
Fs = 1200;            % Sampling frequency                    
T = 1/Fs;             % Sampling period       
L = length(EEGdata);  % Length of signal
t = [0:(L-1)]*T;        % Time vector

%now using bandpass chebyshev type 2 filter
[A,B,C,D] = cheby2(10,40,[4 10]/600);
d = designfilt('bandpassiir','FilterOrder',20, ...
    'StopbandFrequency1',4,'StopbandFrequency2',10, ...
    'StopbandAttenuation',40,'SampleRate',1200);
sos = ss2sos(A,B,C,D);
% fvt = fvtool(sos,d,'Fs',1200);
% legend(fvt,'cheby2','designfilt')

%filtering original data
EEG_filt = filtfilt(d,EEGdata);

figure();
subplot(2,1,1)
plot(t,EEGdata); title('EEGdata.csv before filtering'); grid on; axis tight; xlim([0 20]);
xlabel('Time in seconds'); ylabel('Amplitude/\muV');
subplot(2,1,2)
plot(t,EEG_filt); title('EEGdata.csv after filtering'); grid on; axis tight; xlim([0 20]);
xlabel('Time in seconds'); ylabel('Amplitude/\muV');

I have attached the results of the plots obtained as well as the magnitude response of the filter.

[Cris LaPierre]

Could you expain more on why your results are incorrect, or perhaps describe what you think the results should look like?

[Rudy Steel]

I am not understanding the plot I obtained after filtering. I would assume I should obtain the same signal along the entire x-axis (the first 20seconds) with the difference now that the result would be cleaner

Answer 1

[Cris LaPierre]

You have a ringing artifact because you have such a high order filter. Notice that the y axis of your filtered data is scaled by 10^5. You can scale this to see what you expect.

EEGdata = load('EEGdata.csv');
Fs = 1200;            % Sampling frequency                    
T = 1/Fs;             % Sampling period       
L = length(EEGdata);  % Length of signal
t = [0:(L-1)]*T;        % Time vector

%now using bandpass chebyshev type 2 filter
[A,B,C,D] = cheby2(10,40,[4 10]/600);
d = designfilt('bandpassiir','FilterOrder',20, ...
    'StopbandFrequency1',4,'StopbandFrequency2',10, ...
    'StopbandAttenuation',40,'SampleRate',1200);
sos = ss2sos(A,B,C,D);

%filtering original data
EEG_filt = filtfilt(d,EEGdata);

figure();
plot(t,EEG_filt); title('EEGdata.csv after filtering'); grid on; axis tight; xlim([0 20]);
xlabel('Time in seconds'); ylabel('Amplitude/\muV');
ylim([-10 10])

[Cris LaPierre]

Things improve if you drop your filter order

EEGdata = load('EEGdata.csv');
Fs = 1200;            % Sampling frequency                    
T = 1/Fs;             % Sampling period       
L = length(EEGdata);  % Length of signal
t = [0:(L-1)]*T;        % Time vector

%now using bandpass chebyshev type 2 filter
[A,B,C,D] = cheby2(10,40,[4 10]/600);
d = designfilt('bandpassiir','FilterOrder',8, ...
    'StopbandFrequency1',4,'StopbandFrequency2',10, ...
    'StopbandAttenuation',40,'SampleRate',Fs);
sos = ss2sos(A,B,C,D);

%filtering original data
EEG_filt = filtfilt(d,EEGdata);

plot(t,EEG_filt); title('EEGdata.csv after filtering'); grid on; axis tight; xlim([0 20]);
xlabel('Time in seconds'); ylabel('Amplitude/\muV');
ylim([-10 10])

[Rudy Steel]

Yes, I in fact did not consider the scale of the y-axis, I also now considered reducing the order of the filter and you are right the results do improve. Thank you