output filter from bandpass function

Hi, I am using the bandpass function in the following code:

Fs = 10; 
t = 1:(1/Fs):200;
s = sin(2*pi*0.1*t) + sin(2*pi*0.5*t);
[y,d] = bandpass(s,[0.06,0.12],Fs);

y2 = filter(d,s);

According to matlab documentation d is the Bandpass filter used in the filtering operation, returned as a digitalFilter object. So I expect y2 to be equal to y1, however they are very different. My questions are: 1. Why y2 is different than y1? 2. How can I use d to obtain exactly y1? Thanks,

Answer 1

[Star Strider]

The filter function will only return the same result as ‘y’ (or ‘y1’) for linear-phase FIR filters such as those produced by the fir1 function. For IIR filters, the default design of the bandpass function, you must use the phase-neutral filtfilt function, as I suspect the bandpass function uses by default.

I will let you explore those functions at your leisure.

Your code (with my slight modifications) becomes:

Fs = 10; 
t = 1:(1/Fs):200;
s = sin(2*pi*0.1*t) + sin(2*pi*0.5*t);
[y,d] = bandpass(s,[0.06,0.12],Fs);

y2 = filtfilt(d,s);

figure(1)
plot(t, s,    t, y2,    t, y)

figure(2)
plot(t, y2, '-',    t, y, '--')