How can I use the function fir1 and filtfilt together?

Question

Daniela Conforti 2019-3-20

回答： Walter Roberson 2019-3-20

I need to use function fir1 for a bandpass filter and then I need to add a Zero-phase digital filtering with filtfilt.

I write:

d= fir1(20, [0.42 0.64]) %It's a 20-th order FIR bandpass filter and 0.42 and 0.64 my cut-off frequencies

y2= filtfilt(d,y); %y is my signal

It not works for filtfilt and the error is: Not enough input arguments.

Probably I need to use also the sample rate fz but I don't know where.

NB I don't want to use the function designfilt.

Thank you for help

Answer 1

Walter Roberson 2019-3-20

[b, a] = fir1(20, [0.42 0.64]);
y2 = filtfilt(B, A, y);

The two-output form of fir1 is not well documented. It turns out to always return 1 for a, so you could also use

d = fir1(20, [0.42 0.64]);
y2 = filtfilt(d, 1, y);