How to use the fast fourier transform to calculate the output of a transfer function?

Hi, I have to calculate Y=X*H in the frequency domain and then taking it back in the time domain. I was trying to do it so:

X = fft(x);
H = ones(1, B);
Y = X*H;
y = ifft(Y);

The issue is that X is a 100x1 colummn vector, H is a 1x100 row vector, and so y is 100x100 matrix while I'd want a vector also for the output. What should I do?

[Valerio Marcucci]

PS: I use H as the transfer function of a passband filter. Is it right so?

Answer 1

[Honglei Chen]

I assume your X, H, and Y are all in frequency domain? In that case, your H has to match the size of X and it should be a element wise multiplication, not a matrix multiplication, so you need to use .* instead of *. For example,

X = fft(x);
H = ones(size(X));
Y = X.*H;
y = ifft(y);

HTH

[Valerio Marcucci]

Thank you so much! I did as you said and it worked!