Hi. I'm trying to create a low-pass filter for a large collection of audio files to make some room on my hard drives. I'm using the butter filter as follows:
fn = fs/2;
fc = 125/fn;
fz = 135/fn;
[n, wn] = buttord(fc,fz,3,60);
[b,a] = butter(n, wn, ftype);
But the 'b' output component is always made up of zeros (I just get blank graphs on the freqz plots).
I suspect I have misunderstood something along the line, but I am overall aiming for a filter that reads in the 32kHz audio file, and removes all frequencies over 125Hz, as I do not need them for my analysis. Any advice would be greatly appreciated.