Re: problems implementing 8th order bessel-filter

Started by Jeff Brower December 11, 2007

> hi, I'm trying to low-pass filter my input signal with an 8th order
> bessel filter, cut-off at 2500Hz.
> my code goes like that:
> [b,a] = besself(8,1250/5000);
> filteredSignal = filtfilt(b,1,inputSignal);
> Problem: the filtered vector doesn't show ANY difference in shape
> compared to the original input signal, except that it has a much
> lower magnitude. If I substitute besself with butter it does
> show a nice filtering effect, but I need a bessel filter.
> I tried it with higher orders and lower cut-off freqs, but no
> effect whatsoever.
> what am I missing here?

My guess is there is some effect, but you have to look closer -- maybe run a sine
wave as your input signal.

Bessel filters require extremely high filter orders to approximate equivalent
Butterworth filters. At lower filter orders, Bessel roll-off can be exceedingly slow
-- it might appear that nothing is happening, depending on your measurement