Hi tu you all!
I have a problem of distortion while filtering.
I have a signal with bandwith of 10 KHz centered on a carrier frequency 
of 30 KHz without noise.
I'm trying filtering that with a butterworth filter of signal bandwith of 40 KHz centered on 30
KHz using

[Bf1,Af1]=butter(5,[wn1 wn2]);
y=filtfilt(Bf1,Af1,x);

But if i try to compare x and y they are different even if the filter should be non distorting
for the signal.

If i consider 10*log10(mean(x.^2)/(mean((x-y).^2)) i find something like 15 dB and the errors
are concentrated on the firsts and lasts samples.

Why?
How can I create a not-so-distorting filter?

thanks,

Fortuna