Thread Subject: help with filtfilt

Iim using the application filtfilt to filter a signal in Matlab:

m=10;
b=ones(1,m);
a=m;

Signal_filtered=filtfilt(b,a,original_signal)

Problem:i know that the more m is higher, the more the filter is efficient, but not all the signal need to be filtered.

for example if the signal vary on a interval between [-180?,180?].
the part between -180? et 10? doesn't need to be filtered, it's already smooth, but the second part contains a lot of perturbation and needs to be filtered.

how can i use this filtfilt application in a way that it can itself identify the part that need to be filtered and doesn't touch the smooth part?

i'm asking this because when i fix m=50;
the second part is well filtered but the smooth part changes its value, which it is wrong.

Any suggestions?