filtfilt and filter order

I use a butterworth filter with the filtfilt function. How does the reverse filtering affect the total filter order? If I use a second order butterworth, does the filtfilt function act like a fourth order butterworth (without the shift) or has the reverse filtering no effect on the total filter order? Thnaks for your advice.

Answer 1

[Star Strider]

As I understand the filtfilt documentation, the effect is to produce a result (filtered signal) of a filter of double the order of the filter you give it. It has no effect on the filter itself. (The advantage if filtfilt is that it is phase-neutral.) So at least as I read the documentation, it would act as if your filter was a fourth-order filter in your example.

Answer 2

[p.antunes]

I have a follow up question.

When filtfilt is used as y = filtfilt(b,a,x), such that filter is defined by "a" "b", filtfilt filter the data with a filter with double the order specified when building "a" "b".

However, when filtfilt is used as y = filtfilt(SOS,G,x), such that filter is defined by "SOS" "G", the the filter order duplication will not occur. Is this a correct ?