Why is my filtered data obtained by the FILTER function in Filter Design Toolbox 2.2 (R13) severely attenuated at the beginning of the signal?

I have data whose frequencies are mostly in the passband of my designed filter. After filtering, my output's first 100 points are attenuated by the filter when they should not be.

Answer 1

[MathWorks Support Team]

This is due to the phase shift introduced by your filter. Filters are known to produce a concept called Group Delay in the filtered signal. This causes your signal to be delayed by varying amounts depending on the phase response of the filter. To avoid group delay in your filtered signal, you need to have a zero-phase filter. This can be done in MATLAB by using the function FILTFILT instead of the function FILTER.