Hi, Sam-

For most RSFC parameters, one needs to specify a range of frequencies that define the "low frequence fluctuations" (LFFs) of interest. A typical range of LFFs in resting state might be approx. 0.01 - 0.1 Hz, for example.

fALFF is a ratio:
(the sum of amplitudes of those LFFs) / (sum of amplitudes of all frequencies in time series).

Bandpassing is the mechanism for selecting out the LFFs, and so it is necessary to bandpass your data to calculate LFF. You don't have to use the output bandpassed time series later, but you have to do it beforehand. Note that if you are calculating RSFC parameters, then you can*not* have bandpassed your time series earlier, nor censored your data (e.g., removed volumes of high motion).

I suppose you could use 3dTrproject to get rid of orts, but have you processed these time series already, for example using afni_proc.py? Then your data should already have been detrended, likely.

--pt