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fScan Reference Manual, Chapter 2 (CmdLineOptions:Expr): Bandpass

Bandpass -- Apply a bandpass frequency filter to input data

usage: -expr bandpass flag slowfreq fastfreq scalefact

The BANDPASS function will filter out both low and high frequency signals from a time series data set. Usually, frequencies slower than SLOWFREQ and faster than FASTFREQ will be filtered out.
For convenience, frequencies are typically expressed as wavelengths in seconds, so a slow frequency cut-off would be something like 30-60s and a high frequency something like 2-3s, depending on what frequency wavelengths you want to preserve. If SLOWFREQ is faster than FASTFREQ, then BANDPASS acts as a notch filter to remove frequencies between SLOWFREQ and FASTFREQ (instead of frequencies outside that range).

Note: For temporal filtering, the data set time dimension should have the correct data sampling units (e.g. TR interval) specified; if the time unit is 0 (or negative) a 1s sampling interval is assumed.

SCALEFACT is a scaling factor; FFT output intensity values will be divided by SCALEFACT. Default SCALEFACT is 1000.

The FLAG argument can be used to select filtering options.

  • 1 - frequencies are in Hertz (default: frequencies are seconds)
  • 2 - 2-d spatial filtering [not implemented yet] (default is temporal filtering)
  • 4 - frequencies are number of cycles across time course (default: seconds or Hertz)
  • 8 - make output wordsize same as input (default: output is FLOAT32)
  • 16 - filter out 0 frequency (default: 0 freq mean signal is preserved)
  • 32 - no filtering (test FFT → reverse FFT)


   -expr bandpass 0 30 3            ; filter out frequencies below 30s (0.033 Hz) and above 3s (0.33 Hz)  
   -expr bandpass 1 0.033 0.33      ; same as above, specified in Hertz 
   -expr bandpass 4 10 100          ; filter out frequencies below 10 cycles and above 100 cycles  
   -expr bandpass 0 10 40           ; filter out frequencies between 10s (0.1 Hz) and 40s (0.025 Hz) 

See Also:
fScan Home,
fScan Manual,
Expression Command,
Manual Help

jvs/fscan/manual/chapter2/expression/bandpass.txt · Last modified: 2014/08/04 16:03 (external edit)