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--- jvs:fscan:manual:chapter2:expression:bandpass [2011/09/18 21:28]
voyvodic
+++ jvs:fscan:manual:chapter2:expression:bandpass [2023/02/23 18:43] (current)
@@ Line 1: / Line 1: @@
 **fScan Reference Manual, Chapter 2 (CmdLineOptions:Expr): Bandpass**
 ===== Bandpass -- Apply a bandpass frequency filter to input data =====
- **usage:  -expr bandpass flag minval maxval scalefact**\\
+ **usage:  -expr bandpass flag slowfreq fastfreq scalefact**\\
-The BANDPASS function will filter out both low and high frequency signals.
+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.
-Frequencies are typically expressed as wavelengths in seconds
+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).
-MINFREQ and above MAXFREQ.
+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.
-If MINFREQ is greater than MAXFREQ, then BANDPASS acts as a notch filter to remove frequencies between MAXFREQ and MINFREQ.
-SCALEFACT is a scaling factor; output intensity values will be divided by SCALEFACT. Default SCALEFACT is 1000.
+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.
+The FLAG argument can be used to select filtering options.\\
 FLAG:
-  * 1 - frequencies are in time axis units (i.e. TR's) (default: frequencies are Hertz)
+  * 1 - frequencies are in Hertz (default: frequencies are seconds)
-  * 2 - 2-d spatial filtering (default is temporal filtering) [not implemented yet]
+  * 2 - 2-d spatial filtering [not implemented yet] (default is temporal filtering)
-  * 4 - make output wordsize same as input (default: output is FLOAT32)
+  * 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)
+Example:\\
+<code>
+   -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)
+</code>
 **See Also:**\\

Brain Imaging & Analysis Center

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