BFIL

BFIL uses a bilinear transform to perform ARMA filtering. Zero phase filtering is done by two passes of minimum phase filtering in opposite temporal directions. The command line parameters are:

 bfil infile itype npoles fcenter bwidth ifaz 
  
 infile  =input file name 
 itype  0=low-pass filter, cut off freq= fc (-3dBv)
        1=band-pass filter, center frequency= fc
        2=high-pass filter, cut off freq= fc (-3dBv)
 npoles  =number of poles in filter
          (6dB/octave)/(pair of poles)
 fcenter =center frequency Hz    
 bwidth  =band-pass filter bandwidth (-3dB) Hz 
 ifaz    1=minimum phase  0=zero phase filter

Examples of filtering with BFIL are shown in Figure 93:

(A) Low-Pass Minimum phase, 12 Hz cut-off, 4 poles
bfil c008.seg 0 4 12. 1
(B) High-Pass Minimum phase, 48 Hz cut-off, 4 poles
bfil c008.seg 2 4 48. 1
(C) Band-Pass Minimum phase, 24 Hz center, 24 Hz band-width, 4 poles
bfil c008.seg 1 4 24. 24. 1

**Figure 93:** BFIL: Input data are same as in Figure 91A.
$\includegraphics[scale=.9]{Figurebfil.pdf}$

Another way to band-pass filter is to run the data twice, once through a low-pass, and then through a high-pass filter, choosing cut-off frequencies to produce a band-pass.