SUMMARY
Applies an IIR lowpass filter.
SYNTAX
LOWPASS {BUTTER|BESSEL|C1|C2},{CORNER v}, {NPOLES n},{PASSES n},{TRANBW v},{ATTEN v}
INPUT
BUTTER: Apply a Butterworth filter. BESSEL: Apply a Bessel filter. C1: Apply a Chebyshev Type I filter. C2: Apply a Chebyshev Type II filter. CORNER v: Set corner frequency to v. NPOLES n: Set number of poles {range: 1-10}. PASSES n: Set number of passes {n=1: causal, n=2: zero-phase}. TRANBW v: Set the Chebyshev transition band width to v. ATTEN v: Set the Chebyshev attenuation factor to v.
DEFAULT VALUES
LOWPASS BUTTER CORNER 0.4 NPOLES 2 PASSES 1 TRANBW 0.3 ATTEN 30.
DESCRIPTION
See the BANDPASS command for definitions of the filter parameters and descriptions on how to use them.
EXAMPLES
To apply a four-pole Butterworth with a corner at 2 Hz.:
SAC> LOWPASS NPOLES 4 CORNER 2To apply a two-pole two-pass Bessel with the same corner.:
SAC> LP N 2 BE P 2A Butterworth causal low-pass filter will time shift (forward) the waveform by an amount that depends on the corner frequency. The following macro will time-shift the data.:
SAC> setbb wf $1 SAC> setbb ts $2 SAC> r %wf SAC> ch b (%ts + &1,b&) SAC> write %wf%-TSIf the macro is named time-shift.m, the waveform file named XXX and the time shift -0.25, the following sequence will time-shift the data and output a file named XXX-TS. This macro will fail if IZTYPE is IB.:
SAC> m time-shift.m XXX -0.25
ERROR MESSAGES
- 1301: No data files read in.
- 1306: Illegal operation on unevenly spaced file
- 1307: Illegal operation on spectral file
- 1002: Bad value forcorner frequency larger than Nyquist frequency.
- See Chapter 4 of Rabinerand Gold, Theory and Application of Digital Signal Processing, Prentice-Hall, 1975 for a discussion of IIR filters.
SEE COMMANDS
BANDPASS
LATEST REVISION
Version 101.6