SAC Command Reference Manual

LOWPASS

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 to n {range: 1-10}.
PASSES n:Set number of passes to n {range: 1-2}.
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.:

u:  LOWPASS NPOLES 4 CORNER 2

To apply a two-pole two-pass Bessel with the same corner.:

u:  LP N 2 BE P 2

A 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.:

setbb wf $1
setbb ts $2
r %wf
ch b (%ts + &1,b&)
write %wf%-TS

If 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