# 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 2To apply a two-pole two-pass Bessel with the same corner.:

u: 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.:

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