SAC Command Reference Manual

DECIMATE

SUMMARY

Decimates (downsamples) data, including an optional anti-aliasing FIR filter.

SYNTAX

DECIMATE {n},{FILTER {ON|OFF}}

INPUT

n:Set decimation factor to n. Range is 2 to 7. This command may be applied several times if a larger decimation factor is required.
FILTER {ON}:Turn anti-aliasing FIR filter on.
FILTER OFF:Turn anti-aliasing FIR filter off.

DEFAULT VALUES

DECIMATE 2 filter on

DESCRIPTION

This command is used to downsample data after it has been read into memory. An optional finite impulse response (FIR) filter is applied to the data as it is being decimated to prevent aliasing effects normally associated with downsampling digitized analog signals. These filters also preserve the phase information. The application of these FIR filters often produces undesirable transients at each end of the data so the results should be checked graphically. Turning the anti-aliasing filter option off should only be done when the accuracy of the high frequency response is unimportant, such as when plotting.

EXAMPLES

To reduce the sampling rate by a factor of 42:

u:  READ FILE1
u:  DECIMATE 7
u:  DECIMATE 6

HEADER CHANGES

NPTS, DELTA, E, DEPMIN, DEPMAX, DEPMEN

ERROR MESSAGES

  • 1003: Value out of allowed range at symbol
    • Range on decimation factor is 2 to 7.
  • 1301: No data files read in.
  • 1306: Illegal operation on unevenly spaced file
  • 1307: Illegal operation on spectral file

Note The decimation by 7 filter has occasionally been unstable.

LATEST REVISION

May 15, 1987 (Version 10.2)