SUMMARY

Differentiates data in memory.

SYNTAX

DIF {TWO|THREE|FIVE}

INPUT

TWO:	Apply a two point difference operator.
THREE:	Apply a three point difference operator.
FIVE:	Apply a five point difference operator.

DEFAULT VALUES

DIF TWO

DESCRIPTION

The two-point algorithm is:

OUT(J)= (DATA(J+1) - DATA(J))  / DELTA

The last output point is not defined by this algorithm. It is also not a centered algorithm. SAC takes care of these problems by decreasing the number of points in the file (NPTS) by one and by increasing the begin time (B) by half the sampling interval (DELTA).

The three-point (centered two-point) algorithm is:

OUT(J)= 1/2 * (DATA(J+1) - DATA(J-1)) / DELTA

The first and last output point is not defined by this algorithm. SAC decreases NPTS by 2 and increases B by DELTA.

The five-point (centered four-point) algorithm is:

OUT(J)=2/3 (DATA(J+1) - DATA(J-1)) / DELTA -
 1/12 * (DATA(J+2) - DATA(J-2)) / DELTA

The first two and last two output points are not defined by this algorithm. SAC applies the three-point operator to the second points from each end, decreases NPTS by 2, and increases B by DELTA.

ERROR MESSAGES

• 1301: No data files read in.
• 1306: Illegal operation on unevenly spaced file

HEADER CHANGES

NPTS, B, E, DEPMIN, DEPMAX, DEPMEN

LATEST REVISION

January 15, 1985 (Version 9.10)