[SAC-HELP] a problem while downsampling sac data with decimate

[Arthur Snoke]

This thread was in late May.  Partly as a result of it, I looked more 
closely at the INTERPOLATE command.  For version 101.4, we replaced the 
truncate with a round off when calculating NPTS, but since then I found 
other things that should probably be changed.  If one uses the current 
version carefully, it should work satisfactorily, so a patch is not 
required.  However, I thought I should post to this list to see if anyone 
has any problems with my proposed changes.  I am attaching a PDF version 
of the revised HELP file.

In the current version, there is a default DELTA of 0.025 s.  Hence, if 
one enters INTERPOLATE NPTS 4096, the end time will change to a value that 
depends on how the input DELTA compares to 0.025.  I think that the start 
and end times should not change with INTERPOLATE, and do not understand 
the need/use of a default DELTA.  in the revised version, if NPTS is 
called for as an argument, DELTA is calculated -- just as if DELTA is 
input, NPTS is calculated.

I do not understand when one would use BEGIN.  It seems to me more logical 
to use CUT before calling INTERPOLATE.  I have left it in, so it can be 
used with either NPTS or DELTA.

In the current HELP file, there are logical arguments for NPTS and BEGIN 
that are not needed to be mentioned explicitly.

Another calling argument is EPSILON.  It is incorrectly described in both 
the current HELP file and the source code.  It is effectively a "water 
level" that gives a lower limit to the ratios that are used in the Wiggins 
procedure.  I have used this procedure for many years, both in my own 
programs and in those written by others.  I have never seen a reason to 
vary EPSILON and tests show that for a reasonable time series there is no 
effect on changing EPSILON by factors of 100.  I have left it in, but not 
advertised it in the HELP file.

One final comment about this interpolation routine:  When Wiggins wrote t 
in 1976, it was intended to be used for hand-digitized data.  Typically, 
one included extrema and (perhaps) inflection points.  A difference between 
this scheme and cubic splines is that the output file will not have greater 
extrema than the input data points.  It is also not as smooth as a cubic 
spline interpolation because second derivatives are not forced to be 
continuous.  This scheme works well for data if the DELTA is not changed 
too much.  Following up on the original thread, it should probably not be 
done if one is down-sampling by a large amount both because the fit will 
probably not be very good and because there is no anti-aliasing filtering 
done within INTERPOLATE.  For such a case, it is best to use DECIMATE.

Please comment -- especially if you disagree with anything I have said.

Arthur Snoke
snoke at vt.edu
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