It sounds like things are working and it is not a SAC bug. However, the
LP help file is fairly terse. It references you to the BANDPASS help file
for more details about the options, such as what happens when one does a
"passes 2" option. Also perhaps of interest is that when one does a
"passes 2" option, the number of poles is effectively doubled as the same
filter is run twice, just reversing the phase the second time.
http://www.iris.edu/software/sac/commands/bandpass.html
As Fiona mentioned, there is a phase shift accompanying the causal filter
that leads to a time delay. In my non-SAC filter programs, I accompany a
low-pass filter with a time shift. I have not tried to do a time shift
within SAC -- and am not sure if one can. If it can be done, please
"remind" me.
Arthur
On Tue, 19 Jun 2012, Pablo.Palacios<at>bristol.ac.uk wrote:
Thanks a lot Fiona,
Its works well!
P.
On Tue, June 19, 2012 4:44 pm, Fiona Darbyshire wrote:
Hi,
I don't count myself as an expert, but one thing to check is the
number of passes in the filter command you are using. If you just do
"lp co 5" you will get a Butterworth filter that gives a small
phase-shift, which would translate into the time-shift you observe. To
get a zero-phase filter, you need "lp co 5 p 2" i.e. 2 passes per
application of the filter.
Hope that helps; if you are already using "p 2" then there must be a
different problem, and someone else will have to advise.
Fiona Darbyshire.
On Tue, Jun 19, 2012 at 3:20 AM, <Pablo.Palacios<at>bristol.ac.uk> wrote:
Hi,
I am using sac 101.4 (06/07/2010) on linux and 101.5c (02/01/2012) on
mac
The following case occur in both versions.
The file attached shows a signal z(t) that was filtered three times with
bp co 5. According to the SAC manual the filter used is a Butterworth.
It
definition has only a real component in frequency domain and non-shifted
signal is expected after use it. However, the plot attached shows the
cross-correlations between the original and its filtered signals, used
up
to three times in series (a cascade filter). As is possible to see, each
filter is giving me a shifted signal, delays. At the bottom of the page,
the same data was filtered up to three times, with other program (R),
with
the same Butterworth equation programmed by me.
Although the shift is small (few samples) in case to compute parallel or
series filter, this case could affect the results.
So, my question is if actually sac is using a Butterworth filter, or
another. Or if this is a bug.
Best wishes,
Pablo
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Fiona Darbyshire
Arthur Snoke
