Thread: Re: Decimate Filter ON/OFF

Started: 2019-11-03 01:54:00
Last activity: 2019-11-03 01:54:00
Topics: SAC Help
Onur Tan
2019-11-03 01:54:00
Dear Susan,

In my previous email, I gave the 1000sps output of a seismometer. I have also 200sps output from the seismometer. So, There are two observed data set with 200 and 1000sps at the same time.
the seismogram after decimation 5 with FILTER OFF option is similar with the 200sps output of the seismometer.

Actually I cannot realize my problem with sine example.
However, I generate seismogram with FUNCGEN. I see the aliasing on the P-phase after "decimation 5 FILTER ON" command. Small decimation number (i.e. 2 or 3) do not generate clear aliasing.

My question is that does ON/OFF switch of the decimate work properly in the source codes xdecimation.c, decim.c etc. ?


On 3 Nov 2019, at 00:39, Suzan van der Lee <suzan<at>> wrote:


The decimated signal with "filter OFF" is aliased. You can tell by comparing the results for the energy bursts at 0.25 and .55 s. The first one has some high- and some lower frequency content, while the 2nd one is only rich in high frequency signals. Therefore, decimation should show some energy for the first burst at low frequencies, while the 2nd burst should barely show up at all. You get that (correct) result with "filter ON" but the second burst shows up way too strongly with "filter OFF".

You should try the same procedure on a sine wave with a frequency of 300 Hz, and then again on one with a frequency of 60 Hz. I hope that will illustrate why you need to always have "filter ON". Use funcgen to make the sine wave signal.

Begin forwarded message:

From: Onur Tan <xonurtan<at> <xonurtan<at>>>
Subject: [IRIS][sac-help] Decimate Filter ON/OFF
Date: November 2, 2019 at 2:54:33 PM CDT
To: SAC Help <sac-help<at> <sac-help<at>>>

Dear all,

I have 1000sps data. When I apply decimation (#5) with option FILTER ON, I see strange waveform. However, FILTER OFF option gives much appropriate waveform.
I am confused with FILTER option.

best regards
Onur Tan

PS: SAC 101.6a on Ubuntu

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