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

[weitao wang]

Thanks Milton for quick reply.

Since in our later data processing the phase plays an important role, I am
wondering if the interp or decimate will keep the phase in low freqency, for
example  lower than 0.4Hz. It seems that the decimate cmd performs an
anti-alias filter, but document on interp does not show that. By comparing
the origin data and interped and decimated ones in a short time gap, say two
seconds, we may find all the decimated data points can be found in the
origin data but interped one adds some new points different from ori data.

So will interp pre-filter the origin data to low frequency just same as
decimate cmd ? Are both "phase-keeping" cmd ?

thanks again for all your suggestions

weitao
2010/5/25 Milton P. Plasencia Linares <mplasencia at ogs.trieste.it>

>
> Hi Weitao,
> Yes, when you decimate (5-5-2-2) the header is 9.99999E-01,
> Sac 101.3b 64bits - Fedora Linux.
> I have a test and attach the ps file.
> I downsampling using decimate and interpolation commands,
> (see figure). Values defaults for decimate applied anti-aliasing
> FIR filter. Interpolation use the Wiggins method.
> I see that changing header with 'ch delta 1.0' no affect the
> quantity or form of data.
>
> In the figure, in this case, I think that interpolate does a
> better job, at least not "filtered" completely the EQ signal.
>
> I wait this help you.
>
> Cheers,
>
> Milton
>
> **********************************
> Milton P. PLASENCIA LINARES
>
> Dipartimento Centro di Ricerche Sismologiche
> Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS
>
> Borgo Grotta Gigante 42/C
> (34010) Sgonico - TRIESTE - ITALIA
> Tel: +39-040-2140136
> Fax: +39-040-327307
>
> E-mail: mplasencia at ogs.trieste.it
>
> ASAIN (Antarctic Seismographic Argentinean Italian Network)
> *********************************
>
>
>
> Quoting "weitao wang" <wangwtustc at gmail.com>:
>
>   Hi All
>>
>> When I tried to downsample a sac data using decimate command,I encountered
>> a
>> curious problem on the delta change.
>> Suppose we have one data with samplng rate 100hz, and we want to
>> downsample
>> to 1hz.
>> the cmd I used is
>> decimate 5, now delta=0.05;
>> decimate 5  now=delta=0.25
>> decimate 2  now delta=0.5
>> decimate 2  now  delta=0.999999
>>
>> in the final step the delta is 0.999999 instead of 1.0. Since our later
>> processing need to check the consistent of  sachdr.delta and we have some
>> LHZ sac with delta=1.0, we need to make the downsampled-data's delta to be
>> 1.0 ,not 0.9999999.
>>
>> Is there any way to aviod the 0.99999 thing ? Or we can forcely change
>> delta
>> to 1.0 using  ch delta 1.0 without  bad effects for later processing ?
>>
>> And there is another question, can sac cmd  " interp "be used to downsamp
>> origin data ? is it differ from decimate ?
>>
>> thanks for all your help in advance.
>>
>> wt
>>
>> --
>> This email was Anti Virus checked by INOGS Antivirus filter.
>>
>>
>
>
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