<div><br>Thanks Milton for quick reply.</div>
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<div>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. </div>
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<div>So will interp pre-filter the origin data to low frequency just same as decimate cmd ? Are both "phase-keeping" cmd ?</div>
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<div>thanks again for all your suggestions</div>
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<div>weitao<br></div>
<div class="gmail_quote">2010/5/25 Milton P. Plasencia Linares <span dir="ltr"><<a href="mailto:mplasencia@ogs.trieste.it">mplasencia@ogs.trieste.it</a>></span><br>
<blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8ex; PADDING-LEFT: 1ex" class="gmail_quote"><br>Hi Weitao,<br>Yes, when you decimate (5-5-2-2) the header is 9.99999E-01,<br>Sac 101.3b 64bits - Fedora Linux.<br>
I have a test and attach the ps file.<br>I downsampling using decimate and interpolation commands,<br>(see figure). Values defaults for decimate applied anti-aliasing<br>FIR filter. Interpolation use the Wiggins method.<br>
I see that changing header with 'ch delta 1.0' no affect the<br>quantity or form of data.<br><br>In the figure, in this case, I think that interpolate does a<br>better job, at least not "filtered" completely the EQ signal.<br>
<br>I wait this help you.<br><br>Cheers,<br><br>Milton<br><br>**********************************<br>Milton P. PLASENCIA LINARES<br><br>Dipartimento Centro di Ricerche Sismologiche<br>Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS<br>
<br>Borgo Grotta Gigante 42/C<br>(34010) Sgonico - TRIESTE - ITALIA<br>Tel: +39-040-2140136<br>Fax: +39-040-327307<br><br>E-mail: <a href="mailto:mplasencia@ogs.trieste.it" target="_blank">mplasencia@ogs.trieste.it</a><br>
<br>ASAIN (Antarctic Seismographic Argentinean Italian Network)<br>*********************************
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<div class="h5"><br><br><br>Quoting "weitao wang" <<a href="mailto:wangwtustc@gmail.com" target="_blank">wangwtustc@gmail.com</a>>:<br><br></div></div>
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<div class="h5">Hi All<br><br>When I tried to downsample a sac data using decimate command,I encountered a<br>curious problem on the delta change.<br>Suppose we have one data with samplng rate 100hz, and we want to downsample<br>
to 1hz.<br>the cmd I used is<br>decimate 5, now delta=0.05;<br>decimate 5 now=delta=0.25<br>decimate 2 now delta=0.5<br>decimate 2 now delta=0.999999<br><br>in the final step the delta is 0.999999 instead of 1.0. Since our later<br>
processing need to check the consistent of sachdr.delta and we have some<br>LHZ sac with delta=1.0, we need to make the downsampled-data's delta to be<br>1.0 ,not 0.9999999.<br><br>Is there any way to aviod the 0.99999 thing ? Or we can forcely change delta<br>
to 1.0 using ch delta 1.0 without bad effects for later processing ?<br><br>And there is another question, can sac cmd " interp "be used to downsamp<br>origin data ? is it differ from decimate ?<br><br>thanks for all your help in advance.<br>
<br>wt<br><br></div></div>--<br>This email was Anti Virus checked by INOGS Antivirus filter.<br><br></blockquote><br><br><br>----------------------------------------------------------------<br>This message was sent using IMP, the Internet Messaging Program.<br>
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