wiki:ShmDocMenuWorkFilter
Last modified 14 years ago Last modified on 02/03/2009 05:16:34 PM

Menu entry Filter

Applies filter to traces on display. The filter dialog box opens to offer available filters. A description of the elements of the filter dialog box follows below. After clicking on OK the filtering is done on all traces of the display if no traces have been selected before (may take some time, depending on number and length of traces and on the CPU power of the host). The resulting traces are displayed on screen. The original traces remain in memory but are not visible any more. When the filter is changed, the previously computed filter output traces are deleted and a new filter operation is started using the original traces. Deselecting the filters (filter None) restores the original (unfiltered) traces on screen. Once a filter is selected in remains active until it is explicitly switched off in the filter dialog box or a Read New is executed (see command Read). Read Again keeps the filter, that means the traces read in are filtered automatically.

SHM knows three kinds of filters, FFT filters, recursive filters and tabulated filters. While tabulated filters should be used only under special circumstances, filter operations described here may be performed with either of the other two types. Practically there are some contraints, however. In general, the recursive filters are faster in computation time, in particular for long traces. The computation time for recursive filters grows linearly with the number of sample points, N, while the FFT filters need time proportional to N*log(N), due to the FFT operation. In some cases the FFT filters turn out to be numerically more stable than recursive ones. FFT filters have one filter file for all possible sample rates of the input traces, recursive filters need a separate file for each rate (see filter files of SHM). The program package supplies utility programs to create FFT/recursive filters for several standard filters, special filters have to be created manually by the user. In general, creating FFT filter files is much easier than computing recursive filter coefficients.

If one or more traces have been selected (by left-clicking on the station name or number) before calling the filter dialog box, it opens up with a restricted list of available filters. The chosen filter (usually a Butterworth highpass, lowpass or bandpass) then is applied to the selected traces only and remains active as an additional filter to these traces until it is switched off explicitely. This is useful e.g. for noisy traces to cut out the noise signals in addition to simulation filters applied. But be aware that such an additional Butterworth may change the simulation filtering severely and affect amplitude measurements on simulated seismograms. To mark such traces with additional Butterworth filters visually, they are displayed in a different colour (controlled by configuration parameters colour_addfil_red, colour_addfil_green, colour_addfil_blue, default is dark violet). Different additional filters may be chosen for different traces. To switch off additional filters select the appropriate traces again and choose no filter (None) in the filter dialog box.

The filter dialog box

This dialog box has the following elements

  • List of radio buttons with available filters. None switches of the filter option, all other selections set a new filter. A description of the filters follows.
  • Corner frequencies and order of filter. This refers only to Butterworth Bandpass (BP), Lowpass (LP), and Highpass (HP) filters. BP reads lower bound, upper bound and order, LP reads lower bound and order, HP reads upper bound and order. On all other filter selections these values have no meaning.
  • Name of the filter. Is set automatically when a filter is selected. When selecting filter Other this field has to be edited manually (this option is not tested very well).
  • autocut value option box. Usually filtered traces start with ringing at the beginning of the trace, caused by the discontinuity at the beginning of the input trace. The length of this filter noise depends on the dominant frequency of the filter. The length given here is cut off from each filtered trace so that the ringing does not appear on the screen. Known bug: this causes a shift of the time window selected by Set Time Window.

List of available filters:

  • None: switches off the filter option (see above).
  • Standard BP: Despite the name this is a simulation filter emulating a GRF record with BP from 0.8 to 2.2 Hz.
  • WWSSN-SP: simulation filter of this standard instrument
  • LRSM-SP: simulation filter, useful for more highfrequent signals
  • Wood-Anderson: simulation filter, mainly used for magnitude determinations
  • KIRNOS: simulation filter, similar to displacement
  • SRO-LP: simulation filter, useful for signals of long periods, surface waves
  • WWSSN-LP: simulation filter, not as longperiodic as SRO-LP
  • LRSM-LP: simulation filter
  • Butterworth BP: Butterworth Bandpass, see above
  • Butterworth LP: Butterworth Lowpass, see above
  • Butterworth HP: Butterworth Highpass, see above
  • Other: manual filter preparation, not really tested, for experts only :)

Whether SHM uses FFT or recursive filters may be set (temporarily) in Setup (parameter filter type) or (permanently) in Configure (parameter filter_type). The Read New option of command Read can apply a default filter after reading a new data set automatically. This also may be changed in Setup (parameter default filter) or Configure.

For further information please see the topics

back to documentation index