The Filter Design Window

The Filter Design Window

This window allows the user to design Butterworth filters for use in the associated WaveView window. The filter design is split into two sections, a high-pass and a low-pass filter. Used in combination, a bandpass filter results.

The left mouse-button selects the high-pass corner, and the right mouse-button selects the low-pass corner. For each filter a separate order can be selected. Note that due to the instability of high-order filters, any order >4 will be split into cascaded 4th order filters. E.g. a 14th order filter will be implemented as four filters with order 4-4-4-2. A side-effect of this is that the 3dB corner will be slightly different for a selected filter order greater than 4.

Up to two data sections can be selected from a WaveView window, and the PSDs will be displayed in the Filter Design Window (not referenced to the dB scale on the left). This can be useful for comparing the differences in two signals, which can aid filter design. E.g. examining the difference in the spectra pre-event and during event will show the frequencies involved in the event. The high- and low-pass corners can then be visually positioned for optimum filtering of the event. See Waveview Data Selection for more info. The FFT-Window size selector drop-down list can be used to select the window size (in samples) for the PSD calculation. A larger window increases the long-period result, but loses resolution at high frequencies. Conversely, a small window increases resolution at high-frequency, at the cost of long-period range.

Sometimes, the filtered result can have very small signal levels, which when converted back into integer samples can lose resolution. In this case, applying Gain to the filtered result can maintain the resolution. Use the "Gain" input-box to control this setting. If both high- and low-pass filters are selected, the gain is divided equally between the two, so that the overall gain matches the preset value.

Designed filters can be saved for quick re-selection at a future time. Once the filter has been desgined, press the Save button. You will be prompted for a name for the filter, with a default already selected which describes the selection. You can change this description if necessary. Saved filters can quickly be reselected from the drop-down list of the filter button in the WaveView windw. This drop-down list can also be used to delete saved filters.

The Cancel button closes the window without applying the filter to the WaveView window.

The OK button applies the filter, and closes the window.

The Apply button applies the filter, but doesn't close the window.

If you check the Auto-Apply box, any changes made to the filter design will instantly be applied to the WaveView window. This way, you can interactively see the results of design changes.

Note that sps selector is used only for display purposes, and to control the range of frequencies available, and does not have any influence on the actual filtered result. (the maximum frequency is 1/2 of the sps value, and the range of frequency is four decades.)

See Also:

Data Selection