FFT Filter Dialog

The FFT Filter Dialog can be used on a plot or on a matrix. When the active window is a 2D plot this dialog can be used to apply a one dimensional FFT filter to any of the curves in the active plot layer, see the filtering section for more details.

If the filter type is set to Low Pass FFT Filter QtiPlot removes all the frequencies above a cut-off frequency from the input signal, while keeping the other frequencies unaltered. You can select the cut-off frequency of the filter using the Frequency cutoff input box.

Figure 5-74. The FFT Filter -> Low Pass... dialog.

If the filter type is set to High Pass FFT Filter QtiPlot sets to zero all the frequencies below the specified cut-off frequency. You can decide if the DC offset (zero frequency) should be kept or not in the filtered signal by checking the Keep DC Offset box.

Figure 5-75. The FFT Filter -> High Pass... dialog.

If the filter type is set to Band Pass FFT Filter QtiPlot cuts both high and low frequencies present in an input signal. You can select the curve to filter and both the low and high cutoff frequencies of the filter. Also, you can decide if the DC offset (zero frequency) should be kept or not in the filtered signal by checking the Keep DC Offset box.

Figure 5-76. The FFT Filter -> Band Pass... dialog.

If the filter type is set to Band Block FFT Filter QtiPlot removes a band of frequencies from a signal while leaving those frequencies above and below the stop band. You can select the curve to filter and both the lower and upper stop-band frequencies of the filter. Also, you can decide if the DC offset (zero frequency) should be kept or not in the filtered signal by checking the Keep DC Offset box.

Figure 5-77. The FFT Filter -> Band Block... dialog.

After performing the operation QtiPlot creates a new table containing the filtered data, and adds a new curve to the current layer. The curve is a plot of the filtered data.

If the active window is a matrix the FFT Filter Dialog can be used to apply a two dimensional FFT filter, for more details see the following article by Paul Bourke: http://paulbourke.net/miscellaneous/imagefilter/.

Figure 5-78. The FFT Filter Dialog dialog box for a matrix.

Please note that the input matrix will be modified in order to store the filtered data, but you can revert all the changes by pressing the Undo button.