Geopsy: Taper

Taper, reached from the Waveform menu, applies a tapering function (a.k.a. apodization function, a.k.a. window function) to the signals in the active signal viewer. The applied taper is a tapered cosine window (Tukey window). By clicking on Taper, a pop-up window appears with the following options:

• "Time Limits" allows to specify the time interval out of which the signal will be set to 0. The checkbox "Use only the properties of the first signal" forces to use the same tapering function for all signals (the one computed for the first signal), whatever the properties of the other signals (starting time, length...).
• "Width" allows to give the width, in % of the time interval, of the slope of the taper as also defined for the cosine taper filter but in the frequency domain.

Since the Fourier Transform of a boxcar function (too simple tapering window, equivalent to no taper) is a sinc function, the Fourier Transform of a signal without applying an appropriate Taper will be the multiplication of the true Fourier Transform by the sinc function. On the contrary, appropriate tapers have a Fourier Transform closer to a Dirac function that distorts less the Fourier Transform of the signal. It is therefore necessary to apply a taper before going to the frequency domain.

"Taper" modifies the temporary signals in the active viewer.

See also: Window function (Wikipedia)