BSL PRO Software Guide - Biopac

298 Part EBlackman 0.42323 0.49755 0.07922 0.00000Blackman -61 0.44959 0.49364 0.05677 0.00000Blackman -67 0.42323 0.49755 0.07922 0.00000Blackman -74 0.40217 0.49703 0.09392 0.00183Blackman -92 0.35875 0.48829 0.14128 0.01168Hamming 0.54000 0.46000 0.00000 0.00000Hanning 0.50000 0.50000 0.00000 0.00000Kaiser-Bessel 0.40243 0.49804 0.09831 0.00122Rectangle 0.00000 0.00000 0.00000 0.00000Window functions are used for three purposes in the Biopac Student Lab PRO software:a) Window functions are applied to the impulse response in the FIR digital filtering functions.b) Window functions are applied as part of the FFT function.c) Window functions are applied as part of the Derivative transformation.Digital filteringWhen a window is used in digital filtering, the impulse response of the filter (rather than the data itself) is modified.When the impulse response smoothly approaches zero at both the beginning and end of the data, this worksrelatively well.When the impulse response is not so well behaved, edge effects occur. To minimize edge effects:— Window, or force the edges of the impulse response to smoothly approach zero. The exact process depends onthe Window selected (see previous page).— Increase the number of coefficients for the FIR filter used to transform the data.FFTThe FFT function also windows data, although the nature of the windowing function is somewhat different in thesense that the window operates on the data. One of the assumptions of the FFT is that the input data is an infinitelyrepeating signal with the endpoint wrapping around. In practice, the endpoints are almost never exactly equal. Youcan check this by choosing the Delta measurement item from the measurement pop-up menus, which returns theamplitude difference between the first selected point and the last. To the extent that the endpoints differ, the FFToutput will produce high frequency components as an artifact of the transformation.By windowing the data, the effects of this phenomenon are greatly diminished. When data are windowed, a windowis moved across the data, much as the smoothing function moves across the data. Whereas the smoothing functionsimply takes the average of a specified number of points, each type of window weights the data somewhatdifferently.Visit the online support center at www.biopac.com