Windowed Sinc filters combine a "sinc" (sin x / x) function with one of ScopeFIR's data window. These filters are sometimes preferred over Parks-McClellan FIR filters for their smother passbands and stopbands.

In the Time domain, the sinc function corresponds to a "brick-wall" filter in the Frequency domain. A brick-wall frequency response is simply an idealized response which has flat gain in the passband, and infinite attenuation in the stopband. However, in practice, it is not possible to realize brick-wall filters because they would have an infinite number of taps. (And let's face it: who has the time to calculate an infinite number of taps?). Therefore, we need to truncate the sinc function to span some practical number of taps. However, truncating the sinc function causes problems of its own. Therefore, we use data windowing to mitigate the effects of truncation.

The Windowed Sinc Editor is shown below:

To design a Windowed Sinc filter:

• Specify the sampling frequency by hitting the Sampling Frequency button in the upper left. You will be prompted to enter the numerical frequency and the units of frequency/time.
• Specify the Number of Taps. As usual, the passband tends to be flatter and the stopband tends to have more attenuation as you increase the number of taps.
• Select a Window Type. The Kaiser-Bessel window is the default. The Kaiser-Bessel window uses a parameter called "Alpha" which controls its shape. You can adjust Alpha to trade attenuation in the stopband against sharpness of the transition at the corner frequency. (Note: other data window types do not use Alpha.) Unlike the Parks-McClellan design method, Windowed Sinc filters do no allow you to specify passband ripple and stopband attenuation, so you must experiment with different window types, and different values of Alpha to achieve your desired response.
• Select a Filter Type from among Lowpass, Highpass, Bandpass, and Bandstop.
• Enter the filter's corner frequencies. Lowpass and Highpass filters use only a single corner frequency--the frequency at which the filter begins to transition between its passband and its stopband. Bandpass and Bandstop filters use a second corner frequency, which, together with the first corner frequency, specifies the passband or stopband. For Windowed Sinc filters, the passband gain will be approximiately 0 dB, and the gain at the corner frequencies will be approximately -6 dB.
• When all specifications have been entered, hit the Design button to actually design the filter coefficients.

Once a filter has been designed, you can check its Frequency Response. If the Automatic option is enabled, ScopeFIR will show you the frequency response at zero, at half the Sampling Frequency, and at the first corner frequency. If the Filter Type is Bandpass or Bandstop, ScopeFIR also will automatically show you the response at the second corner frequency, and at a point midway between the two corner frequencies (which is the center of the passband or stopband). (See Using the Frequency Response List for more information on the Frequency Response list.)