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:
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.)
© 1997-2013 Iowegian International Corporation. All rights reserved.
"ScopeDSP", "ScopeDSP Suite", "ScopeFIR", "ScopeIIR", and the Iowegian logo are trademarks of Iowegian International Corporation