Edit time domain filter dialog

Summary

This file describes how to create and edit time domain sample filters using Edit time domain sample filter dialog.

Setting filters parameters

To create or edit a filter, you need to specify the following parameters:

The image below presents a frequency response of a Chebyshev I high-pass filter. The filter was designed using the following parameters:

Please note how this frequency response fulfills given requirements; for example, the stopband ends at 16 Hz and all the frequencies in the stopband are attenuated by at least 40 dB. The passband begins at 20 Hz and all frequencies in the passband are attenuated—at most—by 3 dB.

High-pass Chebyshev I
                 filter, passband edge frequency 1 = 16 Hz, stopband edge frequency 1 = 20 Hz,
                 passband ripple = 3 dB, stopband attenuation = 40dB

An analogous frequency response graph for a band-stop filter would have two passbands (first: from 0 Hz to passband edge frequency 1, second: begins at passband edge frequency 2), one stopband (from stopband edge frequency 1 to stopband edge frequency 2), and two transition bands. A frequency response graph for a band-pass filter will have two stopbands, one passband, and two transition bands

Important: the maximum frequency that can be set using the frequency controls is the Nyquist frequency which is equal to the half of the signal's sampling frequency.

Drawing filter frequency response

A filter frequency response is a graph showing the filter's output spectrum, that is, how much each frequency's magnitude is attenuated in the signal which passes through the filter

To draw the frequency response of the filter designed using the controls in the Filter parameters panel, press the "Draw filter frequency response" button.

If the filter parameters are correct, the filter frequency response is drawn. Otherwise, a dialog containing error message is shown.

Error messages

The most common error message says that the designed filter order is too big. This happens, when the filter parameters are too strict. The more strict the parameters are, the harder it is for the algorithm to design a filter fulfilling such specifications. For example, it is very hard for the algorithm to design a filter having very narrow transition bands (a transition band is narrow when a passband edge frequency is very close to the corresponding stopband edge frequency), a very small value of ripple in the passband and having all of the frequencies in the stopband strongly attenuated.

If such an error shows up, please release the requirements by modifying the parameters.

Another solution for that error is changing the "Filter family" of the filter. Filters approximated using the Chebyshev approximation functions tend to have smaller filter order comparing to Buttorworth filters having similar filter parameters. What's more, filters approximated using the Elliptic functions have smaller filter order comparing to a Chebyshev filter having similar parameters.