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| 1 | +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| 2 | +<html> |
| 3 | +<head> |
| 4 | +<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> |
| 5 | +<title>Edit time domain sample filter dialog</title> |
| 6 | +</head> |
| 7 | +<body><div><h1>Edit time domain filter dialog</h1> |
| 8 | + |
| 9 | +<h2>Przegląd</h2> |
| 10 | + |
| 11 | +<p>This file describes how to create and edit time domain sample filters using |
| 12 | + Edit time domain sample filter dialog.</p> |
| 13 | + |
| 14 | +<h2>Specifying filter parameters</h2> |
| 15 | + |
| 16 | +<p>To create or edit a filter, you need to specify the following parameters:</p> |
| 17 | + |
| 18 | +<ul> |
| 19 | + <li><b>Filter type</b> - describes what type of filter you want to design.<br/> |
| 20 | + Possible values: |
| 21 | + <ul> |
| 22 | + <li>low-pass</li> |
| 23 | + <li>high-pass</li> |
| 24 | + <li>band-pass</li> |
| 25 | + <li>band-stop</li> |
| 26 | + </ul> |
| 27 | + <li><b>Filter family</b> - is the type of approximation |
| 28 | + function which will be used to design this filter.<br/> |
| 29 | + Possible values: |
| 30 | + <ul> |
| 31 | + <li>Butterworth</li> |
| 32 | + <li>Chebyshev I</li> |
| 33 | + <li>Chebyshev II</li> |
| 34 | + <li>Elliptic</li> |
| 35 | + </ul> |
| 36 | + <li><b>Passband edge frequency 1</b> - is the passband boundary frequency |
| 37 | + which separates the passband from the transition band. |
| 38 | + <li><b>Passband edge frequency 2</b> - if the filter is a bandpass or a bandstop |
| 39 | + filter, then there are two transition bands. In that case passband edge |
| 40 | + frequency 1 separates the passband from the first transition band (and |
| 41 | + its frequency is lower), and the second passband edge frequency separates |
| 42 | + the passband from the second transition band. |
| 43 | + <li><b>Stopband edge frequency 1</b> - is the edge frequency which is the |
| 44 | + boundary between the stopband and the transition band in the filter's |
| 45 | + frequency response. |
| 46 | + <li><b>Stopband edge frequency 2</b> - if the filter is bandpass or bandstop, |
| 47 | + then there are two transition bands. In that case stopband edge frequency 1 |
| 48 | + separates the stopband from the first transition band (and its frequency is |
| 49 | + lower), and the second passband edge frequency separates the passband from the |
| 50 | + second transition band. |
| 51 | + <li><b>Passband ripple</b> - is the maximum frequency response fluctuations |
| 52 | + which is allowed in the passband. For all frequencies in the passband, their |
| 53 | + attenuation must be less or equal than the value set by this control. |
| 54 | + <li><b>Stopband attenuation</b> - the minimum attenuation in the stopband. |
| 55 | + For all frequencies in the stopband, their frequency response magnitude must be |
| 56 | + attenuated at least by the value set by this control. |
| 57 | +</ul> |
| 58 | + |
| 59 | +<p>The image below presents a frequency response of a Chebyshev I high-pass filter. |
| 60 | +The filter was designed using the following parameters: |
| 61 | + |
| 62 | +<ul> |
| 63 | + <li><b>Filter type</b> = high-pass</li> |
| 64 | + <li><b>Filter family</b> = Chebyshev I</li> |
| 65 | + <li><b>Passband edge frequency 1</b> = 20 Hz</li> |
| 66 | + <li><b>Stopband edge frequency 1</b> = 16 Hz</li> |
| 67 | + <li><b>Passband ripple</b> = 3 dB</li> |
| 68 | + <li><b>Stopband attenuation</b> = 40 dB</li> |
| 69 | +</ul> |
| 70 | + |
| 71 | +<p>Please note that the frequency response shown fulfills the requirements given. |
| 72 | +For example - the stopband ends at 16 Hz and all frequencies in the stopband are |
| 73 | +attenuated by at least 40 dB. The passband begins at 20 Hz and all frequencies in |
| 74 | +the passband are attenuated - at most - by 3 dB.</p> |
| 75 | + |
| 76 | +<img src="images/highpassFilterFrequencyResponse.png" alt="High-pass Chebyshev I |
| 77 | +filter, passband edge frequency 1 = 16 Hz, stopband edge frequency 1 = 20 Hz, |
| 78 | +passband ripple = 3 dB, stopband attenuation = 40dB"/> |
| 79 | + |
| 80 | +<p>An analogous frequency response graph for a band-stop filter would have two |
| 81 | +passbands (first: from 0 Hz to passband edge frequency 1, second: begins at passband |
| 82 | +edge frequency 2), one stopband (from stopband edge frequency 1 to stopband edge |
| 83 | +frequency 2), and two transition bands. A frequency response graph for a band-pass |
| 84 | +filter will have two stopbands, one passband, and two transition bands</p> |
| 85 | + |
| 86 | +<p><b>Important - the maximum frequency that can be set using the frequency |
| 87 | +controls is the Nyquist frequency which is equal to the half of the |
| 88 | +signal's sampling frequency.</b></p> |
| 89 | + |
| 90 | +<h2>Drawing filter frequency response</h2> |
| 91 | + |
| 92 | +<p>A filter frequency response is a graph showing the filter's output spectrum, |
| 93 | + that is - how much is each frequency's magnitude attenuated in the signal |
| 94 | + which passes through the filter</p> |
| 95 | + |
| 96 | +<p>To draw the frequency response of the filter designed using the controls |
| 97 | + in the Filter parameters panel, press the "Draw filter frequency response" |
| 98 | + button.</p> |
| 99 | + |
| 100 | +<p>If the filter parameters are correct, the filter frequency response is drawn. |
| 101 | + Otherwise, a dialog containing error message is shown.</p> |
| 102 | + |
| 103 | +<h2>Error messages</h2> |
| 104 | + |
| 105 | +<p>The most common error message says that the designed filter order is too big. |
| 106 | + This happens, when the filter parameters are too strict. The more strict the |
| 107 | + parameters are, the harder for the designer is to design a filter fulfilling |
| 108 | + the specification given. For example - it is very hard for the designer to |
| 109 | + design a filter having very narrow transition bands (a transition band is |
| 110 | + narrow when a passband edge frequency is very close to the corresponding |
| 111 | + stopband edge frequency), a very small value of ripple in the passband and |
| 112 | + having all of the frequencies in the stopband strongly attenuated.</p> |
| 113 | + |
| 114 | +<p>If such an error shows up, please make the parameters less strict.</p> |
| 115 | + |
| 116 | +<p>Another solution for that error is changing the Filter family of the filter. |
| 117 | + Filters approximated using the Chebyshev approximation functions tend to have |
| 118 | + smaller filter order comparing |
| 119 | + to Buttorworth filters having similar filter parameters. What's more, filters |
| 120 | + approximated using the Elliptic functions have smaller filter order |
| 121 | + comparing to a Chebyshev filter having similar parameters.</p> |
| 122 | + |
| 123 | +</div> |
| 124 | +</body> |
| 125 | +</html> |
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