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### 7.14 Some Lesser-Known Filter Characteristics

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The filter types described above are the classic types. Their characteristics depend on particular mathematical relationships such as Bessel functions or Chebyshev polynomials. There is however no rule that you must restrict yourself to using the “official” filters. There are in fact an infinite number of filter characteristics that can be used if necessary. As an example, look at a simple second-order filter. It only has two parameters that define its behaviour—the cutoff frequency and the Q. It is the Q that determines the filter type; we have just seen that a Q of 0.50 gives a Linkwitz–Riley filter, a Q of 0.578 gives a Bessel filter, a Q of 0.707 gives a Butterworth filter, a Q of 0.956 gives a 1 dB-Chebyshev filter, and so on; we have....