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#X text 186 -4 BUTTERWORTH FILTER;
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#X text 24 20 The butterworth filter can be configured for low-pass
\, high-pass \, and shelving \, depending on the placement of the poles
and zeros. For low-pass \, the poles are placed to set the cutoff frequency
and the zeros are at -1 (the Nyquist). Leaving the poles fixed and
moving the zeros then gives shelving filters. In this example \, the
actual filtering is relegated to an abstraction (butterworth3~) which
takes frequencies corresponding to the pole and zero placement.;
#X text 24 147 The butterworth3~ abstraction computes filter coeffients
using control messages \, and so it is not suitable for continuously
time-varying Butterworth filters. For that \, it is often appropriate
to use time-saving approximations \, but precisely which approximations
to use will depend on the way the filter is to be used.;
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