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IIRFilters.TIirFrequencyTransform Enumeration
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Methods for frequency band transformation of IIR filters.

Pascal
TIirFrequencyTransform = (
  ftStateSpaceAnalog,
  ftZeroPoleAnalog,
  ftZeroPoleDiscrete
);
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Description 
ftStateSpaceAnalog 
Analog filter prototype in the zero-pole form is converted to the state-space form. Frequency band transformations are applied first (in the s-domain) followed by a bilinear transform and a finally the filter is converted from the state-space form back to the zero-pole form. 
ftZeroPoleAnalog 
Analog filter prototype is obtained in the zero-pole form. Frequency band transformations are applied first (in the s-domain) followed by a bilinear transform. 
ftZeroPoleDiscrete 
Analog filter prototype is obtained in the zero-pole form. A bilinear transform is applied first to map the filter to z-domain. Frequency band transformations are applied in the z-domain. This method is usefull in cases where the mapping from s-domain to z-domain is not bilinear transformation, but impulse invariance or matched z-transform. 
Description

Defines supported approaches for frequency band transformations of IIR filters. Frequency band transformations are used to convert a normalized prototype analog IIR lowpass filter with a fixed cutoff frequency to arbitrary lowpass, highpass, bandpass or bandstop filter. The frequency band transformation can be applied before or after the filter has been transformed from s-domain to z-domain.

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