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iirlp2xn

Transform IIR lowpass filter to IIR real N-point filter

Syntax

[Num,Den,AllpassNum,AllpassDen] = iirlp2xn(B,A,Wo,Wt)
[Num,Den,AllpassNum,AllpassDen] = iirlp2xn(B,A,Wo,Wt,Pass)
[G,AllpassNum,AllpassDen] = iirlp2bpc(Hd,Wo,Wt), where Hd is a dfilt object
[G,AllpassNum,AllpassDen] = iirlp2bpc(...,Pass)

Description

[Num,Den,AllpassNum,AllpassDen] = iirlp2xn(B,A,Wo,Wt) returns the numerator and denominator vectors, Num and Den respectively, of the target filter transformed from the real lowpass prototype by applying an Nth-order real lowpass to real multipoint frequency transformation, where N is the number of features being mapped. By default the DC feature is kept at its original location.

[Num,Den,AllpassNum,AllpassDen]= iirlp2xn(B,A,Wo,Wt,Pass) allows you to specify an additional parameter, Pass, which chooses between using the "DC Mobility" and the "Nyquist Mobility." In the first case the Nyquist feature stays at its original location and the DC feature is free to move. In the second case the DC feature is kept at an original frequency and the Nyquist feature is allowed to move.

It also returns the numerator, AllpassNum, and the denominator, AllpassDen, of the allpass mapping filter. The prototype lowpass filter is given with the numerator specified by B and the denominator specified by A.

Parameter N also specifies the number of replicas of the prototype filter created around the unit circle after the transformation. This transformation effectively places N features of an original filter, located at frequencies Wo1,...,WoN, at the required target frequency locations, Wt1,...,WtM.

Relative positions of other features of an original filter are the same in the target filter for the Nyquist mobility and are reversed for the DC mobility. For the Nyquist mobility this means that it is possible to select two features of an original filter, F1 and F2, with F1 preceding F2. Feature F1 will still precede F2 after the transformation. However, the distance between F1 and F2 will not be the same before and after the transformation. For DC mobility feature F2 will precede F1 after the transformation.

Choice of the feature subject to this transformation is not restricted to the cutoff frequency of an original lowpass filter. In general it is possible to select any feature; e.g., the stopband edge, the DC, the deep minimum in the stopband, or other ones. The only condition is that the features must be selected in such a way that when creating N bands around the unit circle, there will be no band overlap.

This transformation can also be used for transforming other types of filters; e.g., notch filters or resonators can be easily replicated at a number of required frequency locations. A good application would be an adaptive tone cancellation circuit reacting to the changing number and location of tones.

[G,AllpassNum,AllpassDen] = iirlp2xn(Hd,Wo,Wt) returns transformed dfilt object G with an IIR real N-point filter frequency response. The coefficients AllpassNum and AllpassDen represent the allpass mapping filter for mapping the prototype filter frequency Wo and the target frequencies vector Wt. Note that in this syntax Hd is a dfilt object with a lowpass magnitude response.

[G,AllpassNum,AllpassDen] = iirlp2xn(...,Pass) returns transformed dfilt object G with an IIR real N-point filter frequency response. This syntax allows you to specify an additional parameter, Pass, which chooses between using the "DC Mobility" and the "Nyquist Mobility." In the first case the Nyquist feature stays at its original location and the DC feature is free to move. In the second case the DC feature is kept at an original frequency and the Nyquist feature is allowed to move.

The coefficients AllpassNum and AllpassDen represent the allpass mapping filter for mapping the prototype filter frequency Wo and the target frequencies vector Wt. Note that in this syntax Hd is a dfilt object with a lowpass magnitude response.

Examples

Design a prototype real IIR halfband filter using a standard elliptic approach:

[b, a] = ellip(3, 0.1, 30, 0.409);

Move the cutoffs of the prototype filter to the new locations Wt1=0.25 and Wt2=0.75 creating a real bandpass filter:

[num, den] = iirlp2xn(b, a, [-0.5 0.5], [0.25 0.75], ...
pass');

Verify the result by comparing the prototype filter with the target filter:

fvtool(b, a, num, den);

iirlp2xn has created the desired bandpass filter with the cutoff locations specified in the command.

Arguments

VariableDescription
B

Numerator of the prototype lowpass filter

A

Denominator of the prototype lowpass filter

Wo

Frequency values to be transformed from the prototype filter

Wt

Desired frequency locations in the transformed target filter

Pass

Choice ('pass'/'stop') of passband/stopband at DC, 'pass' being the default

Num

Numerator of the target filter

Den

Denominator of the target filter

AllpassNum

Numerator of the mapping filter

AllpassDen

Denominator of the mapping filter

Frequencies must be normalized to be between 0 and 1, with 1 corresponding to half the sample rate.

References

Cain, G.D., A. Krukowski and I. Kale, "High Order Transformations for Flexible IIR Filter Design," VII European Signal Processing Conference (EUSIPCO'94), vol. 3, pp. 1582-1585, Edinburgh, United Kingdom, September 1994.

Krukowski, A., G.D. Cain and I. Kale, "Custom designed high-order frequency transformations for IIR filters," 38th Midwest Symposium on Circuits and Systems (MWSCAS'95), Rio de Janeiro, Brazil, August 1995.

See Also

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