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Transform IIR lowpass filter to IIR complex bandstop filter

`[Num,Den,AllpassNum,AllpassDen] =
iirlp2bsc(B,A,Wo,Wt)[G,AllpassNum,AllpassDen] = iirlp2bsc(Hd,Wo,Wt)`

where `Hd` is a `dfilt` object

`[Num,Den,AllpassNum,AllpassDen] =
iirlp2bsc(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 a first-order real lowpass to complex bandstop frequency
transformation.

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

This transformation effectively places one feature of an original
filter, located at frequency -W_{o}, at the required
target frequency location, W_{t1}, and the second
feature, originally at `+`W_{o},
at the new location, W_{t2}. It is assumed that
W_{t2} is greater than W_{t1}.
Additionally the transformation swaps passbands with stopbands in
the target filter.

Relative positions of other features of an original filter do
not change in the target filter. This means that it is possible to
select two features of an original filter, F_{1} and
F_{2}, with F_{1} preceding
F_{2}. Feature F_{1} will
still precede F_{2} after the transformation.
However, the distance between F_{1} and F_{2} will
not be the same before and after the transformation.

Choice of the feature subject to the lowpass to bandstop transformation is not restricted only 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.

Lowpass to bandpass transformation can also be used for transforming other types of filters; e.g., real notch filters or resonators can be doubled and positioned at two distinct desired frequencies at any place around the unit circle forming a pair of complex notches/resonators. This transformation can be used for designing bandstop filters for band attenuation or frequency equalizers, from the high-quality prototype lowpass filter.

`[G,AllpassNum,AllpassDen] = iirlp2bsc(Hd,Wo,Wt)` returns
transformed `dfilt` object `G` with
a bandstop magnitude 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.

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 `W`_{t1}`=0.25` and `W`_{t2}`=0.75` creating
a complex bandstop filter:

[num, den] = iirlp2bsc(b, a, 0.5, [0.25, 0.75]);

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

fvtool(b, a, num, den);

The last command in the example plots both filters in the same window so you can compare the results.

Variable | Description |
---|---|

B | Numerator of the prototype lowpass filter |

A | Denominator of the prototype lowpass filter |

Wo | Frequency value to be transformed from the prototype filter. It should be normalized to be between 0 and 1, with 1 corresponding to half the sample rate. |

Wt | Desired frequency locations in the transformed target filter. They should be normalized to be between -1 and 1, with 1 corresponding to half the sample rate. |

Num | Numerator of the target filter |

Den | Denominator of the target filter |

AllpassNum | Numerator of the mapping filter |

AllpassDen | Denominator of the mapping filter |

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