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allpasslp2bs

Allpass filter for lowpass to bandstop transformation

Syntax

```[AllpassNum,AllpassDen] = allpasslp2bs(Wo,Wt) ```

Description

`[AllpassNum,AllpassDen] = allpasslp2bs(Wo,Wt)` returns the numerator, `AllpassNum`, and the denominator, `AllpassDen`, of the second-order allpass mapping filter for performing a real lowpass to real bandstop frequency transformation. This transformation effectively places one feature of an original filter, located at frequency -`W`o, at the required target frequency location, Wt1, 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. This transformation implements the "Nyquist Mobility," which means that the DC feature stays at DC, but the Nyquist feature moves to a location dependent on the selection of `W`o and `W`t.

Relative positions of other features of an original filter 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. After the transformation feature `F`2 will precede `F`1 in the target filter. 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.

Examples

Design the allpass filter changing the lowpass filter with cutoff frequency at `W`o`=0.5` to the real bandstop filter with cutoff frequencies at `W`t1`=0.25` and `W`t2`=0.375`:

```Wo = 0.5; Wt = [0.25, 0.375]; [AllpassNum, AllpassDen] = allpasslp2bs(Wo, Wt); [h, f] = freqz(AllpassNum, AllpassDen, 'whole'); plot(f/pi, abs(angle(h))/pi, Wt, Wo, 'ro'); title('Mapping Function Wo(Wt)'); xlabel('New Frequency, Wt'); ylabel('Old Frequency, Wo');```

In the figure, you find the mapping filter function as determined by the example. Note the response is normalized to π:

Arguments

VariableDescription
`Wo`

Frequency value to be transformed from the prototype filter

`Wt`

Desired frequency locations in the transformed 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

Constantinides, A.G., “Spectral transformations for digital filters,” IEEE® Proceedings, vol. 117, no. 8, pp. 1585-1590, August 1970.

Nowrouzian, B. and A.G. Constantinides, “Prototype reference transfer function parameters in the discrete-time frequency transformations,” Proceedings 33rd Midwest Symposium on Circuits and Systems, Calgary, Canada, vol. 2, pp. 1078-1082, August 1990.

Nowrouzian, B. and L.T. Bruton, “Closed-form solutions for discrete-time elliptic transfer functions,” Proceedings of the 35th Midwest Symposium on Circuits and Systems, vol. 2, pp. 784-787, 1992.

Constantinides, A.G., “Design of bandpass digital filters,” IEEE Proceedings, vol. 1, pp. 1129-1231, June 1969.