# dsp.AllpassFilter System object

Package: dsp

Single section or cascaded allpass filter

 Note:   MATLAB® code with `LatticeCoefficients`, `AllpassCoefficients`, and `WDFCoefficients` properties set to cell arrays will error in a future release. Set these properties to array values instead.

## Description

The `AllpassFilter` object filters each channel of the input using Allpass filter implementations. This System object™ supports code generation. To import this object into Simulink®, use the MATLAB System block.

To filter each channel of the input:

1. Define and set up your Allpass filter. See Construction.

2. Call `step` to filter each channel of the input according to the properties of `dsp.AllpassFilter`. The behavior of `step` is specific to each object in the toolbox.

## Construction

`Allpass = dsp.AllpassFilter` returns an Allpass filter System object, `Allpass`, that filters each channel of the input signal independently using an allpass filter, with the default structure and coefficients.

```Allpass = dsp.AllpassFilter('PropertyName',PropertyValue, ...)``` returns an Allpass filter System object, `Allpass`, with each property set to the specified value.

## Properties

 `Structure` Internal allpass filter structure You can specify the internal allpass filter implementation structure as one of | `Minimum multiplier` | `Lattice` | `Wave Digital Filter`. The default is `Minimum multiplier`. Each structure uses a different set of coefficients, independently stored in the corresponding object property. `AllpassCoefficients` Allpass polynomial coefficients Specify the real allpass polynomial filter coefficients. This property is applicable only when the `Structure` property is set to `Minimum multiplier`. Specify this property as either an N-by-`1` or N-by-`2` matrix of N first-order or second-order allpass sections. The default value of this property is `[-2^(-1/2) 0.5]`. The default value defines a stable second-order allpass filter with poles and zeros located at ±π/3 in the Z plane. This property is tunable. `WDFCoefficients` Wave Digital Filter allpass coefficients Specify the real allpass coefficients in the Wave Digital Filter form. This property is only applicable when the `Structure` property is set to `Wave Digital Filter`. Specify this property as either a N-by-`1` or N-by-`2` matrix of N first-order or second-order allpass sections. All elements must have absolute values less than or equal to `1`. The default value for this property is [`1/2, -2^(1/2)/3`]. This value is a transformed version of the default value of `AllpassCoefficients`, computed using `allpass2wdf(AllpassCoefficients)`. These coefficients define the same stable second-order allpass filter as when `Structure` is set to ```'Wave Digital Filter'```. This property is tunable. `LatticeCoefficients` Lattice allpass coefficients Specify the real or complex allpass coefficients as lattice reflection coefficients. This property is applicable only if the `Structure` property is set to `Lattice`. Specify this property as either a row vector (single-section configuration) or a column vector. The default is` [-2^(1/2)/3, 1/2]`. This value is a transformed and transposed version of the default value of `AllpassCoefficients`, computed using `transpose(tf2latc([1 h.AllpassCoefficients]))`. These coefficients define the same stable second-order allpass filter as when `Structure` is set to `'Lattice'`. This property is tunable. `TrailingFirstOrderSection` Indicate if last section is first order Indicate if last section is first order or second order. When you set `TrailingFirstOrderSection` to `true`, the last section is considered to be first-order, and the second element of the last row of the N-by-2 matrix is ignored. When you set `TrailingFirstOrderSection` to `false`, the last section is considered to be second-order. The default is `false`.

## Methods

 clone Create System object with same property values isLocked Locked status for input attributes and nontunable properties release Allow property value and input characteristics changes reset Reset internal states of a System object step Process inputs using allpass filter

For additional methods, see Analysis Methods for Filter System Objects.

For a complete list of analysis methods supported for the `dsp.AllpassFilter` object, enter `dsp.AllpassFilter.helpFilterAnalysis` at the MATLAB command prompt.

## Examples

expand all

### Lowpass Filtering using Two Allpass Filters

Construct the Allpass Filters

```Fs = 48000; % in Hz FL = 1024; APF1 = dsp.AllpassFilter('AllpassCoefficients',... [-0.710525516540603 0.208818210000029]); APF2 = dsp.AllpassFilter('AllpassCoefficients',... [-0.940456403667957 0.6;... -0.324919696232907 0],... 'TrailingFirstOrderSection',true); ```

Construct the Transfer Function Estimator to estimate the transfer function between the random input and the Allpass filtered output

```TFE = dsp.TransferFunctionEstimator('FrequencyRange',... 'onesided','SpectralAverages',2); ```

Construct the ArrayPlot to plot the magnitude response

```AP = dsp.ArrayPlot('PlotType','Line','YLimits', [-80 5],... 'YLabel','Magnitude (dB)','SampleIncrement', Fs/FL,... 'XLabel','Frequency (Hz)','Title','Magnitude Response',... 'ShowLegend', true,'ChannelNames',{'Magnitude Response'}); ```

Filter the Input and show the magnitude response of the estimated transfer function between the input and the filtered output

```tic; while toc < 5 in = randn(FL,1); out = 0.5.*(step(APF1,in) + step(APF2,in)); A = step(TFE, in, out); step(AP,db(A)); end ```

## Algorithms

The transfer function of an allpass filter is defined by:

$H\left(z\right)=\frac{c\left(n\right)+c\left(n-1\right){z}^{-1}+...+{z}^{-n}}{1+c\left(1\right){z}^{-1}+...+c\left(n\right){z}^{-n}}$

given the nontrivial polynomial coefficients in c. The order n of the transfer function is given by the length of the vector c.

`dsp.AllpassFilter` realizes an allpass filter using three different implementation structures: Minimum multiplier, Wave Digital Filter, and Lattice. These structures differ from generic IIR filters such as df1, df1t, df2, df2t, in that they are computationally more economical and structurally more stable [1]. For all structures, a single instance of `dsp.AllpassFilter` can handle either a single-section or a multiple-section (cascaded) allpass filter. The different sections can have different orders but they are all implemented according to the same structure.

### Minimum multiplier

This structure realizes the allpass filter with the minimum number of required multipliers, equal to the order `n`. It also uses `2n` delay units and `2n` adders. The coefficients used by the multipliers are the same as `AllpassCoefficients`, which are equal to the polynomial vector c in the allpass transfer function. The following code shows an example of a second-order section as a Simulink diagram using basic building blocks. You need a Simulink license to generate the actual diagram using realizemdl.

```hap = dsp.AllpassFilter('AllpassCoefficients', [0.1, -0.7]); realizemdl(hap) ```

### Wave Digital Filter

This structure uses `n` multipliers, but only `n` delay units at the expense of requiring `3n` adders. To use this structure, you must specify the coefficients as `WDFCoefficients` in Wave Digital Filter (WDF) form. Obtain the WDF equivalent of the conventional allpass coefficients such as those in vector `c` in the preceding equation, by using the static method `dsp.AllpassFilter.poly2wdf`. You can also use a similar method, `dsp.AllpassFilter.wdf2poly` to convert given WDF-form coefficients into their equivalent allpass polynomial form. The following code shows an example of a second-order section as a Simulink diagram using basic operation blocks. You need a Simulink license to generate the actual diagram using realizemdl. Use these coefficients for a functionally equivalent filter to the previous Minimum multiplier example.

```c = [0.1, -0.7]; w = dsp.AllpassFilter.poly2wdf(c); hap = dsp.AllpassFilter('Structure', 'Wave Digital Filter', 'WDFCoefficients', w); realizemdl(hap) ```

The multipliers used in the filter implementation are a transformation of the WDF coefficients previously provided. The implementation structure around each multiplier depends on the actual value of the multiplier and can vary for different filtering stages among five different options. For example notice how in the preceding diagram, the first and the second stages are realized with two different inner structures. For more details refer to [2].

### Lattice

This lattice structure uses `2n` multipliers, `n` delay units, and `2n` adders. To use this structure, you must specify the coefficients as `LatticeCoefficients` in Lattice form. Obtain these from the conventional polynomial form of the allpass coefficients by using an appropriate conversion function, such as `tf2latc`. The following code shows an example of a second-order section as a Simulink diagram using basic operation blocks. You need a Simulink license to generate the actual diagram using realizemdl. Use these coefficients for a functionally equivalent filter to the Minimum multiplier structure.

```c = [0.1 -0.7]; k = tf2latc([1 c]); hap = dsp.AllpassFilter('Structure', 'Lattice', 'LatticeCoefficients', k); realizemdl(hap) ```

## References

[1] Regalia, Philip A. and Mitra Sanjit K. and Vaidyanathan, P. P. (1988) "The Digital All-Pass Filter: AVersatile Signal Processing Building Block." Proceedings of the IEEE, Vol. 76, No. 1, 1988, pp. 19–37

[2] M. Lutovac, D. Tosic, B. Evans, Filter Design for Signal Processing Using MATLAB and Mathematica. Prentice Hall, 2001