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# dfilt.df2t

Discrete-time, direct-form II transposed filter

## Syntax

Hd = dfilt.df2t(b,a)
Hd = dfilt.df2t

## Description

Hd = dfilt.df2t(b,a) returns a discrete-time, direct-form II transposed filter, Hd, with numerator coefficients b and denominator coefficients a.

Hd = dfilt.df2t returns a default, discrete-time, direct-form II transposed filter, Hd, with b=1 and a=1. This filter passes the input through to the output unchanged.

 Note   The leading coefficient of the denominator a(1) cannot be 0.

The filter states of dfilt.df2t object can be extracted as a column vector with:

```b =[1 2];
a =[1 -0.9];
Hd = dfilt.df2t(b,a);
FiltStates = double(Hd.States);```

The resulting filter states column vector is

$\left(\begin{array}{c}z\left(1\right)\\ z\left(2\right)\end{array}\right)$

## Examples

Create a direct-form II transposed discrete-time filter with coefficients from a 4–th order lowpass Butterworth design:

```[b,a] = butter(4,.5);
Hd = dfilt.df2t(b,a);
```