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# dfilt.df1

Discrete-time, direct-form I filter

## Syntax

Hd = dfilt.df1(b,a)
Hd = dfilt.df1

## Description

Hd = dfilt.df1(b,a) returns a discrete-time, direct-form I filter, Hd, with numerator coefficients b and denominator coefficients a. The filter states for this object are stored in a filtstates object.

Hd = dfilt.df1 returns a default, discrete-time, direct-form I 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.

Image of direct form one filter diagram

To display the filter states, use this code to access the filtstates object.

```Hs = Hd.states       % Where Hd is the dfilt.df1 object and
double (Hs)          % Hs is the filtstates object
```

The vector is

$\left[\begin{array}{c}zb\left(1\right)\\ zb\left(2\right)\\ \cdots \\ zb\left(n\right)\\ za\left(1\right)\\ za\left(2\right)\\ \cdots \\ za\left(n\right)\end{array}\right]$

## Examples

Create a direct-form I discrete-time filter with coefficients from a fourth-order lowpass Butterworth design

```[b,a] = butter(4,.5);
Hd = dfilt.df1(b,a)
```