# Documentation

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# iirftransf

IIR frequency transformation of filter

## Syntax

```[OutNum,OutDen] = iirftransf(OrigNum,OrigDen,FTFNum,FTFDen) ```

## Description

`[OutNum,OutDen] = iirftransf(OrigNum,OrigDen,FTFNum,FTFDen)` returns the numerator and denominator vectors, `OutNum` and `OutDen`, of the target filter, which is the result of transforming the prototype filter specified by the numerator, `OrigNum`, and denominator, `OrigDen`, with the mapping filter given by the numerator, `FTFNum`, and the denominator, `FTFDen`. If the allpass mapping filter is not specified, then the function returns an original filter.

## Examples

Design a prototype real IIR halfband filter using a standard elliptic approach:

```[b, a] = ellip(3, 0.1, 30, 0.409); [AlpNum, AlpDen] = allpasslp2lp(0.5, 0.25); [num, den] = iirftransf(b, a, AlpNum, AlpDen);```

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

`fvtool(b, a, num, den);`

Here's the comparison between the filters.

## Arguments

VariableDescription
`OrigNum`

Numerator of the prototype lowpass filter

`OrigDen`

Denominator of the prototype lowpass filter

`FTFNum`

Numerator of the mapping filter

`FTFDen`

Denominator of the mapping filter

`OutNum`

Numerator of the target filter

`OutDen`

Denominator of the target filter