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# vDSP_DFT_Interleaved_CreateSetupD(_:_:_:_:)

Returns a setup structure that contains precalculated data for forward and inverse, double-precision interleaved discrete Fourier transform (DFT) functions.

```
func vDSP_DFT_Interleaved_CreateSetupD(_ Previous: vDSP_DFT_Interleaved_SetupD?, _ Length: vDSP_Length, _ Direction: vDSP_DFT_Direction, _ RealtoComplex: vDSP_DFT_RealtoComplex) -> vDSP_DFT_Interleaved_SetupD?
```

## Parameters

`Previous`

An existing [`vDSP_DFT_Interleaved_SetupD`](/documentation/Accelerate/vDSP_DFT_Interleaved_SetupD) structure that shares memory and direction with the setup structure that this function returns. Pass `nil` to create an object with newly initialized and allocated memory. To ensure correct operation, if you specify a previous setup structure it must share the same direction as the `Direction` parameter.

`Length`

For complex-to-complex transforms, the number of complex elements.

    For real-to-complex transforms, the number of real elements divided by 2.

`Direction`

A flag that specifies the transform direction. Pass [`vDSP_DFT_Direction.FORWARD`](/documentation/Accelerate/vDSP_DFT_Direction/FORWARD) to transform from the time domain to the frequency domain. Pass [`vDSP_DFT_Direction.INVERSE`](/documentation/Accelerate/vDSP_DFT_Direction/INVERSE) to transform from the frequency domain to the time domain.

`RealtoComplex`

A flag that specifies the transform type. To transform from complex to complex, pass [`vDSP_DFT_RealtoComplex.interleaved_ComplextoComplex`](/documentation/Accelerate/vDSP_DFT_RealtoComplex/interleaved_ComplextoComplex). To transform from real to complex, pass [`vDSP_DFT_RealtoComplex.interleaved_RealtoComplex`](/documentation/Accelerate/vDSP_DFT_RealtoComplex/interleaved_RealtoComplex).

## Return Value

Returns a [`vDSP_DFT_Interleaved_SetupD`](/documentation/Accelerate/vDSP_DFT_Interleaved_SetupD) object, or `nil` if the function fails, either from insufficient memory or because `Length` doesn’t satisfy the above requirements.

## Discussion> Important:
> To prevent potential memory leaks, if the `Previous` parameter is not `nil`, the return value and the `Previous` value must be different variables.

## Discussion

The interleaved DFT operations that the Accelerate framework provides work over collections with specific counts. The maximum number of complex elements that these operations support is 4096, and other supported counts are the result of the formula `f * 2ⁿ` for certain values of `f` and `n`. In the case of real-to-complex, `n` is the number of real elements divided by two, and for complex-to-complex `n` is the number of complex elements.

The following tables show the complete list of supported lengths for different values of `f` and `n`:

### Supported lengths for f = 1

|n |`2ⁿ`|`length` (`f * 2ⁿ`)|
|--|----|-------------------|
|3 |8   |**8**              |
|4 |16  |**16**             |
|5 |32  |**32**             |
|6 |64  |**64**             |
|7 |128 |**128**            |
|8 |256 |**256**            |
|9 |512 |**512**            |
|10|1024|**1024**           |
|11|2048|**2048**           |
|12|4096|**4096**           |

### Supported lengths for f = 3

|n|`2ⁿ`|`length` (`f * 2ⁿ`)|
|-|----|-------------------|
|2|4   |**12**             |
|3|8   |**24**             |
|4|16  |**48**             |
|5|32  |**96**             |
|6|64  |**192**            |
|7|128 |**384**            |
|8|256 |**768**            |

### Supported lengths for f = 5

|n|`2ⁿ`|`length` (`f * 2ⁿ`)|
|-|----|-------------------|
|2|4   |**20**             |
|3|8   |**40**             |
|4|16  |**80**             |
|5|32  |**160**            |
|6|64  |**320**            |
|7|128 |**640**            |

### Supported lengths for f = 9

|n|`2ⁿ`|`length` (`f * 2ⁿ`)|
|-|----|-------------------|
|2|4   |**36**             |
|3|8   |**72**             |
|4|16  |**144**            |
|5|32  |**288**            |
|6|64  |**576**            |
|7|128 |**1152**           |

### Supported lengths for f = 15

|n|`2ⁿ`|`length` (`f * 2ⁿ`)|
|-|----|-------------------|
|2|4   |**60**             |
|3|8   |**120**            |
|4|16  |**240**            |
|5|32  |**480**            |
|6|64  |**960**            |
|7|128 |**1920**           |

## See Also

  <doc://com.apple.accelerate/documentation/Accelerate/understanding-data-packing-for-fourier-transforms>

  <doc://com.apple.accelerate/documentation/Accelerate/reducing-spectral-leakage-with-windowing>


---

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