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# Single-channel biquadratic filters

Filter a single-channel signal with a cascade of biquadratic sections.

## Discussion

The vDSP library implements biquadratic filtering as a cascade of individual infinite impulse response (IIR) filters called *sections.* Each section has its own set of feedback and feedforward coefficients, and implements a direct-form 1 filter.

When the biquadratic filter function executes, the sections execute in sequence. Each section processes the entire input signal and passes its output to the next section for further processing.

> Note:
> The vDSP biquadratic filters work in place. That is, the source and destination pointers may point to the same memory.

## Topics

### Biquadratic filter essentials

  <doc://com.apple.accelerate/documentation/Accelerate/applying-biquadratic-filters-to-a-music-loop>

  <doc://com.apple.accelerate/documentation/Accelerate/creating-an-audio-unit-extension-using-the-vdsp-library>

### Creating a single-channel biquadratic filter setup

[`extern vDSP_biquad_SetupvDSP_biquad_CreateSetup(const double *__Coefficients, vDSP_Length __M);`](/documentation/Accelerate/vDSP_biquad_CreateSetup)

Builds a data structure that contains precalculated data for use by a single-precision cascaded biquadratic filter function.

[`typealias vDSP_biquad_Setup`](/documentation/Accelerate/vDSP_biquad_Setup)

A data structure that contains precalculated data for use by the single-precision cascaded biquadratic IIR filter function.

[`extern vDSP_biquad_SetupDvDSP_biquad_CreateSetupD(const double *__Coefficients, vDSP_Length __M);`](/documentation/Accelerate/vDSP_biquad_CreateSetupD)

Builds a data structure that contains precalculated data for use by a double-precision cascaded biquadratic filter function.

[`typealias vDSP_biquad_SetupD`](/documentation/Accelerate/vDSP_biquad_SetupD)

A data structure that contains precalculated data for use by the double-precision cascaded biquadratic IIR filter function.

### Applying a single-channel biquadratic filter

[`extern void vDSP_biquad(const struct vDSP_biquad_SetupStruct *__Setup, float *__Delay, const float *__X, vDSP_Stride __IX, float *__Y, vDSP_Stride __IY, vDSP_Length __N);`](/documentation/Accelerate/vDSP_biquad)

Applies a single-precision single-channel biquadratic IIR filter.

[`extern void vDSP_biquadD(const struct vDSP_biquad_SetupStructD *__Setup, double *__Delay, const double *__X, vDSP_Stride __IX, double *__Y, vDSP_Stride __IY, vDSP_Length __N);`](/documentation/Accelerate/vDSP_biquadD)

Applies a double-precision single-channel biquadratic IIR filter.

### Setting the coefficients of a single-channel biquadratic filter

[`extern void vDSP_biquad_SetCoefficientsSingle(vDSP_biquad_Setup __setup, const float *__coeffs, vDSP_Length __start_sec, vDSP_Length __nsec);`](/documentation/Accelerate/vDSP_biquad_SetCoefficientsSingle)

Sets single-precision coefficients of the specified single-channel biquadratic filter setup object.

[`extern void vDSP_biquad_SetCoefficientsDouble(vDSP_biquad_Setup __setup, const double *__coeffs, vDSP_Length __start_sec, vDSP_Length __nsec);`](/documentation/Accelerate/vDSP_biquad_SetCoefficientsDouble)

Sets double-precision coefficients of the specified single-channel biquadratic filter setup object.

### Destroying a single-channel biquadratic filter setup

[`extern void vDSP_biquad_DestroySetup(vDSP_biquad_Setup __setup);`](/documentation/Accelerate/vDSP_biquad_DestroySetup)

Destroys a single-precision biquadratic filter setup object.

[`extern void vDSP_biquad_DestroySetupD(vDSP_biquad_SetupD __setup);`](/documentation/Accelerate/vDSP_biquad_DestroySetupD)

Destroys a double-precision biquadratic filter setup object.


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