Overview

ST Biquad Cascade

Discussion

The BIQ module is a cascade (up to 10) of infinite impulse response second order filters. The coefficients of each second order section can be configured independently. The overall BIQ transfer function can thus be designed for many purposes. It implements a direct-form I, with the coefficients normalized over a0. A post-shift parameter per cell allows the coefficients to exceed the range of [-1.0 : 1.0]. The BIQ module supports mono and stereo interleaved 32 bit I/O data. The minimum input frame is 96 and the maximum input frame is 480 stereo samples. Post shift is calculated internally.

Current implementation follows this formula :

y(n) = [b0.x(n) + b1.x(n-1) + b2.x(n-2) + a1.y(n-1) + a2.y(n-2)]*2^post_shift

As a consequence, compared to the canonical formula, a1 and a2 coefficients sign is inverted.

The ordering of coefficients is:

b0

b1

b2

a1

a2

Type Definition

typedef struct _ModuleSTBiquadCascade
{
    ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
    INT32 nAPIResult;                             // Return status for last call into the ST API
    INT32 numStages;                              // Number of stages in the filter. The filter order = 2 * numStages
    INT32 enable;                                 // Block enable
    FLOAT32* coeffs;                              // Matrix of filter coefficients. The size of the matrix is 5 x [no stages]. Each col contains the variables for a biquad arranged as [b0; b1; b2; a1; a2].
    void * pGlobals;                              // Points to the global variables structure
} ModuleSTBiquadCascadeClass;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
nAPIResult	int	state	0	1	Unrestricted
numStages	int	const	0	1	1:1:10
enable	int	parameter	0	1	0:1:1
coeffs	float*	parameter	0	[5 x 1]	Unrestricted
pGlobals	void *	state	1		Unrestricted

Pins

Input Pins

Name: in

Description: audio input

Data type: fract32

Channel range: 1, 2

Block size range: 96:480

Sample rate range: Unrestricted

Complex support: Real

Output Pins

Name: out

Description: audio output