Overview

2nd order smoothly updating IIR filter

Discussion

Smoothly varying 5 coefficient biquad filter. The module is designed to operate on multiple channels and applies the same coefficients across all channels. The state array is allocated to contain 2*numChannels values. The frequency response of the module is determined by the values of the 5 coefficients, and the coefficients come directly from the MATLAB second order representation. The leading a0 coefficient is not supplied and is assumed to equal 1.0. The order of the coefficients must be [b0 b1 b2 a1 a2].

The module is similar to the unsmoothed Biquad module, except that the filter coefficients are smoothed on a block-by-block basis. The module contains two sets of filter coefficients. The coeffcients [b0, b1, b2, a1, a2] are the "target" coefficients that serve as control variables to the module, and the coefficients [current_b0, current_b1, current_b2, current_a1, current_a2] are the instantaneous coefficients that are updated on a block by block basis using exponential smoothing. The current coefficients exponentially approach the control coefficients. The instance variable .updateActive is a Boolean and it controls whether the coefficient smoothing occurs at the start of each block. The variable .updateActive allows the coefficient smoothing to be halted and the target coefficients updated atomically.

The rate of smoothing is controlled by the .smoothingTime instance variable.

Type Definition

typedef struct _ModuleBiquadSmoothed
{
    ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
    INT32 updateActive;                           // Specifies whether the filter coefficients are updating (=1) or fixed (=0).
    FLOAT32 smoothingTime;                        // Time constant of the smoothing process.
    FLOAT32 b0;                                   // Desired first numerator coefficient.
    FLOAT32 b1;                                   // Desired second numerator coefficient.
    FLOAT32 b2;                                   // Desired third numerator coefficient.
    FLOAT32 a1;                                   // Desired second denominator coefficient.
    FLOAT32 a2;                                   // Desired third denominator coefficient.
    FLOAT32 current_b0;                           // Instantaneous first numerator coefficient.
    FLOAT32 current_b1;                           // Instantaneous second numerator coefficient.
    FLOAT32 current_b2;                           // Instantaneous third numerator coefficient.
    FLOAT32 current_a1;                           // Instantaneous second denominator coefficient.
    FLOAT32 current_a2;                           // Instantaneous third denominator coefficient.
    FLOAT32 smoothingCoeff;                       // Smoothing coefficient. This is computed based on the smoothingTime, sample rate, and block size of the module.
    FLOAT32* state;                               // State variables. 2 per channel.
} ModuleBiquadSmoothedClass;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
updateActive	int	parameter	0	1	0:1
smoothingTime	float	parameter	0	10	0:1000	msec
b0	float	parameter	0	1	Unrestricted
b1	float	parameter	0	0	Unrestricted
b2	float	parameter	0	0	Unrestricted
a1	float	parameter	0	0	Unrestricted
a2	float	parameter	0	0	Unrestricted
current_b0	float	state	1	1	Unrestricted
current_b1	float	state	1	0	Unrestricted
current_b2	float	state	1	0	Unrestricted
current_a1	float	state	1	0	Unrestricted
current_a2	float	state	1	0	Unrestricted
smoothingCoeff	float	derived	1	0.06449	Unrestricted
state	float*	state	1	[2 x 1]	Unrestricted

Pins

Input Pins

Name: in

Description: audio input

Data type: float

Channel range: Unrestricted

Block size range: Unrestricted

Sample rate range: Unrestricted

Complex support: Real

Output Pins

Name: out

Description: audio output