Overview

2nd order smoothly updating IIR filter with high precision implementation

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 in coeffs array must be [b0 b1 b2 a1 a2]. In the set function the coefficients will be mapped from Biquad form to High Precision representation [Fb; Gaa; Gab; K; Fa].

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 targetCoeffs are the "target" coefficients that serve as control variables to the module, and the coefficients currentCoeffs 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 _ModuleBiquadSmoothedHP
{
    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 smoothingCoeff;                       // Smoothing coefficient. This is computed based on the smoothingTime, sample rate, and block size of the module.
    FLOAT32* coeffs;                              // Biquad filter coefficients arranged as [b0; b1; b2; a1; a2].
    FLOAT32* targetCoeffs;                        // Array of filter target coefficients. The size of the array is 5 and contains the variables for a biquad arranged as [Fb; Gaa; Gab; K; Fa].
    FLOAT32* currentCoeffs;                       // Array of filter current coefficients. The size of the array is 5 and contains the variables for a biquad.
    FLOAT32* state;                               // State variables. 2 per channel.
} ModuleBiquadSmoothedHPClass;

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
smoothingCoeff	float	derived	1	0.06449	Unrestricted
coeffs	float*	parameter	0	[5 x 1]	Unrestricted
targetCoeffs	float*	derived	1	[5 x 1]	Unrestricted
currentCoeffs	float*	state	1	[5 x 1]	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

Data type: float

MATLAB Usage

File Name: biquad_smoothedHP_module.m

 M=biquad_smoothedHP_module(NAME)
 Creates a second order IIR filter (biquad_smoothed) for use with 
 Audio Weaver. This filter smoothly updates its coefficients on a block-
 by-block basis. This version uses a high precision implementation
 
 Arguments:
    NAME - name of the module.