(8.D.2.5) AGCLimiterCoreV2

Overview

Soft knee gain computer for use in peak limiters

Discussion

The limiter core provides a peak amplitude follower and input/output gain transfer function for use in AGC systems. It is designed to take as its input the output of the BlockStatistics or the Abs module. The module outputs the time varying gain on the output pin which is typically used as input to the AGCMultiplier or a ScalerNControl module

The peak following amplitude detector and the gain transfer function may be calculated at the sample rate, which aids in reducing the gain of fast transients. The module stores the gain computed for the last sample in the block to the currentGain state variable. This variable can be examined to track the behavior of the limiter.

V2 was introduced to allow multichannel inputs which in effect significantly reduces the module processing load compared to using multiple instances of the initial single channel version.

Type Definition

typedef struct _ModuleAGCLimiterCoreV2
{
    ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
    INT32 numChan;                                // Number of channels.
    FLOAT32* threshold;                           // Amplitude level at which the AGC Limiter Core reduces its output gain value
    FLOAT32* gain;                                // Value used to scale the output of the AGC Limiter Core (make up gain)
    FLOAT32* kneeDepth;                           // Knee depth controls the sharpness of the transition between no limiting and limiting
    FLOAT32* ratio;                               // Slope of the output attenuation when the signal is above threshold - derived from the standard compression ratio parameter by the formula slope = 1.0 - (1.0/ratio)
    FLOAT32* attackTime;                          // Envelope detector attack time constant
    FLOAT32* decayTime;                           // Envelope detector decay time constant
    FLOAT32* slope;                               // Internal derived variable which holds the slope of the compression curve
    FLOAT32* sharpnessFactor;                     // Internal derived variable which is used to implement the soft knee
    FLOAT32* attackCoeff;                         // Internal derived variable which implements the attackTime
    FLOAT32* decayCoeff;                          // Internal derived variable which implements the decayTime
    FLOAT32* envState;                            // Holds the instantaneous state of the envelope detector
    FLOAT32* currentGain;                         // Instantaneous gain computed by the block
} ModuleAGCLimiterCoreV2Class;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
numChan	int	const	1	1	Unrestricted
threshold	float*	parameter	0	[1 x 1]	-60:0	dB
gain	float*	parameter	0	[1 x 1]	0:60	dB
kneeDepth	float*	parameter	0	[1 x 1]	0.1:60	dB
ratio	float*	parameter	0	[1 x 1]	1:100
attackTime	float*	parameter	0	[1 x 1]	1:2000	msec
decayTime	float*	parameter	0	[1 x 1]	1:2000	msec
slope	float*	derived	1	[1 x 1]	Unrestricted	dB/dB
sharpnessFactor	float*	derived	1	[1 x 1]	Unrestricted
attackCoeff	float*	derived	1	[1 x 1]	Unrestricted
decayCoeff	float*	derived	1	[1 x 1]	Unrestricted
envState	float*	state	1	[1 x 1]	Unrestricted
currentGain	float*	state	0	[1 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

Scratch Pins

Channel count: 1

Block size: 32

Sample rate: 48000

Channel count: 1

Block size: 32

Sample rate: 48000

MATLAB Usage

File Name: agc_limiter_core_v2_module.m

 M=agc_limiter_core_v2_module(NAME)
 Creates a limiter gain computer with soft knee for use in Audio
 Weaver. It is used in conjunction with the AGCMultiplier to 
 implement full limiter.
 
 Arguments:
    NAME - name of the module.