Overview

Gain computer used to realize soft-knee 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 AbsModule. The module outputs the time varying gain on the mono output pin which is typically used as input to the AGCMultiplierModule. The peak following amplitude detector and the gain transfer function are 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.

Type Definition

typedef struct _ModuleAGCLimiterCore
{
    ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
    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
    FLOAT32 slope;                                // Internal derived variable which holds the slope of the compression curve
    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 currentGain;                          // Instantaneous gain computed by the block
    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
} ModuleAGCLimiterCoreClass;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
threshold	float	parameter	0	-20	-60:0	dB
gain	float	parameter	0	0	-24:24	dB
slope	float	derived	0	0.75	0:100	dB/dB
kneeDepth	float	parameter	0	2	0.1:60	dB
ratio	float	parameter	0	4	0.1:100
attackTime	float	parameter	0	20	0.01:1000	msec
decayTime	float	parameter	0	100	0.01:1000	msec
currentGain	float	state	0	0	Unrestricted	dB
sharpnessFactor	float	derived	1	0.5	Unrestricted
attackCoeff	float	derived	1	0.001041	Unrestricted
decayCoeff	float	derived	1	0.0002083	Unrestricted
envState	float	state	1	0	Unrestricted	dB

Pins

Input Pins

Name: in

Description: audio input

Data type: float

Channel range: 1

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: agc_limiter_core_module.m

 M=agc_limiter_core_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.