Overview

Gain computer used to realize a downward expander (or noise gate)

Discussion

The downward expander core provides a peak amplitude follower and input/output gain transfer function for use in noise gating 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.

When the signal is above the threshold the output gain is 1.0 (no change). When the signal is below the threshold, each dB drop in the input causes the output to drop by ratio dB. For instance, a ratio of 1 results in no additional attenuation and a ratio of 11 results in 10dB more attenuation for each dB that the input drops.

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 expander.

Type Definition

typedef struct _ModuleDownwardExpanderCore
{
    ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
    FLOAT32 threshold;                            // Below this level the module reduces gain
    FLOAT32 ratio;                                // Slope of the gain curve when the signal is below the threshold
    FLOAT32 kneeDepth;                            // Knee depth controls the sharpness of the transition between expanding and not expanding
    FLOAT32 attackTime;                           // Envelope detector attack time constant
    FLOAT32 decayTime;                            // Envelope detector decay time constant
    FLOAT32 currentGain;                          // Instantaneous gain computed by the block
    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* kneePoly;                            // Derived variable for interpolating the soft knee
} ModuleDownwardExpanderCoreClass;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
threshold	float	parameter	0	-20	-100:0	dB
ratio	float	parameter	0	5	0.1:100	dB/dB
kneeDepth	float	parameter	0	0	0:60	dB
attackTime	float	parameter	0	20	0.01:1000	msec
decayTime	float	parameter	0	100	0.01:1000	msec
currentGain	float	state	0	0	-10:5
attackCoeff	float	derived	1	0.001041	Unrestricted
decayCoeff	float	derived	1	0.0002083	Unrestricted
envState	float	state	1	0	-100:1
kneePoly	float*	derived	1	[1 x 3]	Unrestricted

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: downward_expander_core_module.m

 M=downward_expander_core_module(NAME)
 Creates a downward expander (or noise gate) module with a soft knee.
 It is used in conjunction with the AGCMultiplier to implement a complete
 expander.  Arguments:
    NAME - name of the module.