Overview

Gain computer used to realize compressors

Discussion

This module accepts a mono input, takes the absolute value, computes an envelope follower (with settable attack and decay rates), and computes the appropriate gain based on threshold, gain, and slope parameters. The output has a linear gain factor. The computation is performed once per tick. If you have a multichannel input, then use the max_abs_module.m to compute the absolute value across multiple channels.

The envelope follower uses the maximum value of the input within the tick for its calculation. The output gain is smoothed with a one-pole smoother to reduce zipper noise.

Type Definition

typedef struct _ModuleAGCCompressorCore
{
    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 ratio;                                // Specifies the amount of attenuation (and gain) when above (or below) the threshold.
    FLOAT32 attackTime;                           // Envelope detector attack time constant.
    FLOAT32 attackTimeMin;                        // Minimum value of the attack time constant.
    FLOAT32 decayTime;                            // Envelope detector decay time constant.
    FLOAT32 decayTimeMin;                         // Minimum value of the decay time constant.
    FLOAT32 currentGain;                          // Instantaneous gain applied at the end of the block.
    FLOAT32 slope;                                // Internal parameter which holds the slope of the compression curve.
    FLOAT32 sharpnessFactor;                      // Unused variable. Kept in for legacy support
    FLOAT32 attackCoeff;                          // Internal derived coefficient based on the attackTime
    FLOAT32 attackCoeffSmoothing;                 // Internal derived coefficient based on the attackTimeMin
    FLOAT32 decayCoeff;                           // Internal derived coefficient based on decayTime
    FLOAT32 decayCoeffSmoothing;                  // Internal derived coefficient based on decayTimeMin
    FLOAT32 envState;                             // State of the envelope detector one-pole filter - runs at tick rate.
} ModuleAGCCompressorCoreClass;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
threshold	float	parameter	0	-20	-60:0	dB
gain	float	parameter	0	1	0:100	dB
ratio	float	parameter	0	4	1:100	dB/dB
attackTime	float	parameter	0	20	20:100	msec
attackTimeMin	float	parameter	0	20	Unrestricted
decayTime	float	parameter	0	100	20:1000	msec
decayTimeMin	float	parameter	0	20	Unrestricted
currentGain	float	state	0	0	-20:20
slope	float	derived	1	0.75	0:100	dB/dB
sharpnessFactor	float	derived	1	0	Unrestricted
attackCoeff	float	derived	1	1	Unrestricted
attackCoeffSmoothing	float	derived	1	0.001041	Unrestricted
decayCoeff	float	derived	1	0.01324	Unrestricted
decayCoeffSmoothing	float	derived	1	0.0004166	Unrestricted
envState	float	state	1	0	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: agc_compressor_core_module.m

 M=agc_compressor_core_module(NAME)
 Creates a gain computation part of a compressor. It is used in
 conjunction with the AGCMultiplier to implement full compressor.
 
 Arguments:
    NAME - name of the module.