(8.D.2.5) AGCCore
Overview
Slowly varying RMS based gain computer
Discussion
The AGCCore module is a slowly varying dynamics computer based on the RMS level of the signal. The module is used inconjunction with the MultiplierV2 module to create automatic gain controls. The module outputs a linear gain which can be applied to another signal using the MultiplierV2 module.
The first part of the module's processing function computes the RMS signal level over all input samples. The RMS value is then smoothed by a filter with the specified smoothingTime. The smoothed value is converted to dB and this input level feeds the control logic. The control logic computes an output gain so that when multiplied by the input signal the output signal has an RMS level of .targetLevel. .targetLevel is similar to the threshold in a limiter and .ratio has a similar meaning. If the input signal is .ratio dB loader than .targetLevel then the output will actually be 1 db above .targetLevel. Basically, .ratio specifies how hard the AGC will work to achieve the .targetLevel.
The variables .maxGain and .maxAttenuation control the maximum boost and cut that the AGC will allow. Both .maxGain and .maxAttenuation are positive numbers. For example, if maxGain = 10 and maxAttenuation = 20 then the AGCCore will boost by up to +10 dB and cut by up to -20 dB.
When the input level falls below .activationThreshold then the AGC freezes and holds its output gain. If .enableRecovery is checked then when the input signal falls below .activationThreshold then the AGCCore's gain will slew back to 0 dB. The rate of slewing is specified by recoveryRate which has units of dB/sec.
Type Definition
typedef struct _ModuleAGCCore
{
ModuleInstanceDescriptor instance; // Common Audio Weaver module instance structure
FLOAT32 targetLevel; // Target audio level.
FLOAT32 maxAttenuation; // Maximum attenuation of the AGC.
FLOAT32 maxGain; // Maximum gain of the AGC.
FLOAT32 ratio; // Slope of the output attenuation when the signal is above threshold.
FLOAT32 activationThreshold; // Activation threshold of the AGC. The AGC stops updating its gain when the instantaneous input level is below this value.
FLOAT32 smoothingTime; // Response time of the AGC. This controls the RMS smoothing interval and the time constant of the smoothly updating gain.
INT32 enableRecovery; // Boolean which enables the AGC's gain recovery. If enabled, the AGC slows returns 0 dB gain when the input is below the activation threshold.
FLOAT32 recoveryRate; // Rate at which the gain is adjusted when the input is below the activation threshold.
FLOAT32 currentGain; // Instantaneous gain of the smoothing operation.
FLOAT32 oneOverSlope; // Used by the processing function to compute the amount of cut/boost. Equal to 1-1/ratio.
FLOAT32 smoothingCoeffSample; // Sample-by-sample smoothing coefficient for the output gain adjustment. Set via smoothingTime.
FLOAT32 smoothingCoeffBlock; // Block-by-block smoothing coefficient for the RMS measurement. Set via smoothingTime.
FLOAT32 recoveryRateUp; // Recovery rate coefficient when increasing the gain (it is always >= 1).
FLOAT32 recoveryRateDown; // Recovery rate coefficient when decreasing the gain (it is always <= 1).
FLOAT32 targetGain; // Target gain of the smoothing operation.
FLOAT32 energy; // Smoothed energy measurement.
FLOAT32 oneOverNumSamples; // 1 divided by the number of samples in the input pin. Used internally by the algorithm in order to save a divide operation.
} ModuleAGCCoreClass;
Variables
Properties
Name | Type | Usage | isHidden | Default value | Range | Units |
targetLevel | float | parameter | 0 | -20 | -50:0.1:50 | dB |
maxAttenuation | float | parameter | 0 | 100 | 0:100 | dB |
maxGain | float | parameter | 0 | 12 | 0:50 | dB |
ratio | float | parameter | 0 | 10 | 1:100 | dB/dB |
activationThreshold | float | parameter | 0 | -50 | -100:-20 | dB |
smoothingTime | float | parameter | 0 | 100 | 1:5000 | msec |
enableRecovery | int | parameter | 0 | 1 | 0:1 | Â |
recoveryRate | float | parameter | 0 | 2.4 | 1:20 | dB/sec |
currentGain | float | state | 0 | 0 | -12:12 | Â |
oneOverSlope | float | derived | 1 | 0.9 | Unrestricted | Â |
smoothingCoeffSample | float | derived | 1 | 0.0002083 | Unrestricted | Â |
smoothingCoeffBlock | float | derived | 1 | 0.006644 | Unrestricted | Â |
recoveryRateUp | float | derived | 1 | 1 | Unrestricted | Â |
recoveryRateDown | float | derived | 1 | 0.9998 | Unrestricted | Â |
targetGain | float | state | 1 | 1 | Unrestricted | Â |
energy | float | state | 1 | 0 | Unrestricted | Â |
oneOverNumSamples | float | derived | 1 | 0.03125 | Unrestricted | Â |
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: agc_core_module.m
M=agc_core_module(NAME)
Audio Weaver module which calculates the instantaneous gain needed
by an automatic gain control AGC module. This module is used in
conjunction with the agc_multiplier_module.
Arguments:
NAME - name of the module.
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