AGCLimiterCoreV2Fract32

Overview

Soft knee gain computer for use in peak limiters

Discussion

The agc_limiter_core_v2_fract32_module computes a time varying gain which is used to restrict the peak value of a signal.  The module is designed to take its input from the max_abs_fract32_module. The module computes a time varying gain on a sample-by-sample basis, and this gain is later applied by the agc_multiplier_fract32_module.  The output gain signal is in 8.24 format.  The agc_multiplier_fract32_module expects the gain to be in 1.31 format.  Thus, the multiplier must be configured to shift by 7 bits.  The combination of max_abs_fract32_module, agc_limiter_core_v2_fract32_module, and agc_multiplier_fract32_module modules is referred to as a "limiter" and is contained in the script limiter_module.m.

It is also possible to incorporate delay into the limiter effectively allowing it to "look ahead" and anticipate transients.  See the limiter_module.m script for details.

One way to understand the operation of the limiter is to consider the input / output curve shown in the figure below.  The X-axis represents the level of the input signal, in dB.  For simplicity, assume that the input signal is a sine wave.  The Y-axis represents the level of the output sine wave.  In the figure, the limitThreshold is set to -30 dB.  Signals below limitThreshold are unaffected.  Signals above limitThreshold are compressed by the parameter ratio.  In the figure, ratio is set to 3 dB/dB, and thus the output signal gets 1 dB louder for every 3 dB increase in the input signal.  In typical limiter applications, ratio is set to 100 preventing the signal from getting much louder than threshold.

The AGCLimiterCoreV2Fract32 module computes the gain needed to realize the input / output curve shown above.  The gain curve is shown below.  Below limitThreshold, the gain equals 0 dB and thus the level of the signal is unaffected.  After limitThreshold, attenuation is applied.

The outputGain parameter allows you to specify additional gain through the limiter.  This gain is just incorporated into the output signal.

The limiter includes a soft knee which causes the attenuation to kick in gradually.  The knee is a quadratic function extending from limitThreshold – kneeDepth to limitThreshold + kneeDepth.  Due to manner in which the knee is implemented, kneeDepth must always be > 0.  The figure below illustrates the input / output level characteristic with:

limitThreshold = -10

ratio = 100

kneeDepth = 10

Type Definition

typedef struct _ModuleAGCLimiterCoreV2Fract32
{
    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
    fract32* 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
    fract32* attackCoeff;                         // Internal derived variable which implements the attackTime
    fract32* decayCoeff;                          // Internal derived variable which implements the decayTime
    fract32* thresholdFract32;                    // threshold - in 8.24 "base2 dB"
    fract32* gainFract32;                         // gain - in 8.24 "base2 dB"
    fract32* kneeDepthFract32;                    // Knee depth - in 8.24 "base2 dB"
    fract32* sharpnessFactorFract32;              // Internal derived variable which is used to implement the soft knee
    fract32* envState;                            // Holds the instantaneous state of the envelope detector
    fract32* currentGain;                         // Instantaneous gain computed by the block
} ModuleAGCLimiterCoreV2Fract32Class;

Variables

Properties

Pins

Input Pins

Name: in

Description: audio input

Data type: fract32

Channel range: Unrestricted

Block size range: Unrestricted

Sample rate range: Unrestricted

Complex support: Real

Output Pins

Name: out

Description: audio output

Data type: fract32

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_fract32_module.m

 M=agc_limiter_core_v2_fract32_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.