Overview

RMS detector with built-in smoothing

Discussion

This module calculates the smoothed RMS value of the input signal on a block-by-block basis. The input pin can have an arbitrary number of interleaved channels and the RMS calculation occurs over all channels. The output pin has a single channel and a blockSize of 1.

The following algorithm is used internally. For each block, the sum of the square of all input data is computed (the energy), and this includes all channels and the entire block. Next, the energy is divided by the number of samples to yield the average energy. The average energy then passes through a first order lowpass filter with a specified integration time (the smoothingTime). Finally, the square root of the filter output is computed and this serves as the output of the module.

The module also exposes some of the computed signals as state variables. The variable filteredValue holds the output of the module (the smoothed RMS value). The variable instantaneousValue holds the square root of the average energy in the block; no smoothing.

Type Definition

typedef struct _ModuleRMS
{
    ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
    FLOAT32 smoothingTime;                        // Time interval over which to smooth the measurement.
    FLOAT32 instantaneousValue;                   // Instantaneous (unsmoothed) output value.
    FLOAT32 filteredValue;                        // Smoothed output value.
    FLOAT32 a1;                                   // a1 coefficient of 1st order smoothing filter.
    FLOAT32 b0;                                   // b0 coefficient of 1st order smoothing filter.
    FLOAT32 b1;                                   // b1 coefficient of 1st order smoothing filter.
    FLOAT32 xNm1;                                 // Delayed input sample.
    FLOAT32 yNm1;                                 // Delayed output sample.
} ModuleRMSClass;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
smoothingTime	float	parameter	0	1000	0:10000	msec
instantaneousValue	float	state	0	0	-60:10
filteredValue	float	state	0	0	-60:10
a1	float	derived	1	-0.9958	Unrestricted
b0	float	derived	1	0.00418	Unrestricted
b1	float	derived	1	0	Unrestricted
xNm1	float	state	1	0	Unrestricted
yNm1	float	state	1	0	Unrestricted

Pins

Input Pins

Name: in1

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: Filtered RMS output

Data type: float

MATLAB Usage

File Name: rms_module.m

 M=rms_module(NAME, NUMIN)
 RMS detector with built-in smoothing. The module accepts multiple input
 signals and computes a control signal which reprsents the smoothed
 RMS value. 
 
 Arguments:
    NAME - name of the module.
    NUMIN - number of input pins.