Compressor

Dynamic range compressor

• 

Libraries:
Audio Toolbox / Dynamic Range Control

Description

The Compressor block performs dynamic range compression independently across each input channel. Dynamic range compression attenuates the volume of loud sounds that cross a given threshold. The block uses specified attack and release times to achieve a smooth applied gain curve.

Examples

Suppress Loud Sounds

Use the Compressor block to suppress loud sounds and visualize the applied compression gain.

• Open Model

Trigger Gain Control Based on Loudness Measurement

This model enables you to apply dynamic range compression to an audio signal while staying inside a preset loudness range. In this model, a Compressor block increases the loudness and decreases the dynamic range of an audio signal. A Loudness Meter block calculates the momentary loudness of the compressed audio signal. If momentary loudness crosses a -23 LUFS threshold, an enabled subsystem applies gain to lower the corresponding level of the audio signal.

• Open Model

Diminish Plosives from Speech

This model minimizes the plosives of a speech signal by applying highpass filtering and low-band compression.

• Open Model

Ports

Input

x — Input signal

matrix | 1-D vector

• Matrix input –– Each column of the input is treated as an independent channel.

• 1-D vector input –– The input is treated as a single channel.

This port is unnamed unless you specify additional input ports.

Data Types: single | double

T — Threshold (dB)

scalar

Dependencies

To enable this port, select Specify from input port for the Threshold (dB) parameter.

Data Types: single | double

R — Ratio

scalar

Dependencies

To enable this port, select Specify from input port for the Ratio parameter.

Data Types: single | double

K — Knee width (dB)

scalar

Dependencies

To enable this port, select Specify from input port for the Knee width (dB) parameter.

Data Types: single | double

AT — Attack time (s)

scalar

Dependencies

To enable this port, select Specify from input port for the Attack time (s) parameter.

Data Types: single | double

RT — Release time (s)

scalar

Dependencies

To enable this port, select Specify from input port for the Release time (s) parameter.

Data Types: single | double

Output

Y — Output signal

matrix

The Compressor block outputs a signal with the same data type as the input signal. The size of the output depends on the size of the input:

• Matrix input –– The block outputs a matrix the same size and data type as the input signal.

• 1-D vector input –– The block outputs an N-by-1 matrix (column vector), where N is the number of elements in the 1-D vector.

Data Types: single | double

G — Gain applied to each input sample

matrix

Dependencies

To enable this port, select the Output gain (dB) parameter.

Data Types: single | double

Parameters

If a parameter is listed as tunable, then you can change its value during simulation.

Main Tab

Threshold (dB) — Operation threshold

–10 (default) | scalar in the range –50 to 0 inclusive

Operation threshold is the level above which gain is applied to the input signal.

To specify Threshold (dB) from an input port, select Specify from input port for the parameter.

Tunable: Yes

Ratio — Compression ratio

5 (default) | scalar in the range 1 to 50 inclusive

Compression ratio is the input/output ratio for signals that overshoot the operation threshold.

Assuming a hard knee characteristic and a steady-state input such that x[n] dB > Threshold (dB), the compression ratio is defined as $$R=\frac{(x[n]-T)}{(y[n]-T)}$$, where

• R is the compression ratio.

• x[n] is the input signal in dB.

• y[n] is the output signal in dB.

• T is the threshold in dB.

To specify Ratio from an input port, select Specify from input port for the parameter.

Tunable: Yes

Knee width (dB) — Transition area in compression characteristic

0 (default) | scalar in the range 0 to 20 inclusive

For soft knee characteristics, the transition area is defined by the relation

$$y=x+\frac{\left(\frac{1}{R}-1\right)\times {\left(x-T+\frac{W}{2}\right)}^2}{\left(2\times W\right)}$$

for the range $$\left(2\times \left|x-T\right|\right)\le W$$, where

• y is the output level in dB.

• x is the input level in dB.

• R is the compression ratio.

• T is the threshold in dB.

• W is the knee width in dB.

To specify Knee width (dB) from an input port, select Specify from input port for the parameter.

Tunable: Yes

View static characteristic — Open static characteristic plot of dynamic range compressor

button

The plot is updated automatically when parameters of the Compressor block change.

Tunable: Yes

Attack time (s) — Time for applied gain to ramp up

0.05 (default) | scalar in the range 0 to 4 inclusive

Attack time is the time the compressor gain takes to rise from 10% to 90% of its final value when the input goes above the threshold. The Attack time (s) parameter smooths the applied gain curve.

To specify Attack time (s) from an input port, select Specify from input port for the parameter.

Tunable: Yes

Release time (s) — Time for applied gain to ramp down

0.2 (default) | scalar in the range 0 to 4 inclusive

Release time is the time the compressor gain takes to drop from 90% to 10% of its final value when the input goes below the threshold. The Release time (s) parameter smooths the applied gain curve.

To specify Release time (s) from an input port, select Specify from input port for the parameter.

Tunable: Yes

Make-up gain mode — Make-up gain mode

Property (default) | Auto

• Property –– Make-up gain is set to the value specified by the Make-up gain (dB) parameter.

• Auto –– Make-up gain is applied at the output of the Compressor block such that a steady-state 0 dB input has a 0 dB output.

Tunable: No

Make-up gain (dB) — Applied make-up gain

0 (default) | scalar in the range –10 to 24 inclusive

Make-up gain compensates for gain lost during compression. It is applied at the output of the Compressor block.

Tunable: Yes

Dependencies

To enable this parameter, set the Make-up gain mode parameter to Property.

Inherit sample rate from input — Specify source of input sample rate

on (default) | off

When you select this parameter, the block inherits its sample rate from the input signal. When you clear this parameter, specify the sample rate in the Input sample rate (Hz) parameter.

Tunable: No

Input sample rate (Hz) — Sample rate of input

44100 (default) | positive scalar

Tunable: Yes

Dependencies

To enable this parameter, clear the Inherit sample rate from input parameter.

Advanced Tab

Output gain (dB) — Gain applied on each input sample

off (default) | on

When you select this parameter, an additional output port, G, is added to the block. The G port outputs the gain applied on each input channel in dB.

Tunable: No

Sidechain — Enable sidechain input

off (default) | on

When you select this parameter, an additional input port SC is added to the block. The SC port enables dynamic range compression of the input signal x using a separate sidechain signal.

The datatype and (frame) length input to the SC port must be the same as the input to the x port.

The number of channels of the sidechain input must be equal to the number of channels of x or be equal to one.

• Sidechain channel count is equal to one –– The computed gain, G, based on this channel is applied to all channels of x.

• Sidechain channel count is equal to channel count of x –– The computed gain, G, for each sidechain channel is applied to the corresponding channel of x.

Simulate using — Specify type of simulation to run

Interpreted execution (default) | Code generation

• Interpreted execution –– Simulate model using the MATLAB^® interpreter. This option shortens startup time and has a simulation speed comparable to Code generation. In this mode, you can debug the source code of the block.

• Code generation –– Simulate model using generated C code. The first time you run a simulation, Simulink^® generates C code for the block. The C code is reused for subsequent simulations, as long as the model does not change. This option requires additional startup time, but the speed of the subsequent simulations is comparable to Interpreted execution.

Tunable: No

Block Characteristics

Data Types	double | single
Direct Feedthrough	no
Multidimensional Signals	no
Variable-Size Signals	yes
Zero-Crossing Detection	no

Algorithms

The Compressor block processes a signal frame by frame and element by element.

1. The N-point signal, x[n], is converted to decibels:

   $$x_{\text{dB}}[n]=20\times {\log }_{10}\left|x[n]\right|$$

2. x_dB[n] passes through the gain computer. The gain computer uses the static compression characteristic of the Compressor block to attenuate gain that is above the threshold.

   If you specified a soft knee, the gain computer has the following static characteristic:

   $$x_{\text{sc}}(x_{\text{dB}})=\{\begin{array}{cc}{x}_{\text{dB}}& x_{\text{dB}}<\left(T-\frac{W}{2}\right)\\ x_{\text{dB}}+\frac{\left(\frac{1}{R}-1\right){\left(x_{\text{dB}}-T+\frac{W}{2}\right)}^2}{2W}& \left(T-\frac{W}{2}\right)\le x_{\text{dB}}\le \left(T+\frac{W}{2}\right)\\ T+\frac{\left(x_{\text{dB}}-T\right)}{R}& x_{\text{dB}}>\left(T+\frac{W}{2}\right)\end{array},$$

   where T is the threshold, R is the compression ratio, and W is the knee width.

   If you specified a hard knee, the gain computer has the following static characteristic:

   $$x_{\text{sc}}(x_{\text{dB}})=\{\begin{array}{cc}{x}_{\text{dB}}& x_{\text{dB}}<T\\ T+\frac{\left(x_{\text{dB}}-T\right)}{R}& x_{\text{dB}}\ge T\end{array}$$

3. The computed gain, g_c[n], is calculated as

   $$g_{\text{c}}[n]=x_{\text{sc}}[n]-x_{\text{dB}}[n].$$

4. g_c[n] is smoothed using specified attack and release time parameters:

   $$g_{\text{s}}\text{[}n]=\{\begin{array}{cc}{\alpha }_{\text{A}}{g}_{\text{s}}[n-1]+(1-{\alpha }_{\text{A}})g_{\text{c}}[n],& g_{\text{c}}[n]\le g_{\text{s}}[n-1]\\ {\alpha }_{\text{R}}{g}_{\text{s}}\text{[}n-1]+(1-{\alpha }_{\text{R}})g_{\text{c}}[n],& g_{\text{c}}[n]>g_{\text{s}}[n-1]\end{array}$$

   The attack time coefficient, α _A, is calculated as

   $${\alpha }_{\text{A}}=\exp \left(\frac{-\log (9)}{Fs\times T_{\text{A}}}\right).$$

   The release time coefficient, α _R, is calculated as

   $${\alpha }_{\text{R}}=\exp \left(\frac{-\log (9)}{Fs\times T_{\text{R}}}\right).$$

   T _A is the attack time period, specified by the Attack time (s) parameter. T_R is the release time period, specified by the Release time (s) parameter. Fs is the input sampling rate, specified by the Inherit sample rate from input or the Input sample rate (Hz) parameter.

5. If Make-up gain (dB) is set to Auto, the make-up gain is calculated as the negative of the computed gain for a 0 dB input:

   $$M={-x_{\text{sc}}|}_{x_{\text{dB}}=0}.$$

   Given a steady-state input of 0 dB, this configuration achieves a steady-state output of 0 dB. The make-up gain is determined by the Threshold (dB), Ratio, and Knee width (dB) parameters. It does not depend on the input signal.

6. The make-up gain, M, is added to the smoothed gain, g_s[n]:

   $$g_{\text{m}}[n]=g_{\text{s}}[n]+M$$

7. The calculated gain in dB, g_dB[n], is translated to a linear domain:

   $$g_{\text{lin}}[n]={10}^{\left(\frac{g_{\text{m}}[n]}{20}\right)}$$

8. The output of the dynamic range compressor is given as

   $$y[n]=x[n]\times g_{\text{lin}}[n].$$

References

[1] Giannoulis, Dimitrios, Michael Massberg, and Joshua D. Reiss. "Digital Dynamic Range Compressor Design –– A Tutorial And Analysis." Journal of Audio Engineering Society. Vol. 60, Issue 6, 2012, pp. 399–408.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2016a