noiseGate

Dynamic range gate

Description

The noiseGate System object™ performs dynamic range gating independently across each input channel. Dynamic range gating suppresses signals below a given threshold. It uses specified attack, release, and hold times to achieve a smooth applied gain curve. Properties of the noiseGate System object specify the type of dynamic range gating.

To perform dynamic range gating:

Create the noiseGate object and set its properties.
Call the object with arguments, as if it were a function.

To learn more about how System objects work, see What Are System Objects?.

Creation

Syntax

dRG = noiseGate

dRG = noiseGate(thresholdValue)

dRG = noiseGate(___,Name,Value)

Description

dRG = noiseGate creates a System object, dRG, that performs dynamic range gating independently across each input channel.

dRG = noiseGate(thresholdValue) sets the Threshold property to thresholdValue.

dRG = noiseGate(___,Name,Value) sets each property Name to the specified Value. Unspecified properties have default values.

Example: dRG = noiseGate('AttackTime',0.01,'SampleRate',16000) creates a System object, dRG, with a 10 ms attack time and a 16 kHz sample rate.

Properties

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Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the release function unlocks them.

If a property is tunable, you can change its value at any time.

For more information on changing property values, see System Design in MATLAB Using System Objects.

`Threshold` — Operation threshold (dB)
`–10` (default) | real scalar

Operation threshold in dB, specified as a real scalar.

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

Tunable: Yes

Data Types: single | double

`AttackTime` — Attack time (s)
`0.05` (default) | real scalar

Attack time in seconds, specified as a real scalar greater than or equal to 0.

Attack time is the time it takes the applied gain to rise from 10% to 90% of its final value when the input goes below the threshold.

Tunable: Yes

Data Types: single | double

`ReleaseTime` — Release time (s)
`0.02` (default) | real scalar

Release time in seconds, specified as a real scalar greater than or equal to 0.

Release time is the time it takes the applied gain to drop from 90% to 10% of its final value when the input goes above the threshold.

Tunable: Yes

Data Types: single | double

`HoldTime` — Hold time (s)
`0.05` (default) | real finite scalar

Hold time in seconds, specified as a real scalar greater than or equal to 0.

Hold time is the period in which the applied gain is held constant before it starts moving toward its steady-state value. Hold time begins when the input level crosses the operation threshold.

Tunable: Yes

Data Types: single | double

`SampleRate` — Input sample rate (Hz)
`44100` (default) | positive scalar

Input sample rate in Hz, specified as a positive scalar.

Tunable: Yes

Data Types: single | double

Usage

Syntax

audioOut = dRG(audioIn)

[audioOut,gain] = dRG(audioIn)

Description

example

audioOut = dRG(audioIn) performs dynamic range gating on the input signal, audioIn, and returns the gated signal, audioOut. The type of dynamic range gating is specified by the algorithm and properties of the noiseGate System object, dRG.

[audioOut,gain] = dRG(audioIn) also returns the applied gain, in dB, at each input sample.

Input Arguments

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`audioIn` — Audio input to noise gate
matrix

Audio input to the noise gate, specified as a matrix. The columns of the matrix are treated as independent audio channels.

Data Types: single | double

Output Arguments

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`audioOut` — Audio output from noise gate
matrix

Audio output from the noise gate, returned as a matrix the same size as audioIn.

Data Types: single | double

`gain` — Gain applied by noise gate (dB)
matrix

Gain applied by noise gate, returned as a matrix the same size as audioIn.

Data Types: single | double

Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named obj, use this syntax:

release(obj)

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Specific to noiseGate

`visualize`	Visualize static characteristic of dynamic range controller
`createAudioPluginClass`	Create audio plugin class that implements functionality of System object
`parameterTuner`	Tune object parameters while streaming

MIDI

`configureMIDI`	Configure MIDI connections between audio object and MIDI controller
`disconnectMIDI`	Disconnect MIDI controls from audio object
`getMIDIConnections`	Get MIDI connections of audio object

Common to All System Objects

`clone`	Create duplicate System object
`isLocked`	Determine if System object is in use
`release`	Release resources and allow changes to System object property values and input characteristics
`reset`	Reset internal states of System object
`step`	Run System object algorithm

The createAudioPluginClass and configureMIDI functions map tunable properties of the noiseGate System object to user-facing parameters:

Property	Range	Mapping	Unit
`Threshold`	[–140, 0]	linear	dB
`AttackTime`	[0, 4]	linear	seconds
`ReleaseTime`	[0, 4]	linear	seconds
`HoldTime`	[0, 4]	linear	seconds

Examples

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Gate Audio Signal

Open Live Script

Use dynamic range gating to attenuate background noise from an audio signal.

Set up the dsp.AudioFileReader and audioDeviceWriter System objects™.

frameLength = 1024;
fileReader = dsp.AudioFileReader( ...
    'Filename','Counting-16-44p1-mono-15secs.wav', ...
    'SamplesPerFrame',frameLength);
deviceWriter = audioDeviceWriter( ...
    'SampleRate',fileReader.SampleRate);

Corrupt the audio signal with Gaussian noise. Play the audio.

while ~isDone(fileReader)
    x = fileReader();
    xCorrupted = x + (1e-2/4)*randn(frameLength,1);
    deviceWriter(xCorrupted);
end

release(fileReader)

Set up a dynamic range gate with a threshold of -25 dB, an attack time of 0.01 seconds, a release time of 0.02 seconds, and a hold time of 0 seconds. Use the sample rate of your audio file reader.

gate = noiseGate(-25, ...
    'AttackTime',0.01, ...
    'ReleaseTime',0.02, ...
    'HoldTime',0, ...
    'SampleRate',fileReader.SampleRate);

Set up a time scope to visualize the signal before and after dynamic range gating.

scope = timescope( ...
    'SampleRate',fileReader.SampleRate, ...
    'TimeSpanOverrunAction','Scroll', ...
    'TimeSpanSource','property',...
    'TimeSpan',16, ...
    'BufferLength',1.5e6, ...
    'YLimits',[-1 1], ...
    'ShowGrid',true, ...
    'ShowLegend',true, ...
    'Title','Corrupted vs. Gated Audio');

Play the processed audio and visualize it on scope.

while ~isDone(fileReader)
    x = fileReader();
    xCorrupted = x + (1e-2/4)*randn(frameLength,1);
    y = gate(xCorrupted);
    deviceWriter(y);
    scope([xCorrupted,y]);
end

release(fileReader)
release(gate)
release(deviceWriter)
release(scope)

Tune Noise Gate Parameters

Open Live Script

Create a dsp.AudioFileReader to read in audio frame-by-frame. Create an audioDeviceWriter to write audio to your sound card. Create a noiseGate to process the audio data.

frameLength = 1024;
fileReader = dsp.AudioFileReader('RockDrums-44p1-stereo-11secs.mp3', ...
    'SamplesPerFrame',frameLength);
deviceWriter = audioDeviceWriter('SampleRate',fileReader.SampleRate);

dRG = noiseGate('SampleRate',fileReader.SampleRate);

Call parameterTuner to open a UI to tune parameters of the noiseGate while streaming.

parameterTuner(dRG)

In an audio stream loop:

Read in a frame of audio from the file.
Apply dynamic range gating.
Write the frame of audio to your audio device for listening.

While streaming, tune parameters of the dynamic range gate and listen to the effect.

while ~isDone(fileReader)
    audioIn = fileReader();
    audioOut = dRG(audioIn);
    deviceWriter(audioOut);
    drawnow limitrate % required to update parameter
end

As a best practice, release your objects once done.

release(deviceWriter)
release(fileReader)
release(dRG)

Algorithms

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The noiseGate System object processes a signal frame by frame and element by element.

Convert Input Signal to Magnitude

The N-point signal, x[n], is converted to magnitude:

$x_{a} [n] = | x [n] | .$

Gain Computer

x_a[n] passes through the gain computer. The gain computer uses the static characteristic properties of the dynamic range gate to determine a brick-wall gain for signal below the threshold:

$g_{c} (x_{a}) = {\begin{matrix} 0 & x_{a} < T_{lin} \\ 1 & x_{a} \geq T_{lin} \end{matrix} .$

T_lin is the threshold property converted to a linear domain:

$T_{lin} = 10^{(\frac{T_{dB}}{20})} .$

Gain Smoothing

The computed gain, g_c[n], is smoothed using specified attack, release, and hold time properties:

$g_{s} [n] = {\begin{matrix} α_{A} g_{s} [n - 1] + (1 - α_{A}) g_{c} [n] \\ g_{s} [n - 1] \\ α_{R} g_{s} [n - 1] + (1 - α_{R}) g_{c} [n] \\ g_{s} [n - 1] \end{matrix} \begin{matrix} (C_{A} > T_{H}) & (g_{c} [n] \leq g_{s} [n - 1]) \\ C_{A} \leq T_{H} \\ (C_{R} > T_{H}) & (g_{c} [n] > g_{s} [n - 1]) \\ C_{R} \leq T_{H} \end{matrix}$

The attack time coefficient, α_A , is calculated as

$α_{A} = \exp (\frac{- \log (9)}{F s \times T_{A}}) .$

The release time coefficient, α_R , is calculated as

$α_{R} = \exp (\frac{- \log (9)}{F s \times T_{R}}) .$

T_A is the attack time period, specified by the AttackTime property. T_R is the release time period, specified by the ReleaseTime property. Fs is the input sampling rate, specified by the SampleRate property.

C_A and C_R are hold counters for attack and release, respectively. The limit, T_H , is determined by the HoldTime property.

Apply Gain

The output of the dynamic range gate is given as

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

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.

Documentation

noiseGate

Description

Creation

Syntax

Description

Properties

`Threshold` — Operation threshold (dB)
`–10` (default) | real scalar

`AttackTime` — Attack time (s)
`0.05` (default) | real scalar

`ReleaseTime` — Release time (s)
`0.02` (default) | real scalar

`HoldTime` — Hold time (s)
`0.05` (default) | real finite scalar

`SampleRate` — Input sample rate (Hz)
`44100` (default) | positive scalar

Usage

Syntax

Description

Input Arguments

`audioIn` — Audio input to noise gate
matrix

Output Arguments

`audioOut` — Audio output from noise gate
matrix

`gain` — Gain applied by noise gate (dB)
matrix

Object Functions

Specific to noiseGate

MIDI

Common to All System Objects

Examples

Gate Audio Signal

Tune Noise Gate Parameters

Algorithms

Convert Input Signal to Magnitude

Gain Computer

Gain Smoothing

Apply Gain

References

Extended Capabilities

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

See Also

Topics

Audio Toolbox Documentation

Support

Documentation

noiseGate

Description

Creation

Syntax

Description

Properties

Threshold — Operation threshold (dB) –10 (default) | real scalar

AttackTime — Attack time (s) 0.05 (default) | real scalar

ReleaseTime — Release time (s) 0.02 (default) | real scalar

HoldTime — Hold time (s) 0.05 (default) | real finite scalar

SampleRate — Input sample rate (Hz) 44100 (default) | positive scalar

Usage

Syntax

Description

Input Arguments

audioIn — Audio input to noise gate matrix

Output Arguments

audioOut — Audio output from noise gate matrix

gain — Gain applied by noise gate (dB) matrix

Object Functions

Specific to noiseGate

MIDI

Common to All System Objects

Examples

Gate Audio Signal

Tune Noise Gate Parameters

Algorithms

Convert Input Signal to Magnitude

Gain Computer

Gain Smoothing

Apply Gain

References

Extended Capabilities

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

See Also

Topics

Audio Toolbox Documentation

Support

`Threshold` — Operation threshold (dB)
`–10` (default) | real scalar

`AttackTime` — Attack time (s)
`0.05` (default) | real scalar

`ReleaseTime` — Release time (s)
`0.02` (default) | real scalar

`HoldTime` — Hold time (s)
`0.05` (default) | real finite scalar

`SampleRate` — Input sample rate (Hz)
`44100` (default) | positive scalar

`audioIn` — Audio input to noise gate
matrix

`audioOut` — Audio output from noise gate
matrix

`gain` — Gain applied by noise gate (dB)
matrix

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