How A Gate Does Its Thing

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The Music Telegraph
Text 2022-08-16


Figure 1: Expander/Gate plug-in of Pro Toos

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How A Gate Does Its Thing


When an expander is set to infinite attenuation limit, it is called a noise gate. There are circuits that are designed to only do gating. This device acts as an infinite expander that will either pass signal above threshold or not pass it below threshold. In this way, the desired signal is allowed to pass, while background sounds and noises that would otherwise be heard in the pauses in the music are not. This device also is effective in reducing leakage. Expanders can be set to act as goise gates (with an attenuation range of infinity).


The gate (an extreme state of an expander) allows a signal above a pre-selected level (the "threshold") to be passed through to the output at unity gain, without dynamic processing. However, once the input signal falls below the threshold level, the gate circuit shuts down the signal by fully or partially attenuating the output. In this way, the desired signal (above threshold) is allowed to pass, but unwanted signal is not. The gate is useful for noise removal on individual tracks, and can be used for special effects such as cutting off a reverb tail. 



The key parameters of Expander/Gate (see Figure 1 in the above picture):

(In this article, I am dealing with Expander/Gate plug-in of Pro Tools.)

  • Phase Invert (∅): Reverses the phase (polarity) of the input signal, allowing you to change frequency response characteristics between multi-miked sources or to correct for miss-wired microphone cables.
  • Thresh: Sets the threshold level. Signals that exceed this level will pass through. Signals that are below it will be gated in the amount determined by the settings of the Attack, Hold, Release and Range parameters.
  • Attack: Sets the attack time (in milliseconds) of the Expander/Gate.
  • Hold: Specifies a duration (in seconds or milliseconds) that the Expander/Gate will stay open after the initial attack cycle. This can be used as a one-shot function to keep the Expander/Gate open for longer periods of time with a single crossing of the threshold. It can also be used to prevent 'gate chatter', which may occur if varying input levels near the threshold cause the Expander/Gate to open and close very rapidly.
  • Release: Controls how long it takes for the Expander/Gate to close after the signal falls below the threshold level. This adjustment may range anywhere from milliseconds to seconds. 
  • Range: Sets the depth of the Expander/Gate when closed. It has a maximum depth of -80 dB. This is sometimes called the 'attenuation limit'. Setting the Expander/Gate to higher range levels allows more of the gated audio that falls below the threshold to bleed through at all times. This is useful for things such as 'drum leakage' where you may want to suppress the overall drum kit sound by a specific amount, while emphasizing the gated instrument such as a snare.
  • Side-Chain: Side-chain processing allows the Expander/Gate to operate according to the varying amplitude of a separate reference track (the control signal) and use it to trigger gating. 






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