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Audio Concepts: The Usage Of Gates Explained

Knowing not how gates work, not just how to use them, is key in crafting a quality mix.
This excerpt from “Mixing Audio: Concepts, Practices and Tools” was provided by Routledge

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Following compressors, gates are perhaps the second most common dynamic range processors in mixing.

Gates are also called noise-gates – a name that implies their traditional usage as noise eliminators. In the past, tapes and analog equipment were the main contributors to added hiss.

Nowadays, digital technology tends to allow much cleaner recordings. Still, in project studios unwanted noise on a recording might be the outcome of ground loops, a computer fan or airborne traffic. Noise, however, is not the only thing gates are employed to reduce.

Figure 1: The MOTU MasterWorks Gate plugin.

The rumble produced by the floor tom when the rest of the drum kit is played is often gated. The hi-hats spill on a snare track and the headphone spill on a vocal track are just two more examples.

In addition to their traditional role, gates are also used for more creative tasks, like tightening drums, adding punch or applying dynamic movement.

Controls

Figure 2: Inside a gate. The vertical chain shows the main side-chain stages, and the controls link to each stage.
Threshold

Gates affect signals below the threshold – these are either attenuated or muted. Signals above the threshold pass through unaffected, unless some attack is applied.

A gate only cares whether the signal is above or below the threshold; a gate is said to be closed when the signal is below the threshold and open when the signal is above it. Figure 3 illustrates the function of a gate on a snare.

Figure 3: Gate threshold function.

Threshold settings on a gate might seem straightforward – we set the threshold below the wanted signals and above the unwanted signals. In practice, this affair can be tricky since not always a certain threshold meets both criteria.

For example, the snare in Figure 3 lost some of its attack and much of its decay. We could retain more of both by lowering the threshold, but this would cause false triggering by the hi-hats. Solutions for this common issue are discussed later on. In the meantime we should note:

— Lower threshold settings on a gate are often sought after as they help in retaining more of the natural attack and decay.

— The original signal before gating involves a single snare hit and a few hi-hat spikes we wish to eliminate.

— The threshold is set above the hi-hat peaks, so the gate would only open once the snare hits. The gate would close as the snare hit drops below the threshold. After gating, only the loud portion of the snare remains.

Hysteresis

To allow quick gate opening once the signal exceeds the threshold, the level detection on most gates is based on peak-sensing. Level fluctuations are more profound with peak-sensing than with RMS. While fluctuating in level, signals may cross the threshold in both directions many times over a short period of time.

This causes rapid opening and closing of the gate, which produces a type of distortion called chattering (see Figure 4a). One way to overcome this is by having two thresholds – one above which the gate opens, another below which the gate closes.

Figure 4: Hysteresis control on a gate. (a) A gate with single threshold and no hysteresis control. Chattering is introduced due to the rapid opening and closing of the gate, which are triggered by quick level changes. (b) A gate with hysteresis control. The gate only opens as the signal overshoots the opening threshold and only closes as the signal drops below the closing threshold. Level variations between the two thresholds do not cause toggling of the gate state, thus no chattering occurs.

Having two threshold controls would be cumbersome since adjustments to one will call for adjustments to the other. Instead of providing two controls, gates offer a single threshold and a control called hysteresis.

The threshold is the opening threshold, and the hysteresis determines how many dB below it the closing threshold is set. For example, with the threshold set to −20 dB and hysteresis to 5 dB, the closing threshold would pose at −25 dB. Figure 4b illustrates this.

Many gates do not offer hysteresis as an adjustable control, but have a built-in setting fixed between 4 and 10 dB. When hysteresis control is given, these figures provide a good starting point.

Range

Range, or depth, defines the amount of gain applied on signals below the threshold. A range of −10 dB means that signals below the threshold are attenuated by 10 dB.

Often signals below the threshold are considered muted, although in practice this perceived muting is due to heavy attenuation with the typical range of −80 dB. Figure 5 shows the transfer function of a gate, while Figure 6 demonstrates the effect of different range settings.

Figure 5: The transfer function of a gate. (a) A gate with 0 dB range: both below and above the threshold the input–output ratio is at unity gain, and the gate would have no effect on the signal. (b) A gate with −20 dB range: all signals below the threshold are simply attenuated by 20 dB. For example, an input signal at −40 dB will leave the gate at −60 dB. (c) A gate with −80 dB range, which effectively mutes all the signals below the threshold. (d) Again −80 dB range, but with an extended output scale that reveals what happens below the limits of the standard scale. We can see clearer here that an input signal at −40 dB will leave the gate at −120 dB.

The range value can be expressed in either positive or negative values. Depending on the manufacturer, a gain attenuation of 40 dB might be expressed with a range of 40 or −40 dB. This book uses the negative notation. Small range denotes little effect (e.g., −5 dB), while large range denotes more effect (e.g., −80 dB).

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Figure 6: The range function of a gate. When the range is set to −5 dB, signals below the threshold are attenuated, but still heard. With large range, such as −80 dB, signals below the threshold become inaudible

Generally speaking, large range settings are more common in mixing. However, some-times it is only a gentle attenuation we are after. One example would be gating vocals to reduce breath noises between phrases – removing breaths altogether is often perceived as artificial, so these are often only attenuated. Another example involves reducing the room reverb captured on a recording.

Attack And Release

Attack controls how quickly the gate opens, release controls how quickly the gate closes. For example, with the range set to −80 dB, a closed gate would apply −80 dB of gain on the input signal. The attack determines how quickly these −80 dB rise to 0 dB when the gate opens, while the release determines how quickly the gain returns to −80 dB when the gate closes.

The response times on a gate are normally set in milliseconds. Attack times usually span between 0.010 ms (10 ₃s) and 100 ms. Release times are often within the 5–3000 ms range.

As with compressors, both the attack and release times determine the rate of gain change. For instance, a gate might define that response times are referenced to 10 dB of gain change. With attack of 1 ms and a range of −10 dB, it will take 1 ms for the gate to fully open; but with −80 dB of range, it will take the gate 8 ms to open.

The practical outcome of this is that the range affects the overall time it takes a gate to open and close – where appropriate, we can achieve faster attack and release times by dialing a smaller range. In turn, this advocates smaller range settings than a gate might offer – on a busy mix, a range of −40 dB could suffice to make signals below the threshold inaudible.

It is worth noting that both the attack and release controls have an opposite effect on dynamic envelopes from compressors. A longer attack on a gate means that less of the natural attack is retained, a longer release on a gate means that more of the natural decay is retained. Figure 7 illustrates this on a gated snare.

Figure 7: The effect of attack and release on a gated snare. The top row shows different attack times with no release; the drop in gain reduction is shown as a triangle above the graphs. We can see that the longer the attack is the longer it takes gain reduction to drop and the less of the snare’s original attack is retained. Long attack setting causes gain reduction to drop so slowly that none of the signal is audible before the input drops below the threshold. The bottom row shows different release times with no attack; the rise in gain reduction is shown as a triangle above the graphs. We can see that the longer the release is, the slower the gate reduces the signal once it falls below the threshold, so more of the natural decay is audible.

On a gate: longer attack, less of the natural attack; longer release, more of the natural decay. Very short attack and release times can cause audible clicks due to abrupt level changes.

A steep level rise produced by fast attack generates high frequencies, the shorter the attack the higher the frequency. The same applies for release, although the gate operation on the quiet signals below the threshold tends to be slightly less noticeable.

Depending on the gated signal, fast response times might also alter the low and mid frequencies, which essentially means that a gate can affect any part of the frequency spectrum – a side-effect we have to observe. The attack setting has an extra weight when gating transients, especially those with low-frequency content like kicks.

Most of the kick’s character is in its very first cycle, with most of the impact gearing up at the beginning of this cycle. A gate is most likely to reshape this important part of the waveform – either with a short attack that produces sharp level changes or with a longer attack that reshapes the waveform and withholds the kick impact.

One way or another, the kick’s tonality is very likely to change, with the lows, mids and highs all likely to suffer. Ideally, we would like the attack to retain the original shape of the signal’s waveform, which is often only possible with look-ahead (soon discussed). Figure 8 illustrates this.

Figure 8: A gated kick. The top track shows the original kick, and it is hard not to notice the dominant first cycle. The middle track shows the kick after gating with the settings shown on the Digidesign DigiRack Expander/Gate on the right. The attack was set as short as possible and the look-ahead was disabled. You can see the partial loss of initial impact and the steep level climb caused by the gate opening, which generates high frequencies. The bottom track shows original kick after gating with the same settings, but with look-ahead enabled. You can see how the initial attack was not affected.

Very short settings can also cause low-frequency distortion due to the gate operation within the cycles of a low-frequency waveform. Longer attack, release or hold can rectify this issue. In the case of percussive performance, we must consider how the release and hold settings, which affect the length of the sound, lock to the rhythmical feel of the track.

In addition, we want the gate to fully close before the next hit arrives, or successive hits will be gated differently (due to variety in gain reduction during the attack phase).

One issue with the principal operation of gates is that we often want short attack, so more of the signal above the threshold passes through, and short release, so the signal below the threshold is attenuated quickly. These typical short settings are more obstructive for the many reasons explained above.

We have to remember that in many cases the attack and release are applied on large-scale gain changes, with a range of −60 dB or more. Compressors, on the other hand, often work on 20 dB or so, and moreover, gain changes are not as steep due to the gradual development of the input signal.

A gate has no such softening mechanism – it is either open or closed, and there is often quite some gain involved in toggling between the two.

Not always short attack and release settings are appropriate though. Longer times are often used when the gated instrument has long natural attack and release, for example, a synthesized pad that rises and falls in a gradual fashion. Short attack and release will simply trim the parts of the signal that ascend or descend below the threshold. Long settings will keep some gradual sense for both the natural attack and decay.

Longer attack times might let us lower the threshold by a small extent. The reason for this is that false triggers will not be long enough to become audible.

For example, a long attack in Figure 3 would allow the threshold to be slightly below the hi-hat peaks. Although the top of each hit would trigger the gate opening, the slow attack would mean the gate will not fully open by the time the hit drops below the threshold, potentially leaving these hits inaudible.

Hold

Once the signal has dropped below the threshold, the hold time determines for how long the gate’s gain reduction is held unaltered. For example, if 8 dB of gain reduction is applied as the signal undershoots the threshold, a hold period of 2 seconds would mean 2 seconds with 8 dB of gain reduction. Once the hold period is completed, the release phase starts. Gates typically offer a hold period between 0 and 5 seconds.

Hold often replaces release in the task of retaining the natural decay of an instrument. There are two reasons for this.

— First, we can see from Figure 7 that quite a long release setting is needed in order to keep the natural decay. Such a long release is not always practical since it might not end before the next time the signal overshoots the threshold.

— Second, having the release starting right as the signal drops below the threshold causes an escalated decay – the natural decay of a snare, for example, will be superimposed by the artificial descent caused by the release function.

Using hold instead of release lets us keep the fall rate of the natural decay. The hold time can be made to match the interval between two hits, while some release is still used in order for the gate to close without audible clicks (Figure 9).

Figure 9: The difference between release and hold. With release, the natural decay of the instrument is reshaped, which is not the case with hold. The short release after the hold period is applied to prevent clicks.

Hold can be used to retain the character of the natural decay. Like with compressors, longer hold periods can reduce low-frequency distortion.

Longer hold time can also reduce chattering, as the gate is held open while the signal rapidly leaps between the two sides of the threshold. In that sense, the hold facility provides a similar function to hysteresis, so when no hysteresis is offered, hold can be used instead.

In addition, hold might be used to compensate for any look-ahead time that might be in force. For instance, if look-ahead is set to 10 ms, the gate will start closing 10 ms before the signal drops below the threshold. Setting hold time of 10 ms will compensate for this early response.

Look-ahead

This is perhaps the ultimate problem with gates: short attack results in clicks, long attack softens the natural attack and can even repress the beginning of words.

Lower threshold? Cannot be done or spill will open the gate.

Filter the side chain? Done, helps a little.

For this ultimate problem there is an ultimate solution – look-ahead.

As with a compressor, look-ahead lets the side chain examine the input signal slightly before processing takes place. This means that we can dial longer attack times, since the attack phase starts sooner.

By the time the signal arrives to the gain stage, gain reduction has decreased to a degree that allows the leading edge of the signal to pass through the gate with no clicks or envelope reshaping.

The very same principle applies for release – since the release phase starts shortly before the treated signal drops below the threshold, we can dial longer release times.

One of the prime benefits of look-ahead is that it allows longer attack and release times, which make the gate operation less obstructive. A look-ahead function on an analog gate will introduce output delay. Software gates have no such issue, provided auto plugin delay compensation is active. We will soon see an extremely useful look-ahead trick that results in no delay with both analog and digital gates.

Side-chain filters

As with compressors, some gates let us equalize the side-chain signal that triggers the gating. A classic example for where this can be extremely handy is when gating a kick track in order to get rid of spillage like the hi-hats or snare. It goes without saying that both the hi-hats and snare have less energy on the lows than the kick.

So we can filter the highs and mids content from the side-chain, leaving mostly the low-level energy of the kick. In turn, this lets us lower the threshold, so more of the original kick attack can be retained.

Key Input

Gates, just like compressors, let us feed an external signal into the side-chain. On a compressor, the external source input is called external side-chain. On a gate, the same input is often called key input instead (to prevent confusion when a dynamic range processor offers both a compressor and a gate, and the side-chain of each can be fed from a different external source).

We can feed the key input with either a similar signal to the one being gated or a different signal altogether. In the case of the former, an example would involve a gate microphone – an additional microphone on a specific drum, which is later used in mixdown only to feed a gate’s key input.

A gate microphone has one role – capturing the drum with as little spill as possible. Thus, gate microphones (often dynamic/cardioid) are placed as close as possible to the drum skin and are not concerned with faithfully capturing the timbre of the drum.

When we gate a snare, we feed the snare microphone into the gate input and the snare’s gate-mic into the key input. This way, we can dial lower threshold, achieve more accurate gating and have more freedom with the different control settings.

Gate microphones aside, sometimes a specific track will benefit from being gated in respect to a similar track. For example, a kick-out track is better gated with respect to the kick-in microphone, which would normally capture less spill (Figure 10).

Figure 10: Gating kick-out with relation to kick-in. This illustration shows a recording setup, although we usually do this during mixdown with the recorded tracks. The kick-in track is likely to incorporate less spill than the kick-out. While gating the kick-out, feeding the kick-in to the gate’s key input is likely to provide more musical gating. Also, the sound arrives to the kick-in microphone slightly before it does to the kick-out microphone, providing a natural look-ahead. A gate-mic can replace the kick-in for other drums like the snare.

Output level

Although not often provided, the output level control offers a similar functionality to a compressor’s make-up gain – boosting or attenuating the entire signal by a set amount of dB.

Stereo link

The stereo link function on a gate is similar to that on a compressor – it ensures that both left and right channels are gated identically, so no image shifting occurs.

Meters

Some gates provide a gain reduction meter just like compressors. Some have two indicators – one lights up when the gate is open, one when the gate is closed. Some gates add an additional indicator that lights up during the attack, hold and release phases, when the gate is neither fully open nor fully closed.

To purchase Mixing Audio: Concepts, Practices and Tools click on over to the Routledge website.

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