Masterclass - Noise Gates
Modern gates are very sophisticated and it would be a mistake to think that they're only good for removing noise. Paul White explores a few alternative applications.
Gates have come a long way since their invention, with the addition of comprehensive side-chain filtering, external Keying and elaborate envelope control; more sophisticated models can even send and receive MIDI note information, allowing their triggering action to be synchronised to MIDI — or MIDI equipment to be triggered directly from the gate. This type of gate is ideal for such tasks as drum sound replacement, because once the gate is set to trigger to the required sound on tape, the MIDI Out from the gate can be used to drive a sampler or drum machine directly. Or, working with an external MIDI source, an audio signal can be chopped up into precise rhythmic segments simply by feeding in the desired pattern of notes from a MIDI sequencer.
As the majority of gates are not MIDI equipped, however, this article will be based around the Drawmer DS201, partly because of its widespread popularity, and partly because it combines all the features you'd expect to find on a well-specified studio gate; naturally, the techniques described can be applied to any gate offering similar facilities.
Signal ducking is more normally associated with compressors, but a gate with a dedicated ducking function is easier to set up and more predictable in operation. For those not yet familiar with ducking, it is simply a system for using one signal to control the level of another. The oft-cited example is the automatic voice-over, where the speech signal is used to automatically bring down the level of the background music using a ducker. You can hear this effect on the radio every day — the DJ starts to talk and straight away the level of the music drops so that he can be heard. The fact that most of us would prefer to listen to the background music rather than the DJ is a quite different topic altogether.
There are several musical applications of ducking; you may want to pull down a rhythm guitar and keyboard pad part to give a solo instrument more room to breathe, or it may be useful to hold down the level of a reverb or echo return, allowing it to swell back in only when the singer or instrument pauses.
Figure 1 shows how ducking is set up using a gate; note that when controlling a stereo signal:
• The gate must be set to Stereo Link mode.
• The required gate channels must be set to Duck mode.
• The Key Source must be set to External.
The signal to be ducked is fed in via the main audio input (or both in the case of stereo), while the controlling signal (in this example a voice) is fed into one of the Key inputs. Obviously a mic can't be plugged straight into the gate's Key input as this accepts only line level signals. A good place to pick up a suitable signal is from a channel insert send on the mixing console.
Setting up the gate is relatively straightforward:
• The first task is to adjust the threshold so that the gate triggers reliably on the incoming Key (voice) signal. This may be confirmed by monitoring the action of the trigger LEDs.
• The precise amount of attenuation during ducking can be set using the Range control which is calibrated in dBs; a typical setting might be between 5 and 15dB.
• How fast the ducking action occurs depends on the setting of the gate's attack and release times.
• It's worth introducing half a second or so of hold time to prevent the gate from trying to respond to the short pauses between individual words.
• Normally a relatively fast attack, say around 50mS, is sufficient to bring the level under control fairly positively, but without being so fast as to cause clicks. A longer release time can be set so that the levels comes back up smoothly once the speech has ended; try between 0.25 and 1S and adjust by ear until the result feels natural.
Once you've got the hang of this procedure, it can be used to create more space in a mix by pulling down pad and rhythm parts to let vocals or solo instrumental parts breathe. Experiment with attack and release times, as a small amount of noticeable level pumping can add to the power and excitement of a mix. As a rule, in this application, only very small amounts of gain reduction are required, usually just a few dBs.
Earlier, I described how a MIDI gate might be used to chop up a sustained sound in a rhythmic manner. You can also do this using a non-MIDI gate, simply by following this procedure:
• Select External trigger mode
• Feed the gate's Key input with bursts of sound of a suitable duration. (This is simpler if a sequencer is used, as its position and length quantise functions can be used to create very precise bursts of sounds from a synth module.) Any sound will serve as a trigger source as long as it has a fast attack and a fast release.
• The gate's attack and release may then be set to their fastest settings and the hold time set to around 20mS to prevent 'chattering'.
Normally, the Range control would be set to maximum to produce silence between triggered bursts, but it is worth experimenting with the Range setting to see what other effects can be achieved. For example, a range setting of around 10dB might work with a pad sound, which would then appear to play at a low level with the higher-level triggered bursts being superimposed on top. This technique is particularly well suited to programmed bass sounds, which can be processed using part of your drum track to create a very tight, dynamic bass line; you may have heard this in tracks by The Shamen, who use this trick extensively. Figure 3 shows how chopping may be set up.
Chopping may also be extended to treating effects returns — indeed, gated reverb was first devised by gating the sound from a reverberant room using a drum mic as the Key trigger source.
Set your gate to External trigger, switch one channel to Gate and the other to Duck, feed a regular pulse into one of the Key inputs (Stereo Link on) and you have an auto panner. The attack and release times should be set to identical values, between 300mS and 1S, and the trigger pulse fed in at a rate corresponding to twice this time. For example, if you set a half-second attack and release time, then the trigger pulse needs to come along every second to produce a complete pan sweep. Because one channel is set to Duck and the other to gate, the sound level will go up in one channel as it goes down in the other.
When panning a mono signal, this should be split to feed both gate inputs; a stereo signal simply needs plugging into the two gate inputs. The best results are often achieved by synchronising the trigger source (which may be a rimshot or similar transient sound output from a drum machine) to the tempo of the music — again, this is simple where a sequencer is being used. The gate attack and release times are then adjusted to produce a complete sweep between triggers. A nice touch is to pan delays or reverb returns in time with the tempo of the track. Figure 4 illustrates a typical panning setup.
Gates are of little use in cleaning up complete stereo mixes as there will be no periods of absolute silence that will allow them to close. However, the gate can be useful for cleaning up the start of a song so that the first note comes in cleanly from absolute silence. Similarly, when the song finishes, it's nice to have the last note decay into silence. This simply requires a gate set to Stereo Link mode with a fast attack time and a slow release time. The release time will need to be adjusted by ear to suit the decay characteristics of the last note; if the final decay seems cut short, select a longer release time, but if hiss and noise is still audible after the final note has faded, then a shorter release time is in order.
As in all applications, the gate's threshold setting should be as low as possible to allow positive triggering, but without any false triggering from noise. You may find that two different threshold settings are needed — one for the start of the song and one for the end — in which case the correct settings should be established by experimentation and then marked so that the control can be changed to the new setting once the song is playing.
Feature by Paul White
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