If you're swamped in hiss, then you may need this — the full SP on gates and their uses. But listen carefully: I may be asking questions.
Gates are a valuable ally in the fight against noise in the studio - but you need to know their limitations and weaknesses. Paul White explains how they work.
When you first start multitrack recording, the fact that you can get a result at all is a great thrill, but very soon you become more discerning about sound quality, and especially noise, which plagues everyone to a greater or lesser degree. By setting sensible recording levels and by optimising input levels at the mixer and effects units, you can keep noise at a minimum, but with budget equipment, analogue tape and electronic instruments that generate some degree of hum and hiss, some noise is inevitable, even in a professional studio. Noise gates are not a simple fix-it for poor work, but when you've done the best you can, they can be used to make a good piece of work even better. But before you can make the best use of a gate, it really helps to know a little bit about what it does and how it works.
To explain how a gate works, I'll start as I did when examining compressors, by looking at what you might do if you were recording and you didn't have a gate. For example, you might have a vocal track on a multitrack recorder with a little traffic noise from outside, the singer's breathing and the occasional rustle of clothing in the background. Most of the time the singing will hide this, but between phrases when there is a pause in the singing, the background noise is clearly audible because there's nothing else going on to mask it. One solution is to use the appropriate channel fader or mute button to turn off the vocal track at the end of each phrase and then turn it back on just before the next line starts. This ensures complete silence between phrases but it does mean you have to know the song pretty well to turn the gate on and off in the right places. Figure 1 shows how this looks when drawn as a block diagram.
The gate works in a similar way except that it doesn't have to learn the song — it simply has to monitor the signal level and then open and close when the signal crosses a threshold that is set just above the level of the noise. Providing the gate can open really quickly, the amount of signal lost from the beginning of each phrase will be insignificant, and when the phrase ends, the gate closes again. The result is, once again, silence during the pauses — though it must be stressed that a gate can do absolutely nothing to reduce noise in the presence of signal. Figure 2 shows a signal being gated. Note the tiny portion of the attack of the sound that is lost before the gate opens.
A gate that simply snaps open when a signal comes along and then snaps shut again when the signal finishes is not particularly subtle — indeed, the sound of the background noise being abruptly switched on and off can be more disconcerting than having the noise there all the time. What's more, a signal with a long decay could be prematurely cut short as the gate closes.
Modern studio gates have attack and release controls, which help you to optimise the gating action to the signal being treated. The release time control allows the gate to close more slowly when required, so if the sound being treated has a long decay time, the decay time of the gate can be set accordingly. Now when the signal level falls, the end of the signal will be gently faded out rather than chopped off.
The logic behind fitting a release time control is fairly evident from this previous example, but it's not quite so obvious why we need to be able to vary the attack time. If the gate opens instantly, surely it will allow any type of signal to pass through? The answer to this one is yes, the gate will open fast enough to let any signal pass through, but a potential problem occurs when the sound being processed itself has a slow attack. Now what happens is that the sound starts to build up but the gate doesn't open until it reaches the threshold level, then suddenly, the sound is switched on. This can be seen in Figure 3. The result is that the signal has a little step in it where the gate opens, and this is audible as a click. The higher the threshold setting, the bigger the step and the worse the click. To avoid this, we use the attack time control to make the gate open more slowly, so that the sound fades in rather than coming in with a bang — literally. In practice, we would set the gate's attack time as fast as possible and then back it off slowly while listening to the result. When a point has been reached that removes the click, the setting is right.
Just as when using a compressor, the attack and release times of the gate should not be set to their fastest values, because the gate might trigger on individual cycles of the input signal rather than on the overall envelope. Some manufacturers add a Hold time control to force the gate to stay open for a short time before being allowed to close again, the actual hold time being variable from almost instantaneous to a second or more. The hold time control is also useful for creating special effects such as gated reverb — but more of that next month.
So far, we've established that gates have Threshold controls, Attack and Release time controls and sometimes even a Hold control. In fact, the complement of controls so far is similar to that of a compressor, the main difference being that a compressor only operates when the input signal exceeds the threshold, while a gate only operates when the input signal falls below the threshold.
There are occasions when it would be useful to have the gate turn a signal down rather than completely off when it closes — for example, dialogue recorded in a noisy natural environment such as a shopping centre will sound more natural if some background noise is still present between phrases. That's where the Range control comes in, allowing the user to determine exactly how much the signal will be turned down or 'attenuated' when the gate is closed. The range normally varies from 0dB to 60dB or more, so you have plenty of flexibility. In our shopping centre example, the result might sound better with, say, a 6dB drop in background noise during pauses, rather than complete silence. The range control may also be used to set the amount by which the signal is reduced on those gates that double as duckers. Ducking was described last month when we looked at compressors, but many gates provide a similar facility and, arguably, do the job more predictably than compressors.
"Noise gates are not a simple fix-it for poor work, but when you've done the best you can, they can be used to make a good piece of work even better."
And while we're comparing gates and compressors, most dual-channel gates are also fitted with stereo link switches, allowing the two channels to work together when processing a stereo signal. This is less important than it is with compressors as, very often, the signal being gated is mono, but there are occasions, such as when miking an instrument in stereo, when the linked option is useful. Failure to use the linking facility could mean one gate closes before the other, resulting in apparent movement of the stereo image from left to right or vice versa.
A gate is a powerful tool for cleaning up individual tracks on a recording, or the output from a noisy instrument or microphone, but problems occur when the signal we're trying to process is mixed with a different signal. Take the example of a snare drum which has been miked up and recorded. As well as picking up the snare drum, the mic will also pick up some sound from the bass drum, which isn't very far away. If we try to gate the snare drum sound, the chances are that the gate will open on the bass drum beats too. Sometimes careful juggling with the threshold control will solve the problem, but just as often, it doesn't. It was to solve just such a problem that Ivor Drawmer fitted his DS201 noise gate with side-chain filters.
There are two filters on the DS201, and though other manufacturers have now adopted the idea, with the occasional variation, I'll stick to the original DS201 system for the sake of clarity. The two filters are fairly sharp shelving filters, one being high-pass (cuts low frequencies) and one being low-pass (cuts high frequencies). The filters have separate tuning controls allowing their frequencies to be varied — in effect, they are used to set an upper and lower limit to the frequencies that pass through to the side-chain. It is important to understand that these filters don't interfere with the main signal path; they simply affect what the side-chain circuitry 'hears'.
The principle to remember is that if the side-chain can't hear it, it can't trigger from it, so we should adjust the filters so that they pass the greatest amount of the signal we want to trigger from and the smallest amount of the signal that's causing false triggering. In the case of our bass drum and snare drum example, we can use the high-pass filter to remove all the very low frequencies from the side-chain signal so that the gate is triggering from the crack of the snares and not from the thump of the bass drum. A 'side-chain listen' mode is included, enabling the user to monitor the output of the filter as it is being adjusted; once set up, the adjustment of the threshold control should be far less critical.
Conversely, the low-pass filter can be used to prevent high frequency sounds from triggering the gate, and in the case of a drum kit, this could be used to stop the cymbals or hi-hats from inadvertently triggering the drum mics. In this way, not only do the gates remove noise but they also help to maintain the separation between different instruments or microphones working in close proximity. Figure 4 shows how the filters are connected inside the gate.
Next month I'll examine some of the less obvious uses for gates.
Feature by Paul White
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