How It Works: the Noise Gate (Part 10)
What is a Noise Gate? What on earth is a MIDI Noise Gate? David Mellor supplies the answers and explains some standard and not so standard gating techniques, with special reference to the BSS Audio DPR 502 MIDI Noise Gate.
The human ear is a pretty sophisticated piece of apparatus. More sophisticated than the finest microphone, in fact. The ear has the knack of hearing only what you want to hear, that's why you can pick out what certain people in a crowd are saying even though their voices are semi-obscured by other peoples' conversations.
Microphones have little discrimination and pick up everything within their range and coverage angle, whether you want it picked up or not. And once the sound has been converted into an electrical current there is little you can do to sort out wanted sound from unwanted clutter... or is there?
Suppose, for instance, you are miking up a guitarist's amplifier - to record that 'rough and ready' sound, rather than the squeaky clean sound you get when you direct inject into the mixer. I haven't yet heard the amp that doesn't hiss and spit like an angry rattlesnake when the guitarist winds the volume up to 11 to get that subtle nuance of tone (distortion) he is after. Fortunately, when the guitarist is playing, the music he makes covers the noise from the amp. But when he doesn't play, you get a load of musically useless hiss and crackle on tape. The answer to the problem is to have some way of automatically closing the mixer fader on the guitar amp when the musician isn't playing.
We have just invented the noise gate! When the level going into it exceeds a certain threshold (guitar playing), the gate is open and everything gets through. When the level is lower than the threshold (guitar silent), the gate closes up and there is no hiss, hum or crackle at all.
For any type of multitrack recording where microphones are used, a noise gate is essential. The cleaning-up effect it has on the overall sound has to be heard to be appreciated. Note that you would only use it on individual tracks. It would be very rare to use a noise gate on mixed stereo material. Figure 1 shows the basic noise gate set-up.
Basically, noise gates are an open and shut case (!). But they need careful adjustment to achieve the best performance, so here comes a run through of the various noise gate controls, as typified by the BSS DPR 502:
THRESHOLD: A rotary control which sets the level at which the gate opens. Any sound louder than the threshold will trigger (open) the gate. If the level doesn't reach the threshold, the gate stays closed and no sound passes through. With the threshold set on its lowest level, everything gets through. As you increase the level, more and more sound is gated out - the loudest parts being the last to disappear. I find the best way to set the threshold control is to increase it until the sound I want to keep just gets cut off, then I back off a little. The DPR 502 noise gate has an LED bargraph meter which shows the level of the input signal, and also a triangular Gate Open LED. By judging the two together, you can get a good idea of the correct setting.
ENVELOPE: There are three envelope controls, Attack, Hold and Release. As I said earlier, the object of the gate is to have the sound you want to hear mask or cover up the noise, all the time the gate is open. Suppose you are gating a sound which has a slow attack and decay, a synthesized string sound perhaps. If the gate opened instantaneously, either the slow attack would get chopped, or you would hear the background noise come in abruptly. Setting the attack time to a longish value will 'fade in' the gate. The Release control does the same for the tail end of the sound.
Hold sets the length of time for which the gate is fully open. If you're trying to use the gate in a 'transparent' manner - ie. not to let anyone realise that it's being used - then you may not need to use Hold. For special effects, especially that Phil Collins/Hugh Padgham gated drum sound that everyone's after, Hold is invaluable.
RANGE: So far, I have assumed that the gate will be either fully open or fully closed. On multitrack work it's best to keep it this way. The DPR 502 will give 70dB of attenuation to the signal when the gate is closed, which is as fully off as a closed mixing console fader usually gets.
Sometimes, more subtle gating may be useful, especially for live performance work. The Range control lets you set an attenuation of 10dB, 20dB or whatever you like - or no attenuation at all if that's what you really want.
KEY FILTER: The Key Filter and Width controls are used to tune in to particular frequencies to trigger the gate. More on these later.
KEY SOURCE: Lets you derive the triggering signal from a separate input to the DPR 502, rather than the signal being gated.
MODE: For normal use this is set to 'Gate'. Ducking inverts the triggering system so that the gate is open when the signal is below the threshold, and closed when it is above it.
LINK: Makes the noise gate's two channels open and close simultaneously (for gating stereo signals).
Those are the essentials, now for more applications. The modern drum sound relies heavily on noise gates for its punch and power, in several ways.
In the early days of recording, to put three microphones on a drum kit was considered enough. More than enough, some might have said. Things progress however and it was found that, with a great deal of skill and perseverance, you could put a microphone on each individual drum and get a much better sound - or a much worse sound if you didn't have that necessary skill. (One underestimated ingredient to achieving a good multi-miked drum sound is a drummer who is prepared to adjust his kit to accommodate your miking needs. Such drummers are not always available!).
Fortunately, there is a saviour in the form of the noise gate. The problems with multi-miked drums can all be traced back to interaction between the various mics used on the kit. The tom-tom mics will be picking up the snare and vice-versa. The answer is to use a noise gate on each microphone signal, which will ensure that the mic is only 'live' when the drum is actually being hit. This won't get you out of trouble when the drummer insists on hitting the snare and bass drum at the same time (that's his prerogative!) but gating is a powerful technique. Remember that we're talking here about achieving a basic drum sound - I haven't got on to all those exciting effects yet!
Hands up if your digital reverb unit has a 'Gated Reverb' program. Once upon a time you needed a set-up like that shown in Figure 2.
This is just one way of achieving gated reverb and I can guarantee that it will give you a certain quality that you'll never hear on a digital effects unit. Note that I'm not saying that it is always a better effect, but the more effects you have the more imaginative your recordings can be. In many cases this will be just the job, and there are several variations I am sure you can think of.
NOTE: The DPR 502 gate is a line level device, therefore all microphones must be routed via a mixing console.
I mentioned earlier that it is possible to use the external Key Source feature of the DPR 502 to trigger the gate from a second sound input. In this example, what's happening is that the close microphone provides the principle sound for the drum. The room mic gives the reverberation, or ambience. The room mic is fed into the gate to create that 'gated' sound, and to ensure that the ambience doesn't mush up the recording. To get a good trigger for the gate, we take a feed from the close mic, via an auxiliary send on the mixer. Setting the Key Source to 'external' completes the set-up and produces gated reverb - just like that. One alternative would be to use a reverb unit (but not set to 'gated reverb'!) instead of the room mic. You could even gate a delay unit, producing a repeat echo, in a similar way.
The next step up is to use the 'envelope' of one sound to control another sound. As an example, let's have a synth playing a sustained chord, changing only with the pattern of the harmony. You could put a bit of life into this by feeding the synth through the gate and using a drum pattern as the trigger. You could feed, say, a hi-hat playing 16th notes into the external Key In socket and end up with the synth playing 16th notes along with it. Try something like this next time you do a multitrack mix and get a drum track to trigger a gate on one of the other sounds. It's another creative tool.
Back to noise gate theory - we need to know more about how the gate can be triggered. Figure 3 shows the signal paths through the DPR 502. Each channel of the DPR 502 gate has two inputs: the normal input and the Key input (which can be bled off from the normal input, or taken from the Key In socket). The Key input, as you can see, doesn't have an output. It just triggers the gate. The Key input, and the associated filtering and triggering circuitry, is known as the sidechain. An example of what the sidechain does may be helpful...
Suppose you're miking a drum kit and you want separate microphones on the snare and hi-hat. You'll gate both mics, of course. The mics are probably going to be fairly close to each other and the gate on the snare will probably open when the hi-hat plays, and vice-versa, because the level of the two instruments is similar. No matter how precisely you set the threshold, you can't separate the two.
The solution is to modify the signal going into the sidechain to achieve a more reliable trigger. The easiest method is to use the Key Filter and Width controls to home in on the frequency ranges of the two instruments. On the DPR 502, Key Filter sets the frequency and Width sets the range of frequencies the filter covers, from half an octave to full bandwidth. In the case I have outlined, I would set the gate on the hi-hat to '15kHz, 0.5 octave'. This corresponds to a region where the hi-hat is rich in frequencies and the snare is lacking. On the other hand, the snare is high in lower-middle frequencies. A one octave bandwidth at 200Hz should be right for this instrument. The hi-hat has little energy in this range.
There are two points to bear in mind. Firstly, that whatever filtering you do to the Key signal has no effect on frequency content at the audio output of the gate. It's purely an internal means to an end. The second point is that you can use the Check button on the DPR 502, if you wish, to route the sidechain signal to the main output momentarily, so that you can hear the effect of the filter, as an aid to setting up.
An essential feature of a stereo noise gate is the stereo Link switch, which forces both channels to open and close at the same time. In the case of the DPR 502, the controls on the right channel become inoperative and the controls on the left channel serve both.
If you're into effects units, then you will know that any sort of chorusing or flanging effect is liable to be noisy. This is inherent in the way the effect is produced. If it's a stereo chorus you are using, and you want to gate both of the outputs, then you could have a problem. Even if you had all the settings identical on each channel of the gate, they would open and close at different times according to how the two signals varied. This is an effect for which there is no known artistic use - in other words, it sounds dreadful! The Link button is the saviour, because both channels are forced to work identically. If you have set them correctly it will sound wonderful, with no noise.
One more function worth mentioning before I enter the merry world of MIDI is 'ducking'. You know when you listen to a phone-in programme on the radio, and the caller sometimes can't get to say his piece because the presenter just talks over what he has to say, obliterating him completely? The chances are that the radio presenter is making use of a noise gate working in Duck mode.
The caller will act as the main input to the gate, the presenter will be the Key input. When the Key is below the threshold level, the gate is open, and the caller is heard clearly. When the presenter speaks, he goes above the threshold and the gate closes down, by anything from 10dB to 70dB according to the setting of the Range control (or how merciless the presenter wants to be). Now the caller cannot interrupt. Fiendish isn't it?
A more worthy use of ducking is when you have recorded a song where you want to stress the vocal. In the mix, subgroup the instrumental backing down to two channels and feed it through the gate. Use the vocal track as the Key input - linking both channels - and have it duck the backing down by 6dB or so. That way, you can have maximum level on the final master all the time without pushing the faders up and down.
One day all studio equipment will have MIDI - including the cable testers! It's common for effects units to have MIDI, but usually it is just to change programs (as on the Yamaha SPX90).
The BSS DPR 502, however, doesn't have any program memories (gating is a subtle art which would not respond well to such standardising treatments) so this cannot apply. Instead, the DPR 502 responds to MIDI Note-On and Note-Off messages. Note-On corresponds to gate open, Note-Off to gate close. This works both ways, so you could trigger the gate from a MIDI keyboard or you could use the opening and closing of the gate, under normal audio control, to send out MIDI notes. But what can you do with it?
Probably the most useful possibility is to replace less than perfect drum sounds on multitrack tape with more suitable sound samples. You may have tried this before without success, because some samplers that have audio triggering facilities are just not quick enough to respond. Either that or they respond to spurious noises - or don't respond when you want them to! The advantage of giving this MIDI triggering function to a noise gate is that the gate is already optimised for fast and selective response for its normal noise gate function. Having done that, putting MIDI on it is the icing on the cake.
There is more. The DPR 502 responds to the level of the triggering signal, so you can replace real drum sounds with samples and retain dynamics. A signal which just exceeds the gate's threshold level triggers a MIDI Note-On with a velocity value of 10. A signal which hits peak level on the DPR 502's LED bargraph will result in the maximum MIDI velocity of 127.
A fun application which demonstrates the DPR 502's MIDI capabilities is the 'Vocal Drum Trigger' technique - available to anyone with a sense of adventure. Here's what you do...
Plug in one microphone (via the mixer) to both inputs of the DPR 502. You won't use the audio outputs, but connect the MIDI Out to a sampler loaded with snare and bass drum sounds. You'll need to set the MIDI Note Numbers to suit the sampler and the DPR 502. Next, set the Key Filter and Width controls to emphasise bass frequencies on one channel, high frequencies on the other. By making suitable vocal noises you'll find that it is possible to 'sing' a drum track. If you're really committed to new ideas, you could parallel up another DPR 502 and have four drum sounds to play (sing) with. Be bold - try it.
Another useful technique is MIDI-controlled fading. An automated fade-in and fade-out can be performed by passing a stereo signal through the gate and triggering it with a Note-On message from a MIDI keyboard. When the Note-On is received, the stereo source fades up; when the Note-Off is received, it fades down again. The fade times are determined by the Attack and Release settings on the gate. (Make sure to set the Key Source switch to 'external', or the signal will act as its own trigger.) A variation of this is to use the DPR 502 to perform a keyboard-controlled pan. If you feed the same signal to both channels and connect the outputs to separate mixing console inputs, MIDI Note Numbers can be set up so that (say) note C1 pans the signal left, C2 pans it right. The rhythm of the panning is up to the keyboard player. Instead of panning, different EQs could be set up and changed in response to the MIDI keyboard, or under the control of a sequencer perhaps.
Definitely! This article just scratches the surface of what you can do with a good noise gate. They don't appear to be the most exciting pieces of equipment at first glance, but they are in fact among the most valuable tools you can have in your equipment rack. The BSS DPR 502 is one example of a fully specified noise gate (with the added bonus of MIDI). There are other units which are worth investigation, such as the Drawmer DS201, which doesn't have MIDI but is still a first-class unit.
There are cheaper gates, and some compressors incorporate rudimentary gating facilities, but my advice would be to consider a full-function, dedicated gate seriously. With or without MIDI, it will open up a whole new world of effects.
Contact The DPR 502 MIDI noise gate is available from BSS Audio Ltd, (Contact Details).
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Feature by David Mellor
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