Audio gates are what I call an accessory, and pretty far down the list of new gear purchase priorities. But if everything further up the list is okay, then procuring a few gates to insert on percussion and other noisy/buzzy sources is a good thing. This column will address a little history of “noise gates,” how they work, key features to look for when shopping for gates and typical applications.
The Bad Old Days
In the bad old days, noise gates, as they were called, were just electronic mute switches that would unmute their channels when the input signal level reached a specific threshold in dBu. This all worked nicely, but most of the circuitry employed on these early noise gates were nothing more than CB/Ham radio microphone “squelch” circuitry cleaned up as best as could be for live audio usage. They tended to be high in self-noise, had not much headroom before clipping, and were pretty much one knob (threshold level) and an LED to indicate a muted status.
The good news in those days was that you could get a quad channel noise gate reasonably cheap, and a lot of small companies began producing them for both live sound and personal recording applications. Today, the bad news is that a lot of these noisy noise gates are still around, hiding out in various install club systems as a way of spitefully saying, “Here! Here are the gates you requested.” I am hoping that eventually these ancient pieces of “technology” will bump into a real engineer, who will promptly do the dumpster-toss routine on them.
Kudos to Ivor
Back in the United Kingdom in the bad old days, audio tinkerer and sound engineer Ivor Drawmer was tired of using the old noise gates as they could often be unmuted, not by the desired audio source, but by another source with a bit of transient loudness close and loud enough to trigger the gates. So Ivor started to tinker with his prototype of noise gate circuitry, using a modestly narrowed audio frequency filter before the threshold circuit. Thus, the dawn of frequency-conscious gates began, with the Drawmer brand leading the pack and many quality brands adding the feature as well.
Today, the typical frequency-conscious gate for live-sound/recording use is extremely quiet, very versatile, and most of the brands contain similar circuit architecture. Now I am not saying that a Behringer Multigate has the same circuitry as a BSS DPR-504, but the system architecture is the same, with the quality of components different, along with the quality of performance. At present, I prefer the Drawmer brand of gates — not so much because I know Ivor personally, but because I do like the way his “sidechain” performs with minimal gate clicking and the whole control from attack through release. But choosing popular brand gates (dbx, Drawmer, BSS) is like Fords, Chevys and Dodges — everyone finds a brand they are fond of.
Gate Guts
Figure 1 shows a typical noise gate architecture block diagram. The audio signal enters through TRS phone or XLR jacks, is buffered, sent to the VCA, rebuffered and sent back out another set of TRS phone or XLR jacks. The first thing to note is that the VCA (Voltage Controlled Amplifier) is the heart of the unit, and its quality determines the overall fidelity of the audio signal progressing through the gate. Thankfully, dedicated audiophile-grade VCA chips of excellent quality now replace the bad-old-day squelch chips.
The gate sidechain starts by picking off the buffered input signal and processing the audio through a variable bandpass filter circuit to form the frequency-conscious feature. This is where the different brands of gates begin to vary in performance. Some lower-cost frequency-conscious gates will just implement a medium-Q bandpass filter around the lower octaves of the audio frequency spectrum, while more full-featured gates will cas-cade low-pass and high-pass filters to allow the user to tweak in the audio frequencies of interest to trigger the unmuting function. Since most gates are used on percussion sources, the best gates focus on having wide 50 Hz to 500 Hz frequency span on the sidechain filter controls to catch spe-cific drum head tuning resonances as the trigger audio.
After the frequency-conscious filters, the remaining part of the sidechain circuitry is devoted to the threshold detection function along with the ADSR (attack, delay, sustain, release) shaping of the “key” waveform. The key is just a buzzword for the sidechain signal as it progresses from audio to a rectified control signal that follows the audio amplitude, to the chopped up control voltage that opens and closes the VCA and is visualized through the metering LEDs. The threshold control is just the trigger point adjustment where the VCA is commanded to open.
On the backside of the audio signal triggering, the release control permits an adjustable time before remuting the VCA, once the sidechain signal retreats below the set threshold. For drums, the threshold time may be a half-second to several seconds to permit the drumhead sound to decay. For instruments like keyboards, a tight fraction-of-a-second selection of release is useful to reduce hum/buzz or to create a staccato musical effect.
Setting Up a Gate
Yes, setting up a gate could be as easy as dialing in a reasonable threshold, like -20 dBu, and letting the loudest thing nearby to open it up (presuming it is a mic source). But what you really want is some extra time at line check to have the drummer hit the toms (or snare, or kick, etc.) while you PFL the channel with the gate inserted in it, and with the “key listen” switch activated. Referring back to Figure 1, the key listen switch lets you hear the filtered version of the signal that the gate’s sidechain will rectify and unmute to. By adjusting the filter characteristic to each drum’s pri-mary (or secondary) resonance, you can keep adjacent drums from opening up other gates than the desired one.
It is nice to have the time to set up each gate’s frequency-conscious filters at line check, but often time pressure will cause you to either bypass the gate in the beginning of the performance, or at least bypass the key filters out, if there is that option on the gate. Then you get to go back after the mix is settled and key listen to the gates during performance, hoping for a drum fill to catch a hit or two to tweak in the filters of each drum channel. There is never a dull moment for a house engineer when there are new bands every night, and likely each new drummer will have different tunings for each tom.
Application
Given that most gates are provided in pairs, fours or six-packs, the question is: Which inputs get the gate treatment? I always focus on getting the tom-toms gated first, and if there are spare channels, then pursue the snare drum, followed by the kick drum channels. It used to be that a lot of direct inject (DI) keyboard channels used to be notoriously noisy, but thankfully this is mostly a thing of the past.
With percussion rating a priority, setting the release times on the drums becomes a tasteful choice thing. If you have a drummer with badly dampened toms, then shortening up decays under a second makes things sound more pleasant than they really are onstage. I find with a well set up drum kit that I can progressively open the release times from 0.5 seconds to over one second as I go from small toms (6 inches to 10 inches) toward floor toms (16 inches to 24 inches). Too much release time, and the stage noise swells every time a drum-fill starts. And too short release times cause the drums to sound unnatural or synthesized.
Today, with digital consoles, every input can have a gate if desired. However there is a difference between setting -50 dBu floors to drop some hiss out of the overall mix and hard gating drums or other noisy sources. Gates on tom-toms are meant to stay silent if the toms are not hit, then the re-maining open mics are how you are miking the remaining drum kit. Think of how drummers were miked in recording studios in the old days — mostly with a kick drum mic and one or two mics placed over the drummer’s head to pick up the cymbals, snare and toms. Today, with the luxury of individual drum mic channels, it still is best to remember the old technique and let kick and overheads do the heavy lifting, as far as sound rein-forcing and balancing the drum kit sound.
But I have a confession to make: I love gating the toms and letting them open up with near full level on the channel faders. This “drums from hell” sound is enjoyable, but can get old really fast if you overdo it. My compromise is to just do it at the start and trim back the tom fader levels to the point were they are not so objectionable.
Now that you have my secrets on gating, experiment and invent some hot tips of your own.
