Fig. 1. The MasterWorks Gate plug-in from Digital Performer. The gate is inserted on the synth bass track; on the kick drum track, an aux send is routed to Bus 1. Bus 1 is then used as the gate’s “KeySource.”

IN JULY, we looked at basic dynamic processing. This month, we’ll explore advanced techniques for using expander/gates. Our initial discussion of expander/gates was basic: Insert one on a sound, and it can hide unwanted noise when the sound is not playing. When the sound is playing, the noise is still there but it is typically masked by the sound. A gate can be used for some interesting creative effects, but to take full advantage of its capabilities we need to dig a bit deeper.

Any gate incorporates something known as a “gain-control device.” Think of it as a remote control for opening and closing the gate. This remote-control signal can be the same sound we are gating, or it can be a completely different signal. For example, when we gate a snare drum to hide leakage from other parts of the drum kit, two things are happening every time the snare is hit: The snare is passed through the gate’s audio path, and the snare sound is routed through the remote control path, telling it when to open and close the gate.

No one ever said that those two signals had to be the same. Suppose you are trying to gate a snare drum, but the kick drum sometimes causes the gate to open. If you could remove the kick drum from the control signal, the gate would track the snare drum more reliably. This is where the sidechain or “key” filter comes into play. The key filter modifies the control signal, not the audio path. If we set the key filter to cut everything below, say, 250Hz, a lot of the kick drum leakage would be removed from the control path, reducing the tendency for the gate to open on kick hits. Since this signal is different from the audio path, we did not change the sound of the snare. The same technique can sometimes be useful when trying to reduce the instance of cymbals opening tom gates. The “key listen” (or “sidechain listen”) button on a gate lets you temporarily hear the filtered signal.

Many expander/gates (software or hard- ware) provide a separate “key” or “trigger” input, enabling you to use a secondary sound to take control over opening and closing the gate. For example, let’s say you patch a gate on a synth bass but route the kick drum to the gate’s key input (see Figure 1). Even though the synth bass is passing through the gate’s audio path, the kick drum actually opens and closes the gate. If the kick drum does not hit, you won’t hear the synth bass, regardless of what the synth bass is playing. This can be used for some interesting effects where the synth is rhythmically matched to the kick hits. Since the synth is heard only when the kick is played, it sounds like the kick and synth are playing together perfectly.

Fig. 2. The rear panel of the Drawmer DS201 Gate provides audio input, audio output, plus a separate input for a key signal. A front-panel switch turns the key feature on or off.

Next, substitute that synth bass for a low- frequency test tone tuned around 70Hz. Every kick hit opens the gate on the test tone, making your kick sound like a TR808 kick. You can use the gate’s hold and release controls to make the length of the tone as long or as short as you like. A similar approach can be used on a noise signal to trigger the gate from a snare. Every time the snare hits, you’ll get a burst of noise in time with the snare drum. Again, you can either add this to the real drum or use it to replace the snare sound. Trent Reznor will be proud of you.

Here’s how to create a TR808 kick in Pro Tools: First, add an aux track. Make sure that the fader on the aux track is pulled down all the way or you may be in for an unpleasant audio surprise. Insert a signal generator plug-in on the aux track (insert menu > other > signal generator). Slowly bring up the fader on this track and you’ll hear the test signal. It’s pretty useless at this point. Set it to Sine Wave and 200Hz. Insert a gate on the aux track (insert > dynamics > Expander/Gate Dyn 3), making certain that the gate follows the tone generator in the signal path. This means that the signal generator must be in the top insert slot. Adjust the threshold of the expander/gate so that the tone is just muted. Next you need to route the kick drum to the ‘trigger’ input of the gate. Add an aux send to the kick drum track. Set its out- put to Bus 1, set it to pre-fader and bring the aux send fader up. Set the gate’s key input to Bus 1. Click the gate’s sidechain ‘key’ button.

Every time the kick drum is hit, the gate will open, unmuting the tone and giving you a low-frequency burst (You may need to fine tune the gate’s threshold control.) Try tuning the tone down to around 60 or 70Hz. You can turn this into a TR808 kick by lengthening the hold and release of the gate, or keep it short and just use the tone to reinforce the kick. In fact, if you already like the kick drum sound, tune the tone to approximately 40 or 50Hz just to add some impact to the kick drum.

Some engineers have had success using contact pickups on each drum to trigger gates on the microphones. This is a useful technique because the contact pickup is not subject to any leakage, whereas microphones may capture other sounds that might mis-trigger the gates (particularly on loud stages). The contact pickup is placed on the drumhead and patched to the key input (see Figure 2) on the respective drum’s gate. Since the trigger is in physical contact with the head, it will send a signal only when the drum is hit. Leakage is no longer a factor in opening the gate, so the gate reliably opens only when the drum is hit.

Steve La Cerra is an independent audio engineer based in New York. In addition to being an Electronic Musician contributor, he mixes front-of-house for Blue Oyster Cult and teaches audio at Mercy College Dobbs Ferry campus.