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Understanding Gate Filters

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Fig. 1: Gating a snare mic

Two gate features that are eminently useful but often overlooked or misunderstood are the filters and key inputs. Whether hardware- or software-based, all gates incorporate a gain control device and a detector. The gain control device varies the gain of the audio signal, while the detector is sort of a remote-control signal that opens and closes the gate by telling the gain control device what to do; the detector signal could be the same sound as the one you’re trying to gate, or a completely different signal. Take, for example, a gate on a snare drum mic. Every time the snare is hit, two things happen: (1) the snare signal is routed to the detector path, telling the gate when to open and close, and (2), the snare signal passes through the gate’s audio path.

Easy, peasy — see the diagram (Fig. 1) above. Unfortunately, kick drum, toms and cymbals physically close to the snare mic can inadvertently open the gate when the snare’s not being hit. Known as mis-triggering, this might not be a disaster, but it prevents the kit from sounding tight while also allowing those instruments to leak into any sends intended for the snare mic — reverb for example.

Fig. 2: Dynamics (gate) section on a Yamaha CL5

Note that the filter on the gate is not in the audio path. It’s in the detector path, so rest assured that regardless of what you do to the filter, it will not affect the timbre of the audio signal. Most manufacturers provide one filter for the gate, with the ability to choose from high-pass (HPF), low-pass (LPF) or bandpass (BPF) filter types. Fig. 2 shows the Dynamics 1 (gate) section for an input channel on the Yamaha CL5. The drop-down menu for the filter offers three options: HPF, LPF and BPF. Choosing HPF or LPF gives you a simple frequency control; choosing BPF allows you to choose the center frequency and the bandwidth (“Q”) of the filter.

From “On the Digital Edge” by Steve La Cerra, FRONT of HOUSE, June 2024, page 42