The Art of Synth Soloing: Sweep Dual Filters for Soaring Sounds
Thu, 14 Jun 2012

Fig. 1. A dual-filter configuration on the Korg Kronos. Filter B is highpass, and lets all frequencies through by default. With an assigned MIDI control, I can raise its cutoff “floor” slightly to sharpen my sound.
LAST MONTH WE STARTED EXPLORING FILTER TYPES AND DOING BASIC SWEEPS. I expect that you listened to the online examples and got an aural “sense” of what each filter type sounds like. Now, we can get into further expressive tips for using filters in live performance. - JERRY KOVARSKY

Serial Filters

There’s a world of possibilities in sweeping a lowpass filter with differing amounts of resonance. Let’s expand on that and add some more character to your sweeps. The trick is to add an additional filter type into your signal chain, which will only be heard when you sweep it with a different controller than whatever you normally use for cutoff and resonance.

How to set this up will vary depending on your synth and on whether you’re tweaking an existing preset or making your own. Some synths offer a dual-filter design, with a choice of running them in parallel (the pre-filter signal goes through both filters at the same time—sometimes using one filter per stereo side) or serial (the output of one filter feeds the next). Note that I’m talking about having two completely independent filters, each with their own cutoff and resonance controls. Certain synths offer configurations that combine two filter types in a preset chain, under the control of a single cutoff, but we can’t use those for our purposes here.

Assuming a true dual, independent filter design, you want the serial routing. Your first filter would be the usual lowpass, set to your favorite values for cutoff and resonance. The second filter will be a different type—let’s try a highpass. (See Figure 1 above.) It’s important that the highpass cutoff be set to the lowest value, so it’s letting all the frequencies pass through. Set like this, you won’t initially hear any eff ect from the highpass—it’s just in the signal path awaiting further “instructions.” Sweeping your usual cutoff controller (knob, mod wheel, etc.) will only affect the lowpass filter.

Now, you want to set another controller to fully modulate the lowpass cutoff while slightly modulating the highpass cutoff . This could be an unused knob or slider, a ribbon or vector joystick—your choice. I like the effect to be subtle for the highpass, just adding some cool timbre-shaping at the end of my controller gesture. So I’d use a low positive value for that filter modulation assignment, maybe 15 to 45 percent. (See Figure 2 below). This way your highpass cutoff starts at zero, or wide open. As you move your controller, you’re slightly raising that frequency “floor,” which removes some low end from your sound. Go too high and your sound will “thin out” too quickly, likely not cutting through the mix well enough. When it’s just right, though, the effect can actually spotlight the mids and highs.

Fig. 2. Here, the same knob (MIDI CC 17), raises the lowpass filter’s cutoff
a lot (intensity set to +92) as it also raises the highpass cutoff more subtly (intensity at +44). This gesture can further accent my sound even though the lowpass filter might be all the way open.
If it’s offered, you can also add a little resonance to your highpass filter to help accentuate its effects. Now, you can do your traditional filter sweep from the usual controller, or go to your second controller for a sweep that has a little more character. Alternately, you could dedicate the second controller only to highpass sweeps and interact with both as desired.

After experimenting with this, try changing the highpass to a band-reject (a.k.a. notch) filter type, and using more modulation depth/ intensity for how the controller changes its cutoff frequency—around 80 percent works well for me. This adds very cool aural interest to the mid and upper parts of the sweep, precisely when the lowpass filter is fully open and thus has nothing more to add. Setting up a dedicated modulator for a band-reject filter is also fun, as it adds a kind of manual flanger to your sound.

Effect Slot as Filter

How do you do the above when you’re using an emulation of a classic synth that didn’t offer dual filters? That could be a Minimoog clone, a simple virtual analog design, or a DSP-based synth that’s using all of its available filter horsepower to emulate a 24dB-per-octave filter. (Many of my Korg synths do this, for example.) Effects to the rescue!

Since we’re focusing on lead synth playing, I’m assuming most of your sounds are monophonic or that you’re playing one note at a time. So using a filter effect from your synth’s effects section, or a separate filter plug-in on top of a soft synth, will work fine even though it doesn’t represent a true per-voice filter design. On a synth, all of your sound normally goes through the filter, so use an insert or total/master effect here to do the same. Putting the filter effect on an aux send bus is not desirable, since we want no dry signal mixed in, which would weaken your sweeps. As far as performance control goes, we have the same options as earlier, as virtually all software hosts and hardware keyboards let you map effects parameters to MIDI controllers.

Start exploring this, and next month we’ll go even further into other types of effects that will enhance your filter sweeps. Enjoy!


Filter Finder

Here’s a partial list of synths and software that give you the tools to set up the serial filter routing from this column.

Hardware synths: Korg M3, M50, and Kronos (Stereo Multi Mode Filter); Roland Jupiter-80 (Super Filter) and V-Synth GT; Yamaha Motif series (Dynamic Filter).

DAWs: Ableton Live (Auto Filter) Apple Logic and MainStage (Auto Filter); Avid Pro Tools (AIR Vintage Filter); Cakewalk Sonar (ModFilter); MOTU Digital Performer (Multimode Filter); PreSonus Studio One (Auto Filter).

Third party plug-ins: Audio Damage FilterStation; Camel Audio CamelPhat; FabFilter Simplon, Micro, and Volcano; Roger Linn AdrenaLinn Sync; SoundToys FilterFreak; U-he Filterscape; Waldorf D-Pole.

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