The Kitchen Sync

November 20, 2012


Fig. 1. Sync, oscillator tuning, and envelope settings for a model sync patch. Note that the VCF EG (filter envelope) is tasked as a sweep source for the pitch of the slave oscillator (VCO 2) in the VCO Modulation section.
For the last few columns we've been exploring filter sweeps and effects programming. This month, let’s dive in to one of the classic types of synth sounds that can impart a bold timbral character to your playing: synced oscillators.

That Syncing Feeling
The basic science of oscillator sync is that one oscillator (called the master, usually oscillator 1) is set to modulate another (called the slave, usually oscillator 2) so that the waveform cycle of the slave resets every time the master finishes a cycle of its own. When the two oscillators are tuned to different frequencies, the master forces the slave to reset at irregular intervals, which produces interesting harmonic results. Most of you will recognize the sound as “that Cars patch” from the song “Let’s Go.” (Keyboardist Greg Hawkes used a Prophet-5 on the recording, patch 32. Or was it 17? Discuss. . . .)

In a sync sound, the primary pitch is produced by the master oscillator, which is tracking the pitches you play on the keyboard. The tuning of the slave oscillator, which produces the signature tonal character, can be set either of two ways: tuned to a static or fixed pitch, or swept by a modulation source, usually an envelope generator set to sweep and then stop, as opposed to a repeating envelope, or cyclic LFO. Both of these possibilities provide interesting resources for soloing. Let’s go!

Swept Sync
Call up a sync patch on your synth of choice—you’re not likely to find this sound on a basic ROMpler, unless it’s a static sample that you won’t be able to modulate in the ways I’ll describe. So reach for an analog synth, be it real, virtual, or software. The patch may have “sync” in the name, or you may be able to turn on sync manually somewhere in the oscillator or modulation section. Most “classic” sync patches sweep the pitch of the slave oscillator, typically with an envelope. The basic shape is a fast to medium fast attack, a slower decay, and a sustain level of zero so the pitch settles back down. Figure 1 outlines the most relevant settings for a basic sync sound on Korg’s Mono/Poly soft synth.

The Decay value is critical, as it defines the length of your sweep. Every time you play a note, it triggers the envelope shape, and your slave oscillator’s pitch is swept accordingly—classic sync. It can also be interesting to sweep the pitch manually when you hold a note. Depending on the synth you’re using, there are a couple of ways to do this. The simplest is if your synth lets you modulate the envelope sustain level. Raising the value will sweep back up through the whole timbral change, and when finished you can return it back to the zero value. We set the modulation matrix (called “virtual patch” on the Mono/Poly) to use a controller of choice to modulate the Filter EG Sustain point. You’ll want a variable modulator like the mod wheel, a ribbon, a knob, or a slider. Figure 2 shows a simple setup using a controller sending MIDI CC 17—set the Intensity to taste.

If your synth can’t modulate the sustain segment of an envelope, or your patch has a looping envelope that you don’t want to interfere with, you can route a modulation source directly to the pitch/tuning of the slave oscillator. Figure 3 is an example of this using Rob Papen’s Predator.

With either of these approaches, you can now enjoy the originally programmed sweep when you trigger new notes, and then take control manually to repeat that sweep while a note sustains. Setting the amount or depth of the modulation lets you choose how deep you sweep the slave oscillator, and by leaving the controller “up” (not at zero) you can change the start point of the sweep that occurs when you play a new note.

Having a continuous controller assigned to the pitch of the slave oscillator is cool because you can retune its pitch whenever you want. Some players like to have the ADSR parameters assigned to controls on their keyboard so they can completely reshape the sweep in real time. Changing the decay stage can be very effective, too: Shortening the decay and playing new notes will give you a choppy effect that can then be “relaxed” by opening it back to a longer value. All of these concepts are in this month’s online audio examples.

Tuned Sync
The other class of sync sound tunes the slave oscillator to a fixed pitch—one that accentuates harmonics that you like. So how do you interact with this type? One approach I like is to set a modulator to retune the slave pitch to another, equally cool pitch. Modulation routing is the same as before: Set a CC with a modulation amount that produces the desired sound at its highest setting. Then, assign a switch or button to send a value of 127 for that CC when pressed and a value of zero when pressed again. Now, you can hit the button to activate the alternate sync timbre, then hit it again to snap back to the original.

You can use more than one modulator to do this, so you can have three, four, or more different tuning ratios pre-planned, adding a lot of diversity to your performance. The only limit is the modulation capabilities of your synth. Figure 4 is the Mono/Poly again, with four different controllers set to produce varied tunings.

I prefer toggle (a.k.a. latch) over momentary behavior for the switch, since I can “set and forget” the alternate pitch while I’m busy playing, pitch-bending, sweeping filters, and using all the other tools in our growing arsenal. That said, momentary can be a cool effect when you keep pressing the switch rapidly, producing a repeated, alternating timbral effect. So try both and hear what I mean. Until next month, happy soloing!

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