Turn Modular Soft Synths into Signal Processors

Exploit the external input found on some synthesizers

by Craig Anderton

Several software synthesizers offer external inputs for processing audio signals. If you have a bunch of plug-ins, you might wonder how a synth could provide anything particularly useful you don’t already have. However, not only are there synth modules like lowpass filters (as well as possibly some other processors, like ring modulation or overdrive), but you can “play” them with a keyboard and envelopes for effects like gating, as well as do LFO modulation.

We’ll show how to use Arturia’s Moog Modular V to create an envelope-followed filter which you can then gate with a MIDI keyboard, but this only hints at the many possibilities. When your usual collection of plug-ins doesn’t seem to stretch quite far enough, a synth might be just the answer—and modular soft synths are particularly well-suited to signal processing.

Begin by selecting the audio input to be processed. In stand-alone mode, you typically select the audio interface input and output from some kind of preferences (Fig. 1); with a DAW host, the usual procedure is to insert audio you want to process into the instrument track hosting the soft synth.

Fig. 1: The VS-700’s Aux input, which offers a high-impedance option for guitar, is providing the audio input to the Moog Modular.

The Envelope Follower module doesn’t default to being in the synth, so right-click on the label for an Envelope module (e.g., Envelope 1) and select Env. Follow. 1 from the pop-up menu (Fig. 2).

Fig. 2: Replacing modules with the Moog Modular involves right-clicking on an existing module’s label.

With lower-level external ins (e.g., guitar), you’ll need gain. Patch the external audio signal to two mixer module inputs (these should not be linked—i.e., the red button between them should not be lit; see Fig. 3).

Fig. 3: The mixer modules can provide gain for lower-level audio inputs.

Patch one mixer output to a Low Pass Filter audio input; this will filter the input. Patch the other mixer output to the Envelope Follower’s audio input. Next, patch the Envelope Follower Cont Out to the Filter Mod In, then click on the mod in jack and drag up to turn up the modulation amount (Fig. 4)

Fig. 4: Patching between the filter and envelope follower. The blue cables come form the mixer outputs.

Patch the Filter Output to a main Envelope input. If you want to just turn on the envelope and let it run while you play through the envelope follower, set the envelope controls to Attack = 0, and Decay, Slope, and Release to maximum (full clockwise). To gate with a MIDI keyboard, set Attack and Release to 0, and Decay and Slope to maximum (Fig. 5). Of course, you can modify these further so that playing the keyboard adds an attack time, a decay after you release the key, etc.

Fig. 5: The complete envelope follower + gate patch.

Play a key on your MIDI controller, or the Moog Modular’s virtual controller; this will open up the main Envelope, thus opening up the associated VCA and allowing you to hear the audio input. Tweak the filter, mixer, and envelope follower controls as desired.

Note that for the tightest envelope tracking, set the Envelope Follower’s Short/Long control to Short and use the minimum Time Control setting that doesn’t give “ripple” (usually about 10-15 ms). The Time Control is the little “trimpot” below the Envelope Follower’s Threshold control. Also, it’s generally best to turn off keyboard tracking to the filter (choose No instead of K1, K2, K3, or K4 in the filter’s lower right corner).

  Craig Anderton is Editor Emeritus of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.