Sound Design Workshop: LFOs as Envelope

Expand your modulation capabilities with these tips
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In last month’s column, we examined a few methods for turning envelopes into LFOs on synths that support envelope looping. This time, we will invert that process and cover the possibilities lurking within LFOs that can also operate in one-shot mode.

One-shot mode is a neat LFO trick that can be found on several hardware and software-based synthesizers. The first time I encountered it in hardware was in Korg’s monophonic Monotribe ribbon sequencer. As for software, the ubiquitous Xfer Serum included it from its first releases.

Depending on the synthesizer’s implementation, one-shot LFOs can be either basic or mind-bogglingly complex. Here are a few techniques you can use on synths that support it.


Fig. 1

Fig. 1

Korg’s minimalist monophonic masterpiece, the Monologue includes a one-shot LFO for use as a second envelope (see Figure 1). In addition to pitch and filter cutoff, it can also be assigned to oscillator shape, which adds greatly to its usefulness as a modulation resource.

The crucial concept here is that the LFO waveform determines the envelope shape. For example, a sawtooth creates an instant attack followed by decay/release, a triangle delivers a “wow” effect, and the square can provide a burst of parameter modulation within a note. Despite their simplicity, these approaches cover several classic envelope styles. The most important thing to keep in mind is that the envelope time is inversely proportionate to the LFO rate. That is, slow rates deliver longer envelopes and vice versa.

It is also worth mentioning that the Monologue’s LFO always completes its cycle, even in one-shot mode. Because of this, patches with long releases retain their full sawtooth decay, as long as the LFO rate is set to a complimentary value.


Fig. 2

Fig. 2

On the software side, Xfer Serum offers incredibly sophisticated LFOs with up to 64 breakpoints and individually adjustable curves for each segment. While there are only three AHDSR envelopes, there are up to eight LFOs—all of which include the option to operate in one-shot “envelope” mode (see Figure 2).

As with the Monologue above, the overall rate of the LFO determines the speed of the envelope shape, but with such precise control over breakpoints, it’s fairly straightforward to get the contour you’re after.

Fig. 3

Fig. 3

Because the LFOs can also be synced to tempo, you can create intricate rhythmic effects that would be nearly impossible to achieve using more traditional envelopes. Because of their extreme configurability, there’s another trick you can do with Serum’s one-shot option: Step sequences that play once (see Figure 3). This is amazing for adding melodic bursts at the beginning of notes, which is an unusual effect reminiscent of sampled instruments such as the iconic shakuhachi sample that defined Peter Gabriel’s classic “Sledgehammer” in the ’80s.

Here are three tips for quickly configuring Serum’s LFOs:

1. You can copy all settings (including envelope mode) from any LFO by simply Option (or Alt) dragging it onto the “title” of another LFO.
2. Using Serum’s Save Shape feature, you can create a catalog of envelope contours for future use.
3. To design traditional sequences rapidly, hold the shift key when adjusting the LFO shape. This will quantize the breakpoints and slopes into steps. From there, you can finesse the details of the LFO contours.