The world of sound synthesis, and modular synthesis in particular, thrives on movement. As a synthesist, you are constantly looking for inventive ways to add movement to your sounds. In modular synthesis, this movement is provided by control voltages (CV). An LFO, for instance, gives you a cycling control voltage that can be used to modulate different parameters.

The more complex your control voltage is, the more dynamic the movement will be—and by extension, the more control one has over their control voltages, the more expressive their sounds may be. We're reminded of one of synthesist/educator Todd Barton's personal mantras: "You're only as expressive as your control voltages."

There are several possible sources of control voltage in modular synthesis—but today, we're talking about one of our favorites, and one which is often misunderstood: the envelope follower.

Understanding Envelope Followers

At its core, an envelope follower is a module designed to track the amplitude of an incoming signal. In simpler terms, it translates changes in an incoming signal's amplitude into a control voltage, which can then be used to modulate various parameters within a modular system.

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[Above: envelope follower diagrams from the ARP 2600 user manual.]

For example, you could have an envelope follower detect the transients of a dynamic drum loop and convert it into a complex control voltage. That control voltage could alter the loudness of another sound, the pitch of an oscillator, the cutoff frequency of a filter—you could even extract triggers or gates from it in order to synchronize your system to external audio sources. The possibilities are open-ended; and of course, we'll discuss more of them as we go.

Typical Features of Envelope Followers

• Input/Output jacks: There are inputs for capturing the incoming signal and outputs for the generated CV signal. Sometimes, an envelope follower will accommodate the use of external signals from microphones, instruments, or line-level sources by including a dedicated preamp section, which boosts the external signals up to modular level. However, this is not always the case.
• Threshold/Trigger controls: When present, these controls relate to the comparison threshold of an internal comparator which tracks the envelope follower CV output. Typically, this means that when the envelope CV level exceeds a user-definable threshold, a gate or trigger can be produced at a dedicated output. This can be useful for synchronizing events in your modular system to external audio sources—allowing you to advance sequencers, trigger envelopes, randomize parameters, and much more.
• Optionally, Attack and Release parameters: Some envelope followers offer adjustable attack and release times, allowing users to further shape the response characteristics of the generated control voltage. Sometimes, you will see CV inputs for modulating the attack and release parameters, which adds further complexity to the generated CV.
• Envelope gain / sensitivity control: Many envelope followers, such as the Steady State Fate Detect-Rx have a gain knob. This determines the final CV range of the generated envelope. It can be useful with quieter incoming signals, such as those from a guitar or a microphone, acting essentially like a sensitivity control.

Patch Ideas Using Envelope Followers

At its simplest, envelope followers let you incorporate live instruments in your modular synth setup. Run the output of your guitar, for example, through an envelope follower. Use the resulting envelope to modulate the VCA (or any other parameter, for that matter) of a synth voice. Sarah Belle Reid has perfected the art of using envelope followers to combine live instruments and modular synths.

Here are some other use cases of envelope followers that you can play around with:

Sidechaining

Sidechaining is when you duck a signal in response to another incoming signal. Typically, in electronic music, sidechaining is applied to a bass sound in response to a kick drum. By doing so, you make sure that there is enough room in your low end for the kick and bass to breathe. Additionally, when you exaggerate the effect, it gives you a pumping sound that is synonymous with modern dance music.

One of the ways to achieve sidechaining using a modular synth is with an envelope follower. Let’s say we want to sidechain the bass in response to the kick drum. In other words, every time the kick drum hits, the bass should go down in volume. If you can get the amplitude envelope of the kick drum, reverse it, and apply it to the VCA of the bass, the job is done.

First, duplicate the output of the kick drum using either a signal multiple or a splitter cable. This is because we want to hear the kick drum, too, and simultaneously use its amplitude envelope to duck the bass.

Next, take one copy of the kick’s output to your mixer. The other copy goes to the input of your envelope follower. Now, every time the kick hits, the envelope follower will capture the amplitude envelope of the kick drum. Next, we want to reverse this envelope. Take the output from envelope follower and go into a voltage process that allows you to invert and offset signals. Invert the signal and apply a positive voltage offset; take this output to the VCA of the bass sound. Voila, you get sidechaining. If available, adjust the Rise and Fall times of your envelope follower to determine how much and how quickly you want to sidechain the bass. In this patch example. This is one way to patch compressor-like behavior in your Eurorack system.

You can use the same technique to get gated reverbs, too.

Generating Complex Envelopes

Sound synthesis is a lot about movement. The more complex movement you can apply to your parameters, the more lively your sound becomes. Envelope followers can be used to achieve complex movements. For example, use an envelope follower to capture the amplitude envelope of an intricate drum loop. The resulting shape can be supplied to different parameters of your patch. Supply the complex CV to the cutoff of your filter, for instance, to get rhythmic filter sweeps going.

Similarly, you can supply it to a VCA to get dynamic amplitude modulation. Depending on what the VCA is connected to, it can lead to various interesting results.

Extracting Gates from Envelope Followers

Some envelopes have a dedicated gate output and, occasionally, a Threshold control. By adjusting the threshold, you can determine how the outgoing gate corresponds to the amplitude of the envelope follower's CV output: above the threshold, the gate goes high; below the threshold, it goes low again.

As mentioned above, this can be a great way of synchronizing sequencers, triggering random voltages, and much more—enabling a wide variety of interesting effects when used alongside external audio sources or even sound sources within your system.

If your envelope follower doesn't have its own gate output, you can always patch the CV output into a comparator, a special type of module that compares an incoming voltage against a user-definable threshold in order to generate gate signals. This works exactly like the Threshold control described above—so, if your module doesn't have a gate output, don't fret! It's easy to extract gates and triggers from CV signals using comparators.

Creative Envelope Follower Implementations

Modular synths have come a long way from the early days of the Moog and the Buchla. Today, there are several modules that make creative use of the envelope follower. These include:

Make Noise Erbe Verb

A creative reverb module that packs a lot, the Erbe Verb, among other features, has a CV output, which is the result of a built-in envelope follower. This CV goes between 0 and 10 V, the value increasing as the overall signal goes up in amplitude. You can patch this CV back into the module itself, or take it anywhere else in your system. For example, imagine patching the CV out to the size input of the module, and then playing around with the Mix knob for some wild feedback patching.

Make Noise Morphagene and Spectraphon have similar envelope follower functionality. The CV value increases in sync with the average audio levels at the output and input, respectively.

Verbos Multi-Delay Processor

An 8-tap delay module inspired by the prototype Buchla Model 288 Time Domain Processor, the Verbos Multi-Delay has fixed-length envelope follower outputs for each of the tap delays. You can use these envelope followers to quickly create rhythmic patches and generate complex CV signals.

Fixed Filter Banks with Envelope Followers

While the earliest fixed filter banks, such as Buchla 194 and 195, did not have envelope follower outputs, it changed with Buchla 296. Today, there are several modules inspired from Buchla 296, each featuring envelope follower outputs that can be used in interesting ways.

For example, there is Tiptop’s Buchla Model 296t, which is a direct emulation of Buchla 296. There is also Verbos Bark Filter, Zlob Modular VC F3DB, and ADDAC System ADDAC601, each having comparable architectures. Envelope follower outputs in fixed filter banks can be useful for creative feedback patching, among other things.

Here is an example of Sarah Belle Reid using the VC F3DB’s envelope followers to generate complex CV outs.

Amplify Your Creative Expression

Envelope followers can unlock a universe of creative possibilities in modular synthesis. From crafting dynamic sidechain effects to sculpting complex movements for your sounds, and even incorporating live instruments, this module's ability to translate sonic fluctuations into control voltages lets you breathe life and movement into your patches.

Whether you're a seasoned modular explorer or just venturing into the world of CV manipulation, the envelope follower is one of the most versatile tools you can add to your modular system.