Like many of you, I’ve spent countless hours reading Valhalla DSP’s blog when I was first trying to wrap my head around how digital reverbs work. This series is a humble nod to Sean’s Effect-O-Pedia.

Since I spend most of my time at my day job replicating analog effects in software, I’m not here to just go full encyclopedia on what each effect is, there’s plenty of that out there already. Instead, I’ll dig into how each effect is implemented in both the analog and digital worlds, and share a few thoughts (some technical, some philosophical) on what makes them tick.

Preamble done, let’s get started :)

If you’ve checked out my first releases, you’ve probably noticed I have a soft spot for modulation effects. Reverbs and delays felt obvious to me, but modulations took longer to click. They are subtler, more textural. You don’t always “hear” a modulation effect in the same way you hear a reverb tail or some delay repetitions, but once you start exploring how they work, they become addictive.

Maybe most importantly, I love modulation effects because they don’t solve problems. In a plugin world obsessed with fixing things, removing noise, correcting EQ and smoothing transients, modulation is a glorious waste of time. It’s not practical. It just makes things wobble, pulse, and shimmer. And that’s the whole point.

What even is a modulation effect?

Good question. And honestly, there isn’t a single clear definition. Broadly speaking, a modulation is any effect that involves modulating (changing) some aspect of the sound over time. That says everything and nothing at the same time, so it might just be easier to dive into specific types of effects and take it from there.

Tremolo (a.k.a. Amplitude Modulation)

Let’s start with the easy one. In a tremolo effect, the sound characteristic that is modulated is the volume. The modulation source is often periodic, like an LFO, but using more unconventional sources is where things start to get really interesting. With the right modulation source, a tremolo can transform into a rhythmic tool for creating stutter effects, chopped sounds, and evolving textures.

Analog Implementation

• Optical (LDR-based): Uses a photoresistor (LDR) to control the gain of an amplification stage. A light source (usually an LED) drives the LDR, but since the response is kinda slow and far from ideal, the effect has a distinct organic feel, especially with sharp modulating sources like a square wave LFO.
• Transistor-based (FET): A transistor adjusts the gain based on the modulation source, offering a much faster response than LDR-based circuits.
• OTA (Operational Transconductance Amplifier/VCA-based): Similar to FETs but packaged as handy integrated circuit that handles the volume change automatically.

Interestingly enough (at least for me :)), these same techniques are used in analog compressors, which are also a form of amplitude modulation, just with a different modulation source.

Digital Implementation

Super straightforward: multiply your signal by a unipolar modulation source. The tricky part is ensuring the modulation source is smooth enough to avoid unwanted clicks or artifacts.

Harmonic Tremolo

Harmonic tremolo is like tremolo’s more complex cousin. Not the snotty one you’re forced to hang out with at family gatherings, but the cool one with the weird Nintendo games you never heard of before.

Instead of modulating the overall volume, it modulates the volume of different frequency bands separately. Typically, the signal is split into low and high-frequency bands, and their volumes are modulated in opposition: when one increases, the other decreases. This creates a pulsing shift in tonal balance.

Analog Implementation

In vintage guitar amps, this effect is achieved with a clever passive phase-splitting circuit, followed by a tube summing stage that blends the two bands back together. In compact guitar pedals, space and power constraints mean using active op-amp-based low-pass and high-pass filters, followed by an op-amp mixing stage.

Digital Implementation

Essentially the same as the analog method but in the digital domain. Just split the signal, modulate, and mix it back.

The Three Musketeers: Vibrato, Chorus, and Flanger

These are the icons. When most people think of modulation effects, these are the first that come to mind. Or, at least, that’s the case for me. They might be a bit cliché, but when used or implemented in unconventional ways, they never cease to inspire me.

All three effects revolve around modulating a short delay, which results in a varying pitch. The differences lie in the delay time and in how the wet and dry signals are mixed:

• Vibrato: Uses only the delayed signal.
• Chorus: Mixes the dry and delayed signals together.
• Flanger: Similar to chorus but adds a feedback path around the delay for a more pronounced effect.

Analog Implementation

While early modulation effects used electro-mechanical systems like tape machines and rotating speakers, most modern analog implementations rely on BBD (Bucket-Brigade Delay) chips. I won’t go too deep into how BBDs work (that’s a topic for another post, perhaps), but in short, they’re integrated circuits that delay audio using a series of capacitors. The delay time is controlled by a clock signal, which, when modulated by an LFO, produces the signature pitch variation.

Digital Implementation

Digital delay lines. Essentially memory buffers that temporarily store audio and play it back later with a delay. To achieve a continuously varying delay time without artifacts, proper interpolation is essential when reading from the buffer at varying speeds.

Wow, I didn’t expect to ramble on for this long about just a few effects. Let me wrap it up with why am I even writing about this.

Why does this matter?

Because knowing how an effect works changes why you use it. It’s the difference between turning knobs and making choices.

A tremolo isn’t just volume wobble, it’s a rhythmic tool once you realize how to play with the modulation source. A flanger isn’t just an ’80s cliché, it becomes a textural machine when you slow it down and push the feedback. And modulation as a whole stops being a seasoning and becomes an instrument when you know what it’s actually doing.

Scirocco, and Eftilo as a whole, came out of this exact mindset. I’m not interested in recreating the past or chasing specific vintage tones, I want to make effects that are easy to use for everyone, but deep enough to turn into something entirely different in the right hands.

Thanks for reading this first part of The Wave Guide series. I’ll go through more effects in the next one. If there’s something specific you want covered, or if I got something wildly wrong, feel free to reach out.

Until then, stay wobbly.

Ciao!

    ~Jacopo