On control voltage

I've had a busy time since I last updated this blog. I had a sort of frenetic weekend trip to New York City I want to write about, but first I'd like to cover the simpler event I've had in that time, the 2023 Bleep / Blorp Festival of Synthesis and Electronic Music.

đź‘ľ At Bleep / Blorp

My time at the festival was very limited. Our two-year-old had become sick the night before, and I wound up staying at the festival for a relatively short window of time around my scheduled performance, with some of that time carved out to eat with my brother and father, who live in that area. That gave me enough time to attend Eric Lennartson's well-researched presentation on oscilloscope music, briefly poke at a little DIY east-coast eurorack synth system someone had assembled in a Cre8Audio NiftyCase, and then notice after I had repacked my gear following the performance that the whole synthesizer “petting zoo” had been hauled away and I had missed my chance to peruse the rest of it.

I had brought my own eurorack system along for the performance. Mine was much smaller. It contained just a contact microphone module and a USB interface. Here's a video of me using it before the festival:

The contact microphone picks up the sounds of me tapping, rubbing, scratching, or scraping its surface, and detects the amplitude envelope and peaks of that sound—or of an external sound source, if I connect one. The USB interface module simply connects any other modules in my system (in this case, just the contact mic module) to my computer, where I do most of my music production in the VCV Rack application. Over time, I can add more hardware eurorack modules that perform different functions. I used this system for only one piece of my performance set, but I definitely expect to get more out of it over time.

My performance itself was a fun experience. Although I wasn't around long to see other performers, the glimpses of them I did get were very cool. There were some seasoned professionals there alongside neophites like me, and it gave me some inspiration as someone trying to expand my repertoire of musical techniques and ideas. There was a little fuel for gear envy too, of course.

Setup was a breeze; I assembled my gear on a while table during the previous act and then someone deftly and quietly wheeled it into position onstage and plugged my gear into the power and audio hookups. I started a few minutes ahead of my scheduled 20-minute set, as the previous act had departed the stage a little early. I performed five pieces of varying length that I had prepared, well enough I think. I had been worried that for this debut set I would struggle to keep myself on schedule and wrap up in time for the next slot. Instead I found myself having the opposite problem; I had finished this set with more time left than I really intended to, and wanted to do at least one more piece before I left the stage. So I dug up a short, fully sequenced piece in VCV Rack I like, forgetting for the moment that my graphics card really struggles with that particular piece. This doesn't matter for recording, because the recording facilities within VCV Rack perfectly compensates when when its digital signal processing lags behind real time, but for live listening that lag creates a lot of terrible audio stutter. I tried to alleviate this after the piece started by dropping VCV Rack's visual framerate to its minimum value of 10 frames per second, but this only reduced the stutter somewhat. If I had been thinking more quickly I might have tried lowering the audio samplerate too. But no matter. Five out of six pieces without a technical failure isn't so bad.

Here's my favorite piece from the set:

🌊 Bleeping and/or blorping in eurorack

For the uninitiated, eurorack is a modular synthesizer standard based on the A-100 modular synth system created by German manufacturer Doepfer in 1995. The nice thing about eurorack is that the standard is supported by a number of small manufacturers coming up with new and interesting module designs, and those modules can be combined in unforeseen ways to make new sounds or enable new workflows. The eurorack standard makes no physical distinction between the connections that carry different kinds of signals between modules; all of them are conveyed by the same 3.5mm TS patch cables. This means, for example, that a module input jack intended for control voltage (pitch information, amplitude, filter resonance, wavefolding amount) will just as easily accept an audio signal. Or you could even patch an audio-rate pulse wave into the trigger input of a drum module, turning that drum module into a melodic oscillator with a unique waveform.

The humble beginnings of my own eurorack system.

Eurorack systems that professionals and hardcore enthusiasts show off online tend to be sprawling and mostly self-contained; a wall of modules that together handle an entire music production chain from sequencing, live control, and MIDI I/O through mixing, compression, and equalization. It's certainly not the most cost effective way for me to integrate a contact mic with VCV Rack, but it has the potential to not only augment but also gradually replace nearly all the music production I do in software, if I wanted it to and invested a lot of money.

As it is, I'm in no rush to spend a fortune moving my whole workflow off of the computer, so I'm trying to focus on modules that add something new to my current workflow, things like the contact microphone module. The next piece I have my eye on is a simple analog subtractive synth voice. The fun thing about analog circuits is that they can be a little imperfect, unpredictable, and can be sensitive to physical conditions like ambient temperature. This is why an analog oscillator has to be tuned whenever you turn it on. This can add a little subtle spontaneity or “happy accidents” to the sound. For example, oscillators that are just slightly detuned or out-of-phase from one another can produce sonically interesting interference patterns. These kinds of imperfections can be deliberately simulated in a digital system, but occur naturally and effortlessly in the analog world.

I should take a quick moment to explain basic components of a subtractive synth voice, as it hasn't been long since I needed this primer myself. Skip this paragraph if it's old hat for you. Sound is a compression wave in the air, and electrically produced audio signals are fluctuating voltages that control the instantaneous position of a driver in your headphones or speakers; the wiggling motion precisely controlled by those voltage fluctuations creates the sound you hear. You can plot the movement of the voltages in real time using an oscilloscope and detect wave patterns. The “purest” kind of tone or pitched sound is a sine wave with simple, perfectly symmetrical curves that repeat indefinitely. A simple audible sine wave has exactly one audible frequency, and no harmonic overtones. Subtractive synthesis tends to start with certain simple variations on the sine wave: the triangle wave, square or pulse wave, and saw or ramp wave, which are all shaped basically just the way their names suggest. Their deviation from the gentle curve and perfect symmetry of the sine wave produces more harmonic overtones; we tend to experience them as sounding buzzy or reedy and we are hearing more than just that one basic frequency or pitch of the overall wave. The part of a synthesizer that produces these waveforms is the oscillator. The “subtractive” part of subtractive synthesis comes from passing that harmonically rich waveform through a filter, which essentially smooths out the wave into a more sine-like shape, reducing those harmonic overtones. Subtractive synth voices most commonly use a low-pass filter, which removes the harmonic overtones above a controllable cutoff frequency and allow frequencies below the cutoff to pass through. Filters also tend to have a resonance control which allows for boosting frequencies close to the cutoff by a variable amount. Often the resonance control can be pushed to such high levels that even with no input audio signal the filter produces a sine wave at the cutoff frequency; this is called self-oscillation. Usually the cutoff frequency and resonance amount are not fixed in a subtractive synth voice; one or both of them can change according to a control envelope over the course of playing a note, or in some other dynamic way. At the same time, similar dynamic control is applied to a voltage-controlled attenuator or amplifier (VCA) to affect the amplitude or volume of the sound coming out of the filter. All together, this shapes the sound to create different timbres you hear.

I've started building my subtractive synth voice with a used 2hp VCO, a tiny analog oscillator based on the Curtis CEM3340 integrated circuit. I also added an inexpensive passive multiple module, which doesn't connect to the power supply and simply allows me to split signals between multiple cables. Having just the VCO in hardware (and not other parts of the basic subtractive voice chain) is a little challenging because there is a perceptible delay introduced by sending signals on a round trip from software to hardware and back. But it's workable; here's a fun little sequence I recorded developing just one kind of sound I can make with the VCO controlled and processed by software:

The next module I plan to add to complete this synth voice is the After Later Audio Waves—an exact hardware clone of the Mutable Instruments Ripples, a multimode analog filter with strong, self-oscillating resonance and a built-in VCA.

I'm pausing before I buy that filter module, until I've made more credit card payments. Music professionals on YouTube who talk about the potential for reckless spending on eurorack systems say that part of the problem is that individual modules are cheap, so it's easy to lose track of the total cost as one acquired more of them. But they really only mean that many eurorack modules cost less than an entire synthesizer or another piece of equipment that a professional might buy for music production. We're talking about not much less than USD $150 near the low end of the price range, with some of the modules I'm interested in costing two, three, or four times that. Still more than I would comfortably spend on a monthly basis for my avocation. The most expensive part of my current eurorack system by far is the USB interface, and though I bought it second-hand to save money, it was still costly enough that I opted to pay for it in monthly installments. But it will support any modules I add over time.

Conscious of controlling my spending, I intentionally limited myself by opting for a very small portable case to start with: the 4ms Pod 40X, 3U tall, 40hp wide, and 50mm deep. I feel that once I have the filter module in it, I'm going to put a longer pause on acquiring new modules. Most other things I would want to add to my system would either replace existing features of my current software workflow moreso than augmenting it, or are simply too big to add to this tiny case. Having acquired a taste for eurorack, I think I could move my music production workflow to a primarily hardware system. I can see the appeal of a more tactile interface with some creative constraints. But it's something that would take thousands of dollars of investment over time—easily several hundred on an appropriately sized case alone. And I can't reasonably expect any “return on investment” because, at least for now, I don't really get paid for music stuff. So instead I have to consider it as personal, “fun” spending and not as a business expense. With that in mind I'm motivated to take things very slowly with eurorack gear. Frankly, I want to have finished making payments on the USB interface before I even think about buying a bigger case.

🪷 An immutable legacy

I want to acknowledge what an outsized inspiration one particular eurorack module designer-manufacturer has already had on my sounds and techniques, even though I don't presently own a single thing bearing the company's brand: Mutable Instruments. I mentioned earlier that the filter I want to use is a Mutable Instruments clone, but in fact the contact mic module I already have is also a clone of a Mutable Instruments module (Ears, itself an enhancement of the earlier Mikrophonie module designed by one Tom Whitwell). The first patch I made with that contact mic module, featured in the first video above, is built around software clones of Mutable Instruments modules, in particular Rings (a digital resonator module that convincingly simulates the audible vibrations of a wide range of physical objects) and Clouds (a granular “texture synthesizer” which in the video's patch serves as a sort of warbly stereo shimmer delay and reverb). And if I ever move toward creating a more standalone eurorack system, the number one oscillator design I'd want is Mutable Instruments Plaits, a renowned digital “macro oscillator” with numerous melodic and inharmonic/percussive modes.

Mutable Instruments designs are widely popular amongst eurorack musicians, but the company ceased all operations in 2023. This event has an impact on the modular synthesis community comparable to the cancellation of Calvin and Hobbes in 1995. Here was a creative enterprise that forever changed the landscape of its artistic medium and inspired so many successors, and then at the zenith of its popularity its creator was simply done with the work. I think a lot of people, like me, found the announcement that this closure was coming baffling and alarming at first, but once I read more about it I found it completely understandable. Mutable Instruments was always a one-woman operation. Mass-producing the modules required the company's founder and sole employee to contract capable electronics manufacturing companies and diligently supervise and quality-check their output to verify that the finished modules would perform to spec; she explained in an interview that this manufacturing process was highly stressful. She also felt that she was done making the synth designs she wanted to make, needed some time for medical leave, and ultimately wanted to move into a career where she would have some anonymity as a private person. She is already missed and appreciated in this field.

Copyright notice on the back of a eurorack module PCB.

Though Mutable Instruments may be over as a company, clones of the Mutable Instruments modules abound. There are two reasons for this. First, the module designs are simply good: unique, innovative, versatile, well-labeled, an all-around joy to use. Secondly, the designs are all open source, save for the currently proprietary design of the last one in production, the Clouds successor, Beads, which according to its creator's custom will become open source when the sale of units has paid for its research and development. This body of open source designs will remain as an enduring gift to the entire field of modular synthesis. It means DIY modular synth musicians and small manufacturers alike are free to customize those designs or even copy them exactly, provided they don't infringe on Mutable Instruments branding.

Fittingly, the name of Mutable Instruments' creator is already inscribed within my nascent eurorack system, in the copyright notice on the back of the contact microphone module's PCB.

Where not otherwise noted, the content of this blog is written by Dominique Cyprès and licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.