Polyphonic Modular Synth: Modern Approaches

From Chord Oscillators to Physical Modeling

Wes Langill + Jacob Johnson · 06/09/21

Approaching polyphony in Eurorack is a rather daunting task, and there's no one right way to do it. But provided some time is spent thinking about what end result is desired, the open nature of a modular synthesizer means that a wealth of options are at your disposal. This is especially true today, when many of the best Eurorack modules represent the cutting edge of both analog and digital synthesis methods.

Historically, polyphony was a large and costly endeavor on a modular synthesizer, requiring discrete modules for each voice. That means an oscillator, filter, envelope generator, and VCA, and that's just for one voice—multiply that space and financial commitment by four or eight depending on the number of desired voices. But these days, most people seem to be generally content with sacrificing the highest levels of control over polyphonic patches both for the sake of saving HP real estate and exploring the more abstract compositional possibilities of working on a modular synthesizer. And because Eurorack is so replete with digital control and sound generation options, it offers plenty of ways of tackling the issue of polyphony, including chord oscillators, samplers, physical modeling, specialized control sources, and more—all to be discussed below.

Be sure to check out our latest Patch Pals video above where we share some of our favorite quick and easy ways to achieve polyphonic action in Eurorack, and keep reading as we break down some other options in more detail. If this is a totally new subject for you, check out our recent article on general synthesizer polyphony to get up to speed on terminology and concepts.

Sweet Harmony: Chord Oscillators

There's probably no better way to achieve your chordal dreams in one fell swoop than by using a chord oscillator. As we briefly discussed in our video mentioned above, chord oscillators became a popular component of low stress polyphonic Eurorack setups. Though implementations vary, the common thread is that these modules tend to be digitally-based with internal pitch quantization locking into pre-specified harmonic ratios, ranging from basic voicings like major and minor triads to extended chords with tensions. In most modules, a frequency control as found on most oscillators sets the root of the chord, with other controls determining the type or quality of chord, such as major and minor, and voicing, or the arrangement and spread of notes within the chord.

For many, their first encounter with a chord oscillator was, well… the original Qu-Bit Chord. Although fairly limited and large, for a long time there weren't many other options that made generating chords as quick and easy as the Chord. But with the revised Chord v2, Qu-Bit vastly expanded on the original while cutting the size in half, making it much more appealing as other options appeared on the market. Most importantly, it kept the super straightforward interface intact, meaning that even if music theory isn't your strong point, lucious wavetables and suspended 7th chords are only a few knob turns away. The individual outputs for each chord tone and the optional true polyphonic mode are nice touches as well.

Chords can be found in other modules though, not just the Chord, and might be more appealing depending on workflow and compositional preferences. The ever versatile Plaits by Mutable Instruments (as well as its many third-party successors) contains a chord mode that works rather similarly to the Qu-Bit Chord, among the many other things it can do. Another option is the 4ms Ensemble Oscillator, though it's worth mentioning that it will quantize internal pitches to scales rather than chords. Some scales do actually take the shape of triads, and of course chords can be formed by carefully setting the spread control within your desired scale, but the Ensemble Oscillator also goes a step further and breaks free from the confines of western 12-tone equal temperament tunings. Just intonation, 5TET and 7TET, and even some of the tunings explored by composers like Wendy Carlos make an appearance.

The Magic of Multi-Oscillators

Just as the name implies, a multi-oscillator is a single module which contains several VCOs under one shared panel. While many of the same results could be achieved using multiple single oscillator modules, many triple or quad oscillators provide the convenience of normalized 1V/Oct connections between submodules, eliminating the need for buffered multiples.

Closeup of the Minimoog's three-part Oscillator Bank Closeup of the Minimoog's three-part Oscillator Bank

Contrasting with chord oscillators, a lot of multi-oscillators are truly (or mostly) analog through and through. Consider the arrangement of oscillators on a classic synth like the Minimoog—while its three VCOs share internal pitch control input from the keyboard, each oscillator can be set to a different waveform and two of them offer fine tuning controls to deviate from the base pitch of the first oscillator. In this sense, while there's still only one note being played, the frequencies of each oscillator can be offset to create power chords, major or minor triads, or whatever three-note chord voicing you'd like. Many of the multi-oscillators in Eurorack can function in similar ways, but the open nature of patching greatly expands on the possibilities!

As far as actual modules go, Doepfer's A-111-4 Quad Precision VCO is about as straightforward as it gets. Each of the four oscillators offers tuning and independent 1V/Oct input, routings for frequency or pulse-width modulation, sync inputs, and dedicated outputs for triangle, sawtooth, and pulse waveforms. But it's the master section at the bottom which ties everything together, providing the base pitch reference for all four VCOs, cascading pitch and modulation CV inputs onto the other oscillators, and summed mix outputs for all the various waveforms. Sure, this could also be done with individual modules, but at only 18hp and under the $500 price point, you'd be hard pressed to accomplish the same tasks as effectively, considering you would need not just four oscillators, but also a buffered multiple and mixers to account for the internal routings.

But multi-oscillators can certainly get weirder than this. At a quick glance, Instruo's Troika looks to be completely identical between its three oscillators, but on closer inspection it becomes clear that each VCO offers slight differences in how its waveforms are arranged on the crossfaders. So while the Troika may be limited to a single output like our original Minimoog example, the in-between waveform settings make for expressive timbres and fun variation between the three sections.

Of course, because sequencers and 1V/Oct pitch sources are abundant in Eurorack, refining the focus of the module and removing physical knobs while retaining CV inputs can reduce HP size without sacrificing on sound or function. Instruo has another multi-oscillator named Saïch, boasting four sawtooth oscillators for pseudo-supersaw sounds, but the module contains an internal quantizer that can lock oscillators two through four into intervals beyond the base frequency set by the coarse and fine tuning controls. In a way, Saïch thrives in the middle ground between multi-oscillators and chord oscillators.

It should also be noted that in some scenarios, complex oscillators can behave as multi-oscillators, with the added benefit that their sophisticated internal routings can aid in generating some wild timbres. Modules like the Make Noise DPO and Frap Tools Brenso even provide their respective Follow and Integrator circuits to impart variance in the response of both oscillators to one shared voltage source. But most complex oscillators are only two VCOs, so if you want to have your multi-complex oscillator cake and eat it too, there's perhaps no better choice than the Trident from Rossum Electro-Music. With its three oscillators and generous modulation options, you never have to choose between harmony and complex intermodulation, and using both at the same time will lead you to compelling sonic destinations.

However, multi-oscillators aren't restricted to the realm of analog, and, as you might imagine, their digital counterparts push the envelope even further. Synthesis Technology's E370 brings wavetables, cloud detuning, and more to the multi-oscillator format, with the benefits of configurable CV routings, optional chord modes, and more that can only be found on a digital module like this. Other options like Acid Rain's Chainsaw, the Demon Core and its expander from Supercritical Synthesizers, and even the XAOC Devices Odessa with the Hel expander fall into the shared-controls/macro approach of Saïch.

Polyphonic Physical Modeling

The collective voyage of Eurorack modules into more digital waters has inspired makers to explore methods of synthesis and effects processing previously restricted to the realm of software. In particular, physical modeling has become a popular avenue for synthesists to explore, with bells, strings, and other organic sounds serving as a nice counterpart to traditional architectures of synthesis.

Among the more prolific choices, Rings by Mutable Instruments could be a large contributing factor for the popularity of physical modeling in Eurorack. Not only does it offer impressive versatility as a sound source and as a processor, another reason for its abundant success can probably be attributed to designer Émilie Gillet's clever implementation of polyphony, with distinct modes that provide one, two, or four voices. By patching a V/Oct source into the pitch input without using the external gate input to excite the resonator(s), Rings will automatically generate a trigger when it detects a change on the incoming pitch voltage, making it a great choice for smaller systems where there's only room for a small sequencer or just a pitch quantizer.

Turning up the Damping control will increase the decay time for each active voice, so with careful construction of sequences it's totally possible to get interesting strumming and harp-like patches. And Rings isn't limited to string-like sounds as a real harp or guitar would be—different arrangements of the resonator's harmonics can be changed with the Structure control (of course, this isn't even considering what other things can be done with the sympathetic strings or non-linear string models). Regrettably, as of December 2022, Mutable Instruments has permanently ceased production—but their open-source designs mean that alternative versions of Rings are still available. After Later Audio's Resonate and Tall Dog's uRings SE are both clones of the original Clouds design.

But in the time since Rings was released, a few others have joined the ranks of Eurorack physical modeling modules, and most incorporate polyphony in some fashion as well. In particular, while Qu-Bit's Surface doesn't process external sounds like Rings, it does offer up to eight voices of polyphony (double that of what Rings can do), and voices can be triggered and layered across any of its Models, including the drum modes. Similarly, Plonk from Intellijel can load up two of its many models at once, and sending it a trigger while one voice is actively decaying will trigger the other. And of course, our friends at 2hp have a couple of physical modeling options for systems that can't sacrifice a lot of space: Pluck takes care of string-like sounds while Bell handles metallic and wooden percussion.

Achieving Polyphony with Samplers

If a super quick solution is needed, there's no reason that playing samples of chords couldn't be a valid option for a particular patch or composition, especially if those chords would be further processed or manipulated by other modules anyways. This would also circumvent the timbral limitations imposed by a chord or multi-oscillator—what better way to add piano-like chords to a patch than by actually using piano samples? If space in your system is at a premium anyways, a sampler can be a great choice to help out in the occasional polyphonic pinch, without needing to commit to dedicated polyphonic modules.

By far the simplest approach to sample-based polyphony would be playing back full chords from a single-channel sampler module. Triggered playback and pitch transposition are essential for integration into most patches—luckily, modules like the 2hp Play or even Winterbloom's Big Honking Button after some firmware tweaks can do all of these things and more. Both the One and Play also offer the ability to select different samples with CV, so with proper curation and organization of audio files, harmonically coherent sequences of chords could be composed when paired with a sequencer.

But sampler modules in Eurorack are only getting more powerful as time goes on, and today anyone that makes extensive use of samples in their patches is likely to utilize one of the highly capable multi-channel samplers that are available. The largest differences are going to come down to personal workflow and patching needs, but the common thread between many of our favorites like ALM's Squid Salmple, the Rossum Assimil8or, and 1010's Bitbox Mk2 and Micro are that they each offer eight channels (16 in the case of Bitbox Mk2), which is plenty of room for sufficient exploration of drums, layering external sounds, or, for our purposes, polyphony. And of course, if your composition doesn't require the use of every channel for your chords, the sampler can load up any other audio file in those channels to fill whatever roles are missing from your patch.

Compared to the approach using a single channel sampler, the benefit of having additional channels of samples should be clear—use a multi-track gate sequencer or something like Doepfer's A-160-1 and A-161 combo to fire off each chord sample in your desired progression. The various Bitbox models are also MIDI capable, so if you're already using a MIDI-CV converter as part of your setup, extending that MIDI connection from your DAW or hardware sequencer can streamline your physical patching too.

Forming chords with stacked fifths Forming chords with stacked fifths

But another advantage of multiple channels is that we don't need to rely upon samples of full chords to achieve polyphony, and reducing our sample content down to one or two voices per audio file offers more flexibility in generating chords and progressions on the fly. For example, using the same sample of a synth pad playing a perfect fifth on two channels but transposing one up a major third (or four semitones) results in a major seventh chord. Exploring stacked intervals in this way not only results in full fledged harmonic chords like the major seventh example, but several dissonances and neutral chords suggesting neither major or minor tonalities can be found and used to great effect.

Likewise, layering drones of single notes or making use of single-cycle waveforms is a further extension of this concept. This does effectively turn your sampler into a multi-oscillator, but with the distinct advantage of being able to use any sound as source material.

But for the ultimate in polyphonic sample playback, the Eurorack module that's perhaps among the best suited for the task has to be the Disting EX from Expert Sleepers, specifically running its SD Multisample algorithm. If you have any experience with using sample libraries and software instruments like Native Instruments Kontakt, the behavior on Disting EX is largely the same—a collection of related samples from an instrument like a piano, usually one audio file for each individual key, are mapped across a range of notes to be played by a MIDI controller or file. In this case, the Disting EX can read up to three incoming pitch and gate sequences to trigger notes with your favorite control voltage sequencer, and combining this with the internal chord and arpeggiator modes can result in some magical musical ideas. But being the highly flexible and wonderful device that it is, Disting EX can also respond to MIDI data when used with the MIDI Breakout Expander module or messages on the I2C bus, should you be using something like the monome Teletype.

Disting EX comes preloaded with stellar sample libraries from Spitfire Audio specifically curated by Expert Sleepers for the multisample mode, but loading in your own audio files is definitely encouraged as well. In fact, the Disting EX also offers a WAV Recorder algorithm with a multisample recording mode, so capturing audio within your system for the purpose of resampling into a polyphonic instrument is totally feasible and can lead to some highly personalized results.

On the topic of Expert Sleepers, while it's not your typical sampler module, we did want to give a quick mention to their General CV module—a 24-voice polyphonic General MIDI instrument. While technically it uses a microcontroller to synthesize some lovably cheesy emulations of pianos and other sounds rather than using actual samples, its facility as a polyphonic sound source, particularly in its chord and arpeggio modes, was the foundation for how the Disting EX would function in its Multisample mode.

Granular Approaches to Polyphony

Looking for a more abstract approach to polyphony? Similar to working with samplers, granular synthesis opens the door for a number of experimental approaches to polyphonic Eurorack patches. Contrasting with traditional samplers that read from a single point in an audio buffer, most granular processors can generate several simultaneous "grains"—independent snippets of the buffer played back at a time. While grains have historically been rather small due to both the abstract approach to sound design offered and the practical limitations of computer processing power, the wider adoption of granular synthesis, particularly in the realm of ambient music, and the advance of technology has made layers of long grains easily possible.

Perhaps the easiest way to approximate polyphony with a granular sampler or processor is to start by recording an arpeggio or sequence into its buffer. The key here is to experiment with grain duration and density while the buffer is being read back at different points, for although multiple grains might not be generated at the exact same moment in time, overlapping tones can stack and form harmonic structures. For more consistent or predictable performance, this may be more easily achieved using pre-recorded audio on modules that support granularization of external files, but improvised polyphony using continuous live input on a module like the (now-discontinued) Beads from Mutable Instruments can be a ton of fun as well!

As an extension of this approach, the Arbhar from Instruo contains six different buffers (referred to as layers) that can be dynamically recorded to and read from on a whim, almost like a granular version of our previous discussion regarding single and multichannel samplers. If you were to then arbitrarily assign one note to each layer, each note has a ton of additional breathing room to explore beyond holding a static sound at a particular frequency, such as using timbre or amplitude modulations. A dynamic control voltage source into the Layer CV input on Arbhar's included expander can offer a fun way to scan through grains of chord tones. In some ways, this would be no different from dividing one large buffer into several sections, but the pre-prepared delineation of recording space in such a way immediately offers alternative perspectives to inspire new ideas.

For a different yet novel approach to granular polyphony, the Make Noise Morphagene actually has a clever way to generate harmony hiding in one of the front panel controls, given some prior preparation with the SD card and ensuring the module firmware is up to date. By default, maxing out the Morph control would intermittently generate grains at one octave, one octave plus a perfect fifth, and two octaves above the current playback rate, and until January 2020 this was the defined behavior for all Morphagenes. But as of firmware mg203, these intervals can be defined in the "options.txt" file within the SD card as Morph Chord Ratios, thus providing the ability to easily define a predefined chord accessible right on the panel. One special thing about this is that while the included text file contains example values for standard Western musical intervals, the ratios offer five decimal places of precision to allow any modular composer freedom to dip their toe into any tonal or tuning structures their heart desires, such as just intonation or 19-tone equal temperament.

Polyphonic Control Methods

Regardless of your preferred way to generate polyphony, powerful sound possibilities are best complemented by flexible methods of control. Even in monophonic or single voice contexts, controlling any voice in Eurorack offers you a multitude of different options—there are traditional sequencers as well as more abstract, conceptual routes like grouping multiple functions or utilities together to achieve your goals. Approaching sequencing with polyphony in mind is really no different, and there are multiple options for traditional sequencing of multiple voices, and weirder or dedicated polyphonic offerings as well.

Just as Eurorack is filled with an abundance of modern oscillators and effects, powerful methods of digital control for multiple voices exist with polyphony in mind. In particular, the Arpitecht and Triad from WMD (each now discontinued) are a phenomenal pair when it comes to controlling a polyphonic setup. The Arpitecht alone is able to melodically quantize any control voltage input signal and turn it into rich arpeggios with shifting and inspiring rhythms. When paired with the Triad expander, it becomes a truly masterful tool for creating complex chord progressions that are perfectly in key with the rest of your sequence. When paired with modules like the Chainsaw or Demon Core, scanning through the Triad knob will instantly generate musically related and spaced triad chords, taking full advantage of multiple V/Oct inputs.

This method of creating instant chord stacks lends itself well to generative modular workflows, but companies like ACL and Instruo have taken the idea of a chord quantizers to a new level. ACL’s Sinfonion features a startling amount of pitch control, boasting three complete quantizer channels in addition to a chord generator with four outputs and an arpeggiator. Although the Sinfonion’s interface may seem intimidating at first, with a little practice it can be easily mastered and used to perform and create beautifully complex and musical sequences. While the Sinfonion certainly has enough options to generate any chord or sequence you could possibly dream of, Instruo’s Harmonaig approaches polyphony from a simpler, intuitively harmonic angle. The Harmonaig provides four voltage outputs each corresponding to a voice in a chord stack, users can then freely explore Instruo’s modal music system which allows for notes and voicings to be dialed in and restructured at the turn of a knob.

Many traditional sequencers offer multiple lanes or tracks for controlling more than one voice at a time, like the Metropolix from Intellijel or the Malekko Voltage Block. These sequencers offer you multiple tracks of control and allow you to simply program your note values for any given step. This makes it incredibly easy to create chord stacks quickly, and most even include built-in quantizers to make tuning a breeze. Another classic patch for chords is to utilize the multiple lanes of CV on the Make Noise Pressure Points or 0-CTRL, sending all three voltage outputs to any triple oscillator and simply tune up a chord progression for each step of the sequencer. This manual approach does require some time and persistence in tuning each individual note, but there's effectively no limitation on chord qualities, voicings, and progressions.

The fun doesn’t stop there—because modular instruments are so flexible there’s plenty of additional ways to generate polyphic sequences. Getting creative with quantizers, multiples and attenuverters can yield impressive results, and allow you an even further degree of control over how you build your patch.

The beauty of Eurorack (or any modular synthesis paradigm) is that there's often more than one way to achieve your goals, and polyphony is no exception. Sound is only part of the equation, and with something as varied and potentially complex as polyphony, workflow and interaction between synthesist and synthesizer are going to play a vital role in figuring out the best approach for your musical needs.

If chords and harmony aren't your primary means of expression in your patches, there's no need to stress over building sophisticated systems to just have that static chord stab behind your driving beats. As we discussed above, plenty of easy and accessible ways to get polyphony going exist that are well suited for purposes like this. But it should also be incredibly apparent that exciting sound sources and polyphonic methods in Eurorack push the boundaries of historical precedents in music, and we're eagerly looking forward to seeing where polyphony in modular synthesizers will go from here.