A polyphonic Doepfer system, from NAMM 2018

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I must note here that the journey toward a polyphonic Eurorack system is not new—and plenty of instrument designers have come up with their own ways to address it. One of the most thoroughly thought-out approaches comes, as should be no surprise, from the fantastically thorough Dieter Doepfer. Doepfer offers an extensive series of "quad" modules, each offering four of a single function behind one panel. This is a super straightforward way of implementing polyphony in an analog modular system—so if you're looking for a four-voice instrument, checking out his modules will yield loads of options for creating a voice structure. There are lots of other "quad" modules out there that you'll no doubt find handy in this quest—Intellijel's Quadrax and Quad VCA, for instance, make stellar envelope and VCA/mixer choices for a polyphonic setup.

The second potential issue, though, is the matter of decoding control information. Luckily, there are a lot of solutions for this particular problem—but they all hinge around your preferred means of interacting with the instrument. For instance, let's say you want to use a touchplate keyboard like Make Noise's 0-Ctrl or Verbos's Touchplate Keyboard, but you want them to behave polyphonically—your approach might be different from the person who strictly plans to use a sequencer, or an external MIDI keyboard. The problem with many CV-capable modular synth controllers is, of course, that their outputs are monophonic: if a controller has a single CV/Gate output for the entire control surface, then you can't get it to report data about multiple independent "keypresses" (individual notes) simultaneously. (That said, there are workarounds...keep reading for some creative ideas for getting around this problem.)

Most polyphonic MIDI-CV converter modules use a round-robin output scheme with voice stealing—this is a fairly typical approach for providing as satisfactory a polyphonic playing experience as possible. "Round robin" simply means that new keypresses on your controller will be sent sequentially to the next unoccupied CV/Gate output pair in question—so if you hold down one note, subsequent keypresses will be distributed between whatever set of outputs aren't currently occupied with notes that you're still physically playing. "Voice stealing" means that, in situations where you're holding down more keys than your MIDI-CV converter has outputs, new keypresses will terminate the "oldest" notes and take over the appropriate outputs. On several polyphonic MIDI-CV converter modules, these behaviors are somewhat customizable, making it possible to set up according to your personal needs or preferences.

Of course, you don't necessarily need to use a keyboard to make use of polyphony in this context—round robin behavior means that using a monophonic MIDI controller like a wind controller or conventional MIDI sequencer can still make use of this data and distribute it sequentially to each of your individual synth voices.

Omnichords rule. Hear me out

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So the basic patch is really straightforward. Many conventional analog step sequencers have a fixed number of stages and several independently definable voltages that correspond to each step—so whenever the stage changes, you can make several different parameters change all at the same time. Great historical examples of this include Moog's 960, Buchla's 245 and 246, and Serge's TKB—a favorite modern examples is the Make Noise 0-Ctrl, which combines aspects of each of these classic devices into a single, compact unit. There are plenty of devices that can do similar things, but for the sake of this discussion, I'll refer to the 0-Ctrl as an example.

So if you have three oscillators, you can use each row output from the 0-Ctrl to separately control each oscillator's pitch. When you do this, you turn the 0-Ctrl into a simple chord machine—by intentionally tuning each of the values for each stage, you give yourself the ability to quickly and reliably access up to eight distinct chords. This creates something like a modular chord organ (did anyone else ever play a Magnus chord organ? Those things are sick).

Another fun approach is to use something like Doepfer's A-152—by patching three individual gate outputs to the three separate envelopes in this patch, you can come up with a lot of ways of triggering them independently. A particularly fun way is to use the address knob to manually sweep through the output stages—or to use an external slider or joystick to modulate Address via the CV input. With this patch, you've got something like a modular Omnichord—you select your chord by pressing the touchplates on the 0-Ctrl, and "strum" the chord by using your external control source. Of course, using a gate sequencer or unsynchronized square wave LFOs to trigger the envelopes provide alternate variations on this idea—there's a lot of potential territory to explore.

The quite rare Buchla 264 Quad S+H / Polyphonic Adaptor module, via modulargrid.net

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Though fairly easy to accomplish with a set of basic tools, the approach above is a fairly non-standard way of implementing polyphony (as cool as Omnichords and chord organs are). Let's say you wanted something that more directly mirrored the process playing a polyphonic keyboard synth in your analog modular synth...how might you do it? One answer lies in a specialized sort of module that combines multiple Sample & Hold circuits with a sequential switch, specifically for the purpose of performing voice allocation from monophonic control sources.

One of the earliest forms of this sort of module was actually part of Buchla's 200 Series ecosystem in the early 1970s. Buchla's 264 Quad Sample and Hold / Polyphonic Adapter was partly intended to act as a way of interfacing the system's monophonic control sources (such as the 217 Multiple Touch Controlled Voltage Source) and other CV/pulse sources to distribute pitch CV and pulses polyphonically across several voices—acting like a crude voice allocation system. To do that, you'd set the Polyphonic CV Input to "common," select your preferred number of voices, and patch your control source's CV and pulse outputs to the respective Polyphonic Inputs. Every time a pulse is present at the pulse input, a few things would happen: a pulse would be produced at each gate output (in order), and the Control Voltage Output corresponding to the current Pulse Output would update with whatever voltage is present at the Common CV Input. When the next pulse comes along, it advances to the next set of outputs in round-robin fashion, and suddenly, your monophonic keyboard/sequencer/other control source is capable of polyphonically addressing multiple distinct voices.

The catch? As soon as a new voice is triggered, the pulses (gates) for the most recent voice is released—so you can't hold down a chord and expect each independent note to sustain. But if you're using one-shot envelopes that don't have a Sustain stage (or if you're just okay with old notes starting to decay as soon as new notes come along), then this shouldn't be a problem. It's a pretty cool workaround.

Of course, very few of us have a Buchla 264 lying around, but happily, there are some very direct alternatives available in Eurorack format. Intellijel's Shifty is an obvious contender—in fact, this style of voice allocation is its primary purpose. Just set your number of voices, send a CV/Gate pair to its inputs, and you get up to four voices' worth of sequential CV/Gate routing to work with. Shifty also allows you to select the voice allocation direction—so your voices can advance upward, downward, in pendulum motion, or randomly, making for several ways of creating interesting note distribution patterns.

Shifty isn't the only Eurorack module that can do this, though! Another personal favorite is the oft-overlooked and highly multifunctional Doepfer A-152 (mentioned in our "Omnichord" patch above). Referred to by Doepfer as an "Addressed Track & Hold / Switch," it's really no wonder that this module is something of a sleeper...but it is remarkably powerful. You can think of the A-152 as having three primary sections that all share the same controls. The first column of jacks is a 1:8 or 8:1 sequential switch: you can select the current active stage either by advancing with clocks to the external clock input, or you can address with a combination of the manual Address control or the external Address CV input. The other two sections share the same stage selection logic. The second column of jacks is itself a 1:8 sequential switch combined with eight Track & Hold circuits (one for each output); and the final column is a set of gate outputs, where the gate is high at the currently selected output and low at all the other outputs. Likewise, the individual T&H outputs track the input only while that particular stage is selected—otherwise, they hold the voltage that was present whenever that stage was navigated away from.

Original details about the "Fortune" ASR

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As you can see, there are plenty of ways to think about polyphony in modular synthesizers. For now, let's share another peculiar technique somewhat similar in concept to the voice addressing scheme described above: using an Analog Shift Register. Analog Shift Registers aren't exactly a new concept—the first was developed by Fukushi Kawakami (aka "Mr. Fortune") for composer Barry Schrader in 1972, and shortly thereafter became a standard part of early Serge modular system (for more on the history of this technique, check out our articles introducing concepts of Randomness in modular synthesizers, as well as the specific implementations of randomness in vintage Buchla and Serge systems).

An analog shift register is a lot like a Sample & Hold. It features a CV input and gate input...but unlike a typical S&H, it has several outputs. When a gate is received, the first output produces and holds the voltage that is present at the input (like you'd expect)—but the second output produces and holds the voltage that had previously been present at the first output, and the third produces and holds the voltage that had previously been present at the second, and so on. This is a way of "passing" a steady CV along a stream of chained outputs, with each successive output always representative of the prior state of the preceding output. (Note: for those of you similar with the inner workings of BBD-based delays, this should be a familiar concept! This bucket-brigade style behavior is exactly how BBDs work, albeit at a considerably slower clock rate.)

If you want to create a delay-like harmonic effect with an ASR, for instance, try patching up multiple voices (three or four, perhaps) and arrange them such that each voice has gradually darker harmonic content. You could achieve this by using progressively darker waveshapes for each voice (saw, square, triangle, sine, for instance) or by lowpass filtering each progressive voice more intensely. Connect a keyboard's pitch CV output to the ASR's input, and use the keyboard gate to triggers the ASR and to trigger the envelopes for each voice. If you play repeated notes, you should hear the pitches cascade down the line of voices like a peculiar echo. Try playing a melody or sequence, but play each note a few times before advancing to the next note...you should have a steady trail of voices constantly following the current "brightest" note. For a similar "reverse delay" effect, try arranging the voices in the opposite order, creating echoes that get louder and brighter with each successive note.

Creating Arabesque-style harmonies with minimal dissonance can be difficult on an ASR since you have to consider the harmonic implications of several past and future notes as you play. Try, though, working with fewer voices and leaving several ASR stages unpatched. For instance, you could create a two-voice patch that uses the first and third ASR outputs, or first and fourth (if your ASR provides this many stages) rather than using successive stages. This will make stepwise melodies less likely to create unwanted clashes.

Up close with the Verbos Random Sampling's ASR

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There are several ASRs out there in Eurorack format. One personal favorite can be found in Verbos's Random Sampling module. Each of this ASR's stages can actually be treated as individual sample & holds via the per-stage trigger and CV inputs, each normally connected from the prior stage. So you could use it as a single four-stage ASR, as two two-stage ASRs, as a three-stage ASR and separate S&H, or as four fully independent S&Hs. It's wild. That said, its outputs do have some droop and aren't the absolute best for routing pitch CV, so a quantizer might be of use for cleaning up the outputs before sending along to oscillators if harmony is your end goal.

The long-standing star ASR in Eurorack is, yet again, Intellijel's Shifty. Described above as a voice allocation module, Shifty actually also does have a dedicated ASR mode. In this mode, each of the gate outputs fires in sync with one another, allowing an easy means to create the delay and Arabesque-style patches we described above. All in all, Shifty's output voltage are quite solid, and you most often can get away without needing a quantizer on its outputs...so it's by far the simplest choice for doing these ASR patches while maintaining a clear harmonic structure.