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Modular Interfacing

CV & The Audio Interface By Robin Vincent
Published September 2020

Modular InterfacingPhoto: Robin Vincent

Everything you need to know to get your DAW talking to your modular system via your audio interface.

The roads of MIDI between computer and hardware synthesizers are well-trodden, but once you become a devotee to Control Voltage (CV) you need different pathways to find your way from DAW to Eurorack and back again. Let's have a look at what you'll need to pack in your backpack for the journey, along with your sandwiches and a nice flask of tea.

The Theory

Audio signals and CV signals are all variations in electrical potential, and they run along the same sorts of cable. Part of the point of modular synthesis is that there is some crossover between the content of audio and CV signals. An LFO, for example, generates a cycling control voltage that we use to modulate parameters within a synthesizer. As you increase the rate or speed of the LFO, it reaches a frequency that's within the range of human hearing, whereupon we can treat it as an oscillator generating an audio signal. Conversely, an audio signal can be used as a modulation source, and many audio oscillators can be slowed down to a frequency below the threshold of hearing.

The outputs on computer audio interfaces are, as you'd expect, designed to output audio signals. But since audio and control voltage signals are so similar, surely we can also use our DAWs to generate CV signals and our audio interface to pipe these into our modular synths? Well, sometimes. The issue here is that although audio and CV share the same method of delivery, their content can be completely different. Audio voltages are constantly changing, or otherwise we wouldn't hear the results as audio. Crucially, however, a CV can be a static, unvarying value — a DC voltage or, if you like, a signal with a frequency of zero Hertz.

In a system designed to record and play back audio signals, the ability to reproduce signals below 5Hz or so is a mixed blessing. The DAW software itself generally has no problem with very low frequencies, but many audio interfaces are deliberately designed to filter them out; they can't be heard, and they can cause interference or DC offsets that could distort our audio or damage our speakers. And so the biggest obstacle to using our DAW to record, play back or generate CV is an audio interface that filters out DC and very low-frequency signals.

DC-coupling

There are two ways of dealing with this. One is a bit of a fudge, while the other requires that you have the right sort of audio interface. Most audio interfaces have what is called AC-coupled outputs, where a capacitor is used to filter out the extreme low frequencies. This is a disaster for CV, as it cannot output the sort of slow-moving or static values we need.

Outputs without this filter are described as DC-coupled, and are increasingly common in audio interfaces as the use of CV is on the rise. These are capable of generating stable DC voltages and very slowly changing control values, which is exactly what we need to drive a modular synth. One manufacturer whose interfaces have always been DC-coupled are MOTU, while DC-coupled outputs and inputs (but not necessarily both) can also be found on interfaces by RME, Universal Audio, NI, Apogee and PreSonus (who've now made it a feature across their entire range).

However, just because an interface has DC-coupled outputs doesn't necessarily mean it'll be a fully capable source of CV. The Eurorack format specifies a control voltage range of ±5V, but many portable and bus-powered interfaces are not capable of such wide voltage swings. For example, I recently reviewed in this magazine three USB‑powered audio interfaces: the RME Babyface Pro FS, Native Instruments Komplete Audio 6 and MOTU M4. The KA6 could output a range of ±2V, the M4 achieved about ±3V whereas the Babyface Pro could produce the whole ±5V. A good way to get a rough idea of an interface's capabilities in this department is to check the specs to find out the maximum output level on the line outs: NI quote +11.5dBu for the KA6, MOTU +16dBu for the M4 and RME +19dBu for the Babyface Pro.

AC-coupling

If you have an AC-coupled audio interface then you are mostly out of luck — except, as I say, that there is a possible fudge. This involves using amplitude modulation to sneak the low frequencies through the interface by superimposing high-frequency signals on them and then removing the AC component afterwards. This requires a filter, which is a relatively simple circuit to build. Expert Sleepers used to sell a little module that did it, but now they've turned it into an algorithm within their Disting module, which does the same job. Going the other way, you'll need something that will convert your CV into an amplitude-modulated signal that you can sneak past the AC-coupled audio inputs. The Disting again has an algorithm that'll do this, and I've not come across anything else that does.

Expert Sleeper's Disting module (third from the left) allows you to partake of the dark art of AC-coupling.Expert Sleeper's Disting module allows you to partake of the dark art of AC-coupling.

Provided the software you're using (which we'll come onto in a moment) supports an AC mode, then, you can either make a few circuits or buy a number of Disting modules and be all set for sending CV signals from your DAW to your Eurorack and back again. Realistically, though, it's probably much simpler just to get yourself a DC-coupled audio interface.

There's definitely something in the way CV works that offers something worth pursuing.

CV Generation In DAWs

There are several bits of music software that can generate and output control voltage. There's CV Toolkit from Spektro Audio, MOTU Volta, Reaktor Blocks, Dialog Audio SQ4 and Audulus to name but a few. But for this article, I'm going to highlight Bitwig Studio, Ableton Live CV Tools and Silent Way from Expert Sleepers.

Bitwig has a couple of devices within its modular structure that can generate and receive CV. HW CV Out is a simple knob that sends voltage out of a specified audio output. You can then start applying internal modulators like LFOs, step sequencers, randomisers or even MPE expressions to that knob to produce the sort of CV output you're after. You can create fabulously complex forms of modulation within The Grid environment and attach it to that knob to send it to your modular.

The other device is the HW CV Instrument which gives a pitch output and a gate output to provide a melodious signal for sequencing or playing from a keyboard. The octave range of the intended oscillator has to be tuned and calibrated to match the pitch to the voltage. This is done by routing the output of the oscillator back into Bitwig and running a calibration process. This can be a bit fiddly; you need to get input levels right and use quite a bit of trial and error to get it to scan correctly. But, once accomplished, you can send tunes out from Bitwig into that particular oscillator. If you want to try another oscillator then you'll have to recalibrate.

If you want to go in the other direction of controlling instruments or devices within Bitwig from your modular you can add an HW CV In modulator to any device, specify the input, and then direct the voltage to whatever parameter you wish. It's really very straightforward.

Bitwig CV devices have an AC mode and so can generate amplitude-modulated CV signals and interpret them coming in. In theory, then, Bitwig should work well with regular AC-coupled audio interfaces provided you have that bunch of Distings to hand. In practice, however, I had great difficulty getting the calibration of the Bitwig CV instrument to work in AC mode, whereas I had no trouble with the Silent Way plug‑ins.

Live CV Tools

Ableton Live version 10.1 introduced the CV Tools pack of devices for Max For Live, designed to handle the CV side of things. It has a CV Instrument device very much like the one in Bitwig for handling melodic output of pitch and gate signals. A CV Triggers device works like a drum machine, and a Rotating Rhythm Generator can generate all sorts of interesting patterns.

For modulation there's a simple CV LFO which can output various waveforms. There's also a CV Shaper in which you can create any wave shape you like. CV Utility is more like Bitwig's voltage knob: you can apply all sorts of modulation to it from internal devices or MIDI controllers attached to Live. All of these rely on you having a DC-coupled audio interface, as there's no AC mode. For CV coming into Live there's a CV In device that takes any incoming voltage and lets you map it to any software parameter. This has a Pitch mode that can be used with an AC-coupled interface to convert an oscillator pitch into CV in real time, so that's at least something.

With their ES‑1 modules, Disting and DC-coupled Eurorack audio module, Expert Sleepers are undoubtedly the experts in CV and DAW integration. Silent Way is their bundle of CV tools that you can use as plug‑ins in any DAW, and offers a comprehensive range of functions. Silent Way is also available as a Rack Extension for Reason and as virtual modules for VCV Rack opening up both of those modular environments to copious amounts of CV control.

There's a ton of stuff in here for generating voltages as pitch or modulation, gates and triggers, quantising, controllers and followers. Amongst them is the Voice Controller, which does the same Pitch and Gate thing as the devices in Bitwig and Live, but adds a whole bunch of envelopes, LFOs, portamento and detuning. It creates an entire synthesizer voice in voltage which you could connect to all sorts of bits of your modular. Silent Way also has an AC Encoder which will do all the hard work in using amplitude modulation to get through an AC-coupled audio interface.

Making It Work

So, armed with the right software and the right audio interface, is it easy to get your DAW talking to your modular? Yes, for the most part.

The easiest way to get CV flowing into and out of your computer is via the Expert Sleepers ES‑8 or ES‑9 DC-coupled audio interface modules. They connect via USB and become the audio engine for your DAW. The ES‑8 has four inputs and eight outputs and is expandable via ADAT, whereas the ES‑9 has 14 inputs, eight outputs plus ADAT and S/PDIF, the only difficulty being that there are no microphone, instrument or line inputs such as you might need for recording non-modular signals into your DAW. The ES‑9 does at least provide audio outputs for monitoring non-CV audio. Otherwise, DC-coupled audio interfaces like those from MOTU and PreSonus can do the job really well.

All MOTU interfaces are DC-coupled and modular friendly.All MOTU interfaces are DC-coupled and modular friendly.

There's one caveat, which is that if your DC-coupled audio interface has balanced TRS outputs, you'll need what's called a 'floating-ring' cable. If you try to use a standard mono-to-mono patch cable from your interface to your modular, you'll only get the positive half of the signal. To get the full range you need a TRS-to-TS cable that's wired as a floating ring rather than being simply a stereo-to-mono cable. Expert Sleepers can supply them, and you can also use insert cables that have TRS on one end going to two TS jacks. This has the added bonus of giving you positive CV on one jack and an inverted version on the other.

So, to summarise, there are two possible signal paths for driving a modular synth from your computer:

  • Computer > CV-capable software > DC-coupled audio interface > floating-ring cable > modular.
  • Computer > CV-capable software with AC mode > AC-coupled audio interface > Disting > modular.

The Digital Alternative

Whether it's worth the effort to have control voltage running through your digital workspace is really up to you. The alternative is to look at MIDI-to-CV and CV-to-MIDI options that tend to work right out of the box.

You could, for instance, have a Hexinverter Mutant Brain MIDI-to-CV converter module, which will convert a single MIDI connection into 16 Eurorack-compatible outputs from notes to modulation, gates, triggers and clocks. Going the other way, you could use the Befaco VCMC to take CV notes, gates and modulations and map them to MIDI notes, events and controllers. MIDI-to-CV tends to be unipolar, though, and doesn't have the feel of CV control. It feels like you're sequencing MIDI equipment rather than experimenting with the impact, interaction and mixing of voltage. There's definitely something in the way CV works that offers something worth pursuing.

Published September 2020