Controlling Analog Synths with Software Using a DC-coupled Audio Interface

Understanding the Two Types of SignalsWhy Does This Matter to Me?What Will It Allow Me to Do?What Software Can I Use to Control CV?Before Getting StartedWhat Interfaces Will Work?Synthesizing Conclusions

“What in the world is a DC-coupled audio interface?” you might be asking. “And why would I need one?” A DC-coupled audio interface is an interface that will allow you to control analog synths with digital software. It’s the bridge that can connect the seemingly disparate worlds of digital control software and analog synth hardware by using your computer as the translator. How does that work and how can you explore all the possibilities of using your analog synths synced to your DAW? Let’s find out.

Understanding the Two Types of Signals

First, we need to start with some fundamentals. For this discussion, there are two types of signals.

AudioControl voltage

Most audio interfaces are designed to record or reproduce audio signals. Audio signals lie within the audible spectrum between 20Hz (Hertz, or cycles per second) and 20,000Hz (20kHz) and alternate between positive and negative voltages, which is known as alternating current (or AC). Control voltage signals (CV), on the other hand, can be AC or DC and often modulate values in the frequency range below human hearing.

Why does this matter in your audio and analog synth setup? DC control voltage (CV) signals are ultra-low frequency, below the lowest of audio signals. Many audio interfaces will not pass DC (from input to output) because they utilize capacitors in the signal path that filter out DC voltages.

Why Does This Matter to Me?

If you are a hardware synth fan, then here’s the bottom line: If you want to control synth modules with software using CV, then your audio interface must be direct coupled, otherwise it will not work. The CV signals will not pass through your interface. If you want to utilize virtual tools to construct complex and nuanced patches, textures, and patterns and then output those to your synths, then you need an interface that is DC coupled.

What Will It Allow Me to Do?

Have you ever wanted to lock your hardware synth’s tempo to a tempo map in your DAW? Or modulate the pitch via a mod wheel from a USB MIDI controller? Or duck a sustained sound via sidechaining? Maybe modulate a synth parameter (LFO, waveform, etc.) in time with your recorded song? Or trigger or gate a sound at a specific point in a song? If you’ve ever wished you could do that, then software control (via CV) of your analog synths may be the solution.

What Software Can I Use to Control CV?

The capable software is a short list: FL Studio, Softube Modular, Native Instruments Blocks, Reason, Bitwig Studio, and Ableton Suite. The CV tools in each piece of software function differently, including total number of outputs, pitch scale, voltage range, viable parameters, clock resolution, and more. Let’s dive into a few and see what they can do.

FL Studio – Fruity Voltage Controller

FL Studio’s CV controller manages up to eight channels in each instance presented as four pages of L/R stereo outputs. The Main Outputs are the default CV channel, but the additional three pairs can be assigned with unique Left and Right CV controls. Extensive automapping can be used to assign stereo outputs to Mixer Tracks that follow the voltage controller as offsets of the current Mixer Track. Fruity Voltage Controller allows you to choose voltage ranges of +5V, ±10V, or 0V–10V. Additionally, scale can be designated as 1V/oct. or 1.2V/oct. (the latter being used in Buchla systems), and clock resolution can be set between 2 and 96 PPQ (pulses per quarter note).

There are Left and Right channel panels for each output containing identical parameters. This is where you choose the source of your CV function, which include pitch, gate, trigger, velocity, volume, panning, Mod X, Mod Y, clock, and a “None (0V)” parameter. Velocity and volume are both functions of note velocity. These parameters generate voltages between 0 and maximum voltage that reflect internal values from 0 to 127. The velocity subfunction ignores channel volume while the volume subfunction is scaled by channel volume. The “None” parameter allows you to set a custom voltage value. You can also establish voltage offset as well as decide if the offset values are unipolar or bipolar. Scale can also be defined here and applied to Offset and Mod Source values.

Softube Modular Plug-in

Developed with vaunted modular synthesis developer Doepfer, Softube’s Modular plug-in is a virtual modular synthesizer with a diverse, robust array of modules, including seven component-modeled Doepfer modules and 50 stock modules. Mixers, sequencers, multipliers, effects, and more are included with over 400 presets as a jumping-off point. You can expand your virtual repertoire with premium add-ons, including Buchla, Intellijel, 4ms, Mutable Instruments, and more.

In addition to being a virtually limitless modular rig, Softube Modular can be used with hardware Eurorack systems via DC-coupled outputs. Because the plug-in is available in VST, AU, and AAX formats, it’s compatible with most DAWs; however, the capacity to which it can be utilized will depend on the DAW. Within the boundaries of those possible limitations, Modular can conceivably control nearly any analog equivalent of its virtual replication or can be creatively used to dictate disparate parameters on the hardware side.

Native Instruments Blocks

As part of the Reaktor 6 modular DSP lab from Native Instruments, their Blocks platform mimics patch-on-panel modular synthesis. Blocks builds on the synthesis and sound-design capacities of Reaktor as well as elements from synth plug-ins like Monark, Driver, and Massive. Much like Softube’s Modular plug-in, Blocks has the sound-sculpting tools you’d expect from any conceivable rack setup: clock management, LFOs, VCAs, step sequencers, effects, ADSR envelopes, and more. The engine also supports a variety of synthesis options, including additive, subtractive, frequency modulation, and amplitude modulation. Blocks comes loaded with at least 24 modules and 35 pre-programmed preset racks with plenty more available both directly from Native Instruments and through an extensive user community. Other Blocks and Reaktor patrons can upload custom modules for any number of purposes. Native Instruments also offers over 200 partner modules from developers such as TOYBOX, Genki Instruments, ACL, Michael Hetrick, and more.

Because Blocks is a function of Reaktor, it can be used either on its own or piped into another DAW. In either case, the limitations on external modular control are determined only by the software that uses it. Exploration is a driving ethos of the Native Instruments offering, and Blocks is primed to work directly with their Komplete Kontrol S-series keyboards as well as with their Maschine controller right out of the box. This allows for easy integration for regular MIDI-device users and can reduce a lot of the leg work of assigning individual values to outputs on any DC-coupled interface.

Reason

Reason’s support for external CV modulation is quite extensive. It offers a number of utility tools, effects, filters, synthesizers, LPGs, sequencers, monitors, VCFs, and more in the form of plug-ins from numerous developers. It should be noted that many of these plug-ins are emulations of CV functionality. Put another way, they’re virtual representations of voltage-controlled modules. That being said, because of Reason’s unique rack workflow, many of these plug-ins, as well as external VSTs, can be routed through the Spider module to output as CV signals. Spider can also be used as a multiplicator, as well as to sum and invert signals, allowing you to generate complex ideas that can run to multiple parameters, including multiple instruments. Or you could use something like Reason’s Europa wavetable synth as its own voice while simultaneously outputting an elaborate envelope to a parameter on your modular hardware for a synchronized sidechaining of sorts.

Bitwig Studios

Bitwig’s relationship to CV management falls into the same workflow as the rest of their instruments, effects, utility tools, and so on. Their “Hardware” interface objects allow Bitwig to communicate with external devices. Broadly speaking, these handle control of all sorts: audio signals, clock messages, and CV signals. These are broken out into several options.

Clock Out: Sends signals at a predefined interval, a signal only at transport start (in Play mode), a signal only at transport Stop, or a signal with every Note value received.CV Instrument Out: Used to transmit note messages as CV signals out of the system. Pitch CV Out is on one path while another handles Gate Out triggers.CV Out: Distinct from the CV Instrument, this allows you to send CV signals from a designated interface port from a parameter knob.

Bitwig also features a Hardware interface object for CV inputs.

Ableton Suite – CV Tools

Ableton’s modular arrangement capabilities make it a natural contender for CV integration. Originally introduced in Live 10 Suite, the CV Tools Pack comprises 10 devices to send, modulate, generate, and receive CV signals. Even if you don’t have a DC-coupled interface or any modular gear, the tools can introduce modular-style workflow options to reimagine how you make music in Ableton.

Apple

Apple phones and tablets have DC-coupled headphone jacks. Apps like CV Mod, Brute LFO, and Trigger Box are a few capable of generating CV signals. While it might be a bit unconventional, these apps could easily sketch out sound-design concepts or integrate modular hardware into an iPad-centered production setup. CV Mod, for example, is a 2-LFO/VCO oscillator module with a robust suite of compositional possibilities, including eight waveforms, a freehand wavetable grid editor, envelope generators for each oscillator, fine-tuning, and more. These deep controls can be further honed with the addition of something like Audiobus 3, allowing you to send signals through a DC-coupled interface and even control them with an external MIDI controller.

Before Getting Started

By this point, you’ve likely gotten the gears turning a bit on what a DC-coupled interface could do for you, imagining the perfect digital-analog hybrid system. It’s worth clarifying a couple details that will make your life easier. While various plug-ins and DAW modules have a cap on the number of channels they can route, this is only a limitation per instance of use. So, as long as you’ve got the system to manage the CPU load and the outputs for the job, the number of signals you can run from your system is virtually limitless. Additionally, some programs are natively limited to the voltages they process (such as Ableton’s CV Tools). This could be circumvented through the use of a DC offset. If the program goes to 5V but you want to send 6V, then you can send that to a CV mixer with a 1V offset to get 6V.

Another obvious consideration is the cabling required. Hosa CMP-305 and CMP-310 are 5- and 10-foot cables with a 3.5mm plug on one end and an unbalanced (TS) 1/4-inch plug on the other end. TRS cables should be avoided unless you are using an insert cable (TRS 1/4-inch plug on one end and two TS 1/4-inch plugs on the other end). Then you can use GMP-113 adapters to convert the 1/4-inch plugs to 3.5mm plugs. This lets you send positive and negative CV signals out to the tip and ring ends, respectively. This is useful if you want to send a positive CV to open a VCA for a kick-type percussion sound and a negative CV to a VCA to duck a pad.

What Interfaces Will Work?

DC-coupled interfaces are made by companies including PreSonus, Universal Audio, MOTU, Apogee, RME, Lynx, and Native Instruments — USB-powered and rackmounted interfaces with DC-coupled outputs. It’s worth noting that very few brands manufacture hardware with DC-coupled inputs.

Because of the nearly limitless number of creative opportunities and permutations of ideas that can exist, we’ve decided to organize our suggestions around the size and type of DC-coupled interface you might own or decide to acquire.

Compact

PreSonus Studio 68c USB-C Audio Interface

For those with compact, portable, or home studios, having a reliable, multipurpose interface is an integral part of your setup. The Studio 68c is a 6-in/6-out USB-C interface from PreSonus that includes four Class A XMAX preamps as well as a robust MIDI S/PDIF breakout for adapting any number of MIDI devices (cable included). Something this size will be useful to control a small number of CV signals, either for real-time playing or recording, as well as systems where limited CPU-usage is a consideration. Examples might include generating a complex, multi-stage LFO for an analog synthesizer or clock signals to synchronize tempo on a sequencer, arpeggiator, or drum machine with your DAW-based rig. In a performance setting, you’ll find this to be especially helpful when consistency is needed across only a few values without the need for extensive patching once your sound is in motion. Conversely, if you’re doing a fair amount of patching but the outgoing CV signals will continue to originate from the same source while remaining tempo synced, then this is a great choice.

Add to cart Add to listLearn More

Medium

Universal Audio Apollo Twin MKII DUO Heritage Edition

Universal Audio’s Apollo-series interfaces are known for their robust onboard UAD DSP, and this 10-in/6-out interface is a great choice for those with smaller setups who are looking to record external instruments alongside their rackmounted gear. Arrangements might include using CV signals for gate sequencers on a drum machine or using DAW-based tools to modulate a VCA, VCF, or lowpass gate for sidechaining or other rhythmic effects. Similarly, multiple outputs could be used to send the same information from one source to different parameters or to the same relative parameter on different modules, allowing for consistent, complex synchronization. Those with MIDI control surfaces will find this a useful way to control multiple values in real time that might otherwise require additional patching while playing all without the need for supplementary modules.

Add to cart Add to listLearn More

Large

MOTU 624 16×16 Thunderbolt/USB 3.0 Audio Interface

The previous setups will work for those with only a couple modules or who otherwise only want to control a few separate values through their DAW. With the MOTU 624 Thunderbolt/USB 3.0 interface, you have 16-in/16-out capacity that really ups the ante. Having more outputs means more opportunities, right? You can use these outputs to vastly increase the number of modules incorporated into your hybrid rig or to control multiple parameters on a single module from one controller. Anyone with an MPE-capable controller (MIDI polyphonic expression) might use this to map MPE values for crafting soundscapes with simultaneity that’s otherwise impossible — or discouragingly difficult — by analog patching alone. Velocity and sensitivity values could be scaled to control filters, effects, and more. Similarly, a MIDI control surface could be used as a digital matrix to change outgoing signal sources on the fly, making for striking changeups or generally easier control of aural textures of a consistent target source. Moreover, you can easily expand the previously proposed arrangements for a more robust recording or performance system.

Add to cart Add to listLearn More

Extra Large

PreSounus Quantum 26×32 Thunderbolt 2 Audio Interface or Lynx Aurora (n) 32-DNT 32-channel AD/DA Converter

Once you’re working with an interface with this much I/O, your opportunities increase dramatically. Each interface boasts 32 outputs in a rackmounted chassis. This is ideal for heavy-duty studio or performance use, easily controlling parameters across multiple modules or several parameters within a smaller selection of modules. An ambitious choice might be simply to target pitch via CV signals to play 16 synthesizers from a single keyboard (32 outs = 16CV and 16 gates)! Why you’d want to do that is anyone’s guess, but preconstructed parts could easily run to multiple instruments from a DAW, cueing them in sync with each other. The Lynx Aurora would also open the door for CV signals to be received, as well. Though it is an AD/DA converter, this would allow for quite complex interfacing between your modular and digital systems, letting you control values within the DAW while playing from the rack pieces. A modular system of two-way interfacing between synchronized analog and digital systems would only be limited by the I/O capacity (or total interface units) and the CPU of your computer system. Plus, the digital conversion would let you control digital instruments, effects, and more without the limitation of CV processing.

Add to cart Add to listLearn More

Add to cart Add to listLearn More

Synthesizing Conclusions

There’s a good chance you’ve got some brilliant ideas to explore what we didn’t cover, and that’s great! By no stretch is this a definitive list but a way to hopefully spur creativity in your sonic odyssey. If you’re looking to conquer a modular mountain and have some questions about how to do it or feel like picking up some gear for the task, then contact your Sweetwater Sales Engineer or contact us at (800) 222-4700 to plan your voyage.
The post Controlling Analog Synths with Software Using a DC-coupled Audio Interface appeared first on inSync.