A guide to processing audio through your voltage-controlled synths
Fig. 1. The Kenton Pro-2000 MkII is a standalone, multichannel MIDI-to-CV converter.A MODULAR synthesizer is more than just an instrument for playing basses and leads. It can also act as an audio processor that is both highly responsive (via CV or hands-on control) and fully expandable (just add more modules).
Fully integrating a modular synth into the modern studio workflow has never been easier, and utilizing a DAW expands your options exponentially. In addition to traditional MIDI-to-CV converters, there are software applications that generate CVs, allowing you to control module parameters with greater precision from your digital audio workstation, whether for processing or playing an instrument.
Regardless of whether your synth modules are fully analog or a digital hybrid, they are designed to generate and accept voltages. So no matter which module format you have—Frac Rack, Eurorack, Moog-style, etc.—they all play well together, though there might be some cable adapters and level tweaking involved.
In this article, we will look at various ways you can control your modular system when using it to process audio. We will also examine specific products that improve workflow (and some tips on how to get the most out of them).
Brute-Force Audio Processing So you want to run some drum tracks through your favorite filter module? No problem: Just go direct. Simply route a DAW Send to an analog output on your interface to run an audio track to your synth, and then “perform” the processing using the module’s knobs. With another cable, you can return the processed sound to an input on your interface and record it to a audio track. Once you’re finished, simply nudge the processed audio track on the timeline until it’s in sync with the other tracks.
Of course, this type of processing is done by ear and is otherwise unsynchronized with the DAW. And it does not allow you to record the controller movements, tweak them, and play them again with the changes, unless you use a synth module that is designed for that specific purpose. However, in the right situation, this kind of hands-on control of hardware-based processing can inject the excitement of a performance into a track while imparting extra character provided by the synth module.
Fig. 2. MOTU Volta is an AU/MAS plug-in that generates a host of control signals that are synchronized with your host DAW. An interface with DC-coupled audio outputs is required.
The main technical issue you will encounter is level matching between the output of your mixer or interface and the inputs on your modular. Because you’re working with line- level signals, balancing audio levels between products is fairly easy. Some modules even respond well if the input signal is a little on the hot side. Obviously, if your mixer or interface has 1/4-inch I/O and your modular has 3.5mm jacks, you will need adapter cables with the proper connectors on each end.
If you plan to run mic- or instrument- level audio through your modular system in addition to line-level signals, you will want an interface module, such as the Doepfer A-119 External Input/Envelope Follower ($99) for Eurorack systems or the Blacet Research I/O 2225 ($179 assembled) for Frac Rack users. Not only do these types of modules help you attain usable audio levels for processing, they can also provide envelope and gate/trigger signals by following the contour of the input signal.
For example, the Doepfer A-119 has two inputs: Asym for line-level audio and Sym for instrument and mic-level input. The Envelope output tracks the input level and gives you a proportional voltage, which you can use to control a VCA or VCF. The module’s comparator provides a Gate signal (when the input crosses the user defined Threshold level), which you can use with an ADSR envelope generator or other modules that accept gates.
If you’re looking for studio-grade audio interfacing, Cwejman offers the AI-2 ($325), a Eurorack module that has two balanced I/O channels on XLR jacks with gain trims and 3.5mm send and return jacks.
Fig. 3. In order for Volta to calibrate itself to your hardware synth’s oscillators, audio from your modular system is returned to the plug-in through your interface. MIDI-to-CV While it’s fun to have hands-on control over your processing, things get really interesting when you control your modular via MIDI. In addition to helping you play your modular as an instrument, a MIDI-to- CV converter provides control information based on MIDI data that is stored in and synchronized with your DAW. The control information can include gates, triggers, and CVs generated by MIDI sequencers, for example, which can then be used to change parameters such as filter cutoff or modulation depth in time with the MIDI Clock.
MIDI-to-CV conversion can be done with a dedicated module or a standalone device. Kenton Electronics makes a line of MIDI- to-CV converters, including several types of standalone units as well as a Eurorack module (see Figure 1). Doepfer, which helped launch the Eurorack modular craze, offers a variety of MIDI-to-CV modules, as well as the Dark Link ($170), a USB/MIDI-to-CV converter based on the circuit used in its Dark Energy module. (Doepfer also sells the A-192-2 ($190), a CV/ Gate-to-MIDI/USB interface designed for controlling MIDI synths, including soft synths, from your modular.) Other manufacturers of MIDI-to-CV modules include Analogue Systems, Analogue Solutions, Kilpatrick Audio, and Pittsburgh Modular.
Another option is to use a MIDI control surface through a MIDI-to-CV converter to tweak your analog gear when processing audio. This may sound crazy, because the analog gear already has knobs, and CVs generally have a finer resolution than MIDI. But there are times where I have found it made more sense to have several knobs and sliders in one place (within a MIDI controller) when “performing” a patch rather than having to reach in several places to get to knobs that are partially obscured by patch cords. In this situation, you would also have to record your MIDI performance to your DAW if you plan to recall or edit your performance later.
Direct and Indirect Keyboard Control An interesting trend in recent years is the inclusion of CV and gate output jacks on mass-produced keyboard instruments, such as the Akai Professional Max-series USB MIDI controllers and the Arturia MiniBrute and MicroBrute analog synthesizers. Similarly, an instrument such as the Doepfer Dark Energy ($625) desktop synth has USB MIDI input that allows you to translate messages from your DAW to CV’s using the synth’s many 3.5mm top-panel jacks.
These solutions provide a more standardized way to get MIDI-to-CV conversion, which can control audio processing parameters in addition to simply playing a synth voice. But while MIDI- to-CV conversion is very, very handy onstage and in the studio, there is still a higher level of control to be achieved.
Interactive Plug-ins and Specialized Hardware The current state of the art of DAW-to-modular integration involves using software applications and plug-ins to generate CVs, gates, and triggers, which are then sent to your modular system from your audio interface. The result is a level of resolution that surpasses MIDI while providing a higher degree of synchronization to your DAW session. Think of these products as an automation track that can be dynamically created and altered in real time. Software such as Native Instruments Reaktor, Cycling 74 Max/MSP, and Max For Live within Ableton Live are widely used in this regard.
Fig. 4. The Expert Sleepers Silent Way Voice Controller provides everything you need to control an entire synth voice with each plugin instantiation. However, it’s also perfect for manipulating synth-module parameters in sync with your DAW when processing tracks. An important issue that needs to be addressed is how to get DC control voltages to your modular from your computer’s interface. If you’re using a standard audio interface, you will need to determine whether it has AC- or DC-coupled outputs, and then assemble the appropriate cabling. Another option is to use a “break-out box” solution. In order to determine which is best for you, let’s look at two software products that are specifically designed for use with hardware synths and provide self-tuning functionality for calibrating the CV response to match that of your oscillators.
The first mainstream plug-in to offer self-calibration was MOTU Volta ($199), an AU/MAS instrument plug-in that sends DC voltages from any interface with DC-coupled audio outputs (which many of the MOTU interfaces have). Volta can create CV output from MIDI CC messages, ramp automation, or from its own trigger sequencer, step sequencer, LFO, and envelopes (see Figure 2). This gives you enough CVs for a complete synth voice from each instantiation of the plug-in, all of which is synchronized with your host DAW (see Figure 3).
To prevent damage to your DC-coupled audio interface, you will need to use a special cable to connect the audio output to your modular when transmitting CVs. The cable must have a balanced plug on the end that goes to your interface and a 2-conductor TS plug on the other for your modular—TRS (with the ring floating) to TS, or XLR to TS, with the XLR’s pin 2 going to the tip and the other two pins to the sleeve. These cables are easy to make, but are also available for purchase. In a pinch, you can also use one end of an insert cable.
Because the voltage level from the output of a DC-coupled interface can be anywhere from ±2 to ±9VDC, depending on its make and model, there is some unpredictability when it comes to the tuning range you can achieve. You can use Volta (or any other software that sends CVs) with a standard voltmeter to test and confirm the output voltage of your particular interface. But if you have an AC-coupled interface, Volta is out of the picture.
Expert Sleepers Silent Way ($59) is a suite of 19 plug-ins (AU, VST, AAX) that sends CV/gate/trigger signals and features self- calibration capabilities, but it offers a greater number of interfacing options. A trial version of the plug-in suite is available online.
While some of Silent Way’s plug-ins are used only for specific interfacing situations, others provide the basic functionality that modular users need, with the ability to synchronize the output with the host software’s tempo. In addition to the normal synth-like features of an LFO, a stepped LFO, a pitch and envelope follower, and a multistage trigger/envelope, Silent Way offers CV-to-MIDI conversion, CV- to-OSC, a clock plug-in that can generate DIN sync, and a DC module for creating steady-state signals. Silent Way can also accept CVs using the CV Input plug-in, and then process the signals using the Function plug-in, which offers several mathematical functions with offset and scaling options.
Of special note is the Silent Way Voice Controller, a virtual instrument that provides the requisite voltages to control a complete synthesizer voice—pitch CV, trigger, gate, and envelopes, along with LFO modulation, detuning, glide, and transposition (see Figure 4). It calibrates to your hardware oscillator (using either the V/octave or Hz/V standard, the latter of which is for products by Metasonix, Korg, and Yamaha), and the calibration data can be saved as a text file for later use (or loaded into the Silent Way Quantizer plug-in). A version of the Voice Controller is available as a Rack Extension for Propellerhead Reason.
Fig. 6. The Expert Sleepers ES-3 Mk3 connects to your interface’s ADAT Lightpipe port and provides 8 channels of ±10VDC control signals from Silent Way or similar software running on your computer. You can also send audio through any of the module jacks.
Although you will get the best results if you interface Silent Way with an Expert Sleepers ES-series Eurorack module, the plug-in can send its DC voltages through the audio outputs of AC- and DC-coupled interfaces, as long as you use the proper cabling. If you decide to use the audio jacks of an AC-coupled interface, Expert Sleepers recommends that you use the Silent Way AC Encoder plug-in with a rectifier cable, which is simply a floating ring cable (as above), but with the addition of a diode and a capacitor (see Figure 5). You can increase the voltage range by adding more diodes/capacitors in a voltage multiplier configuration. Information about making these cables and circuits can be found at expert- sleepers.co.uk/siwaacencoder.html.
If you want to use your interface’s audio outputs, but you don’t want to hassle with the custom cables, the Expert Sleepers ES-1 accepts standard AC-coupled audio output on 1/4-inch jacks ($215) or a DB-25 connector ($195). As above, you will need to use the Silent Way AC Encoder plug-in with this setup, but you will get a greater voltage range than your interface’s outputs offer because the ES-1’s amplifier design is specifically calibrated for use with modular systems.
The most efficient way to get ±10V, DC- coupled analog signals from your software— whether it’s Silent Way, Volta, Reaktor, or Max/ MSP—is to connect one of the digital outputs on your audio interface to one of Expert Sleepers’ D/A-converter modules, all of which are in the Eurorack format and have 3.5mm output jacks. The ES-3 Mk3 ($325) accepts a TOSLINK connection (from your interface’s ADAT Lightpipe port) and provides 8 channels of CV or audio, while the ES-4 ($295) accepts S/PDIF input using a coaxial jack to provide 5 signals (two of which can be calibrated CVs). (The ES-4 module also requires the ES-4 Controller plug-in that is part of the Silent Way suite, or the ES-4 Mac/MSP External, in order to operate.) Expander modules, with an additional 8 gate outputs, are available for the ES-3 and ES-4 and connect to the host module’s circuit board via ribbon cables.
Not only do the ES-3 and ES-4 provide output levels that are best suited for modular synthesizers, they free up your interface’s audio outputs, which you would have otherwise used for sending DC signals (see Figure 6). Additionally, the ES-5 ($150) expander module is required if you want to use Silent Way to deliver MIDI. The required 3.5mm- to-DIN cable is available from Expert Sleepers.
When you are ready to send audio or CVs from your modular to your DAW, the Expert Sleepers ES-6 ($175) will do the trick. It accepts input through six 3.5mm jacks and interfaces with your computer using an ADAT Lightpipe connection. You can add two more input channels with the ES-7 ($120) expander module, which connects to the ES- 6’s circuit board behind the panel. The ES-6, itself, requires an ES-3 module to provide the required clock signals.
Innerclock Systems provides several break- out box solutions for synchronizing your DAW with your modular and other hardware synths and drum machines. For example, the Sync-Gen IIPro ($750) is a standalone unit that connects to your interface’s audio outputs and provides phase-synchronized MIDI Clock, DIN sync and analog triggers using the company’s VST/AU/ RTAS/AAX-compatible software. The Sync- Gen IILS ($650) offers similar functionality in a Eurorack module. Other products designed for high-resolution synchronization are in the works from this company.