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Expert Sleepers Silent Way

(CV) Control Voltage-Generating Plug-ins [Mac/PC]
By Paul Ward

If you dream of controlling your analogue synths from within your DAW software, it may be that your audio interface will let you do just that — with a little help from this plug‑in suite.

Left: Silent Way Voice Controller is the heart of the package, and is capable of outputting up to six control signals.Left: Silent Way Voice Controller is the heart of the package, and is capable of outputting up to six control signals.

MIDI‑to‑CV interfaces aren't exactly thick on the ground these days. Those of us who were around at the time of the great MIDI revolution might have a device at our disposal, but for those buying into vintage analogue technology, or those keen on purchasing or building their own analogue modular system, software (initially MOTU's Volta, and now a system from Expert Sleepers) may offer an attractive and, arguably, more flexible alternative.

It is a tribute to Bob Moog's creative genius that his innovative use of 'voltage control' is so ingrained in our understanding of analogue synthesis. The concept is disarmingly simple; generate a voltage in one 'piece' of electronics, then apply that voltage to another. A fundamental example would be the voltage output from a musical keyboard which, when fed into an oscillator's pitch control input, is used to create musical notes. Feed the same keyboard output voltage into the control input of a filter's cutoff frequency, and the filter can be made to open wider as keys are pressed further up the keyboard. Envelope generators, LFOs and analogue sequencers all operate on the same principle: create a voltage and apply that voltage as a controller somewhere else.

In the early days of musician‑friendly synthesizers, the ability to pass control voltages between synths to allow them to work together was not lost on savvy users. Sometimes this could work well, particularly if the synths in question came from the same manufacturer, but in other cases the results were not quite as expected, especially as manufacturers used different standards.

With the arrival of MIDI, manufacturers finally agreed on one standard for instrument interconnection, but in the new digital world it seemed our beloved voltage‑controlled synths were out in the cold. Thankfully it wasn't long before MIDI‑to‑CV interfaces began to appear, and pre‑MIDI synths were able to join the new age of MIDI‑driven music production.

Despite enormous strides in the authenticity of emulations, the attraction of a genuine analogue device endures. Today, this is reflected in a second‑hand market where prices continue to amaze and dismay in equal measure, and in a burgeoning underground industry of new modular analogue synth designs. Clearly, voltage‑controlled synthesis is not going away anytime soon, so the question arises as to how best to incorporate these instruments into today's DAW‑dominated world, where even the MIDI port seems to be slipping from 'must have' to 'may have'.

The Silent Solution

Right: Voice Trigger reproduces the gate and envelope features from Voice Controller in a smaller and neater package.Right: Voice Trigger reproduces the gate and envelope features from Voice Controller in a smaller and neater package.

Expert Sleepers' Silent Way is one answer. It consists of a suite of DAW plug‑ins designed to generate control voltages from an audio interface, basically turning its audio outputs into CV and Gate generators. The potential for such technology goes well beyond connecting up a typical monosynth of yesteryear, and Expert Sleepers see their product as providing the beating heart of a powerful modular synthesis system. Silent Way is, in theory, capable of driving anything from a humble Roland SH101, Sequential Pro One or Korg MonoPoly to a giant Moog System 55 or home‑brewed modular dinosaur.

At the time of writing, the plug‑ins in the Silent Way suite consist of Silent Way DC, Silent Way LFO, Silent Way Trigger, Silent Way CV Input and Silent Way Voice Controller, with more modules on the way. They will run as VST or Audio Units plug‑ins, requiring Mac OS 10.4.11 or higher, or Windows XP SP2 or later.

The major limitation facing any software‑based CV generator, including Silent Way, is the ability of your audio interface to maintain a stable DC voltage at its outputs. Alas, most of them can't, including my own RME Fireface 800. Expert Sleepers are maintaining a database of compatible interfaces, along with user reports on the results achieved with them, so take a look at the web site to see if your interface and synth are amongst them. At present, the only range of interfaces that offers the necessary DC‑coupled outputs are those made by Mark of the Unicorn, nearly all of whose products suit. The SOS offices managed to drum up a MOTU 828 interface for me to use during the course of the review, and I can report that this interface worked like a charm in this context.

If there's a catch to this technology, you just read it. Just before we went to press, however, Expert Sleepers announced a solution that should work with most audio interfaces that have balanced outputs. It requires an additional plug-in called Silent Way AC Encoder and specially adapted cables, and we'll try it out as soon as we can!

The Voice Of Truth

Silent Way DC simply outputs a continuous DC voltage, but one that can be automated, allowing you to create slow filter sweeps and the like if your synth has the appropriate CV inputs.Silent Way DC simply outputs a continuous DC voltage, but one that can be automated, allowing you to create slow filter sweeps and the like if your synth has the appropriate CV inputs.

Installation on my PC was as simple as it gets, involving simply copying the Silent Way DLL files into my plug‑ins folder. In demo mode, the plug‑ins are fully functional, but will time out after 15 minutes. A single registration permits installation on three computers. I experienced no crashes or any other unpleasant activity during the review period.

To get things started, I chose first to get the SIlent Way Voice Controller plug‑in up and running. Setting it up can be a little daunting, but the web site does a good job of guiding you through, with tutorials for a number of popular DAWs. I was using Cubase 5, and I found the textual description enough to prompt me along the way, though video guides are also available. Once you have grasped the basic concept, the process is simple enough, but it helps to have a good understanding of your DAW's routing capabilities and what you are trying to achieve. I can imagine a few people brought up on the instant gratification of VST Instruments struggling at first.

Voice Controller is capable of generating six control voltages, including a gate signal, pitch and envelope voltages. I linked the outputs of the MOTU 828 into my Minimoog's gate and pitch CV inputs, and after the brief calibration process I was up and running. Calibrating Voice Controller consists of feeding the connected synth's audio output back into the plug‑in, so that Voice Controller can see how its voltage outputs relate to the synth's pitch. This is achieved by simply clicking a calibration button on the plug‑in, whereupon a rising series of voltages is output from the plug‑in and a mapping is automatically created, aligning voltages to MIDI note values. A small graphic gives an overview of the mapping achieved; I found myself wanting to edit the mapping curve 'just to see', but this isn't possible at present, though Expert Sleepers have it on the 'soon to be released' list.

Voice Controller will cope with both Volt‑per‑octave and Hz/Volt synths. My own dear Moog Taurus pedals are (strangely, for Moog) of the latter variety, so it was pleasing to have this flexibility. MIDI‑to‑Hz/V converters were never as easy to find as their Volt‑per‑octave cousins; I have sold on some perfectly good synths partly because their Hz/V system made them hard to integrate with the rest of my system.

The range of available output voltage is dependent on your audio interface. If the interface features balanced outputs (as do most), you'll need a special cable to tap the voltage from two of the three wires, and it is also possible to tap a second, inverted copy of the CV. I used a few simple stereo‑jack‑to‑twin‑mono‑jack adaptors.

In the case of my combination of MOTU 828 and Minimoog, I managed to achieve a consistently accurate four‑and‑a‑half‑octave span of 54 semitones from F#‑1 to B3, which I felt to be a reasonable working range. I noticed that the mapping began wrapping back on itself at higher pitches, producing correct notes, but in the lower octave. Once calibrated, the triggering and pitch response was reliable and consistent. I habitually like my monosynth keyboard to respond with highest-note priority, but there are times when lowest and newest are more appropriate. Happily, Voice Controller optionally allows any of these, and also has a retrigger on/off switch — something I wish my Minimoog had had from day one!

Further Adventures

Wish your synth had more modulation sources? Silent Way LFO will add them.Wish your synth had more modulation sources? Silent Way LFO will add them.

Once I had the basics working, I decided to bring more CVs into play. Voice Controller contains an amazingly flexible voltage mixing matrix, allowing combinations of pitch, velocity and the three envelopes to be mixed in varying amounts to each of the six CV outputs. These CVs can also be offset by a fixed amount. To prevent audible clicks when faced with rapid CV changes, variable 'smoothing' can be applied to the CV outputs, creating either a slew-rate limit (reducing the speed of voltage change), or applying a low‑pass filter to the voltage output.

The voltages generated for both gate and trigger (a transient pulse related to note‑on) are variable. I abused the trigger voltage to add a slight filter spike to the attack of my Sequential Pro One, and it worked well for some uber‑aggressive bass sequencing.

I had great fun plugging Voice Controller's envelope CV into my Mini's VCA and VCF. I found it useful to modulate the envelope voltages with velocity information. The three envelope generators can be tweaked as simple ADSR devices, or you can go into more detail for each envelope, with a range of multi‑stage controls for both time and shape. Although my old MIDI-CV box does allow me to pass a velocity offset voltage, it certainly doesn't allow me to create and modulate envelope voltages, so this was a truly rewarding experience! Inspiring stuff indeed.

The level of control in Voice Controller is impressive. For the implementation of both fixed‑time and fixed‑rate portamento alone, Expert Sleepers have made it into my Christmas card list! It dismays me that so many synth manufacturers seem content to supply their instruments with only one or the other, when it can make an enormous difference to the way a synth part is played.

But what about polyphony? Well, Voice Controller has that covered too. By instantiating a number of Voice Controller plug‑ins, assigning one as 'master' and the others to individual voice numbers, I was able to use my six‑voice Oberheim Xpander via its CV inputs. In a fully fledged, multi‑oscillator modular system, this will be a very powerful feature.

Trigger Happy

Silent Way is not only a MIDI‑to‑CV converter: the CV Input plug‑in can record control voltages output from your synths. Silent Way is not only a MIDI‑to‑CV converter: the CV Input plug‑in can record control voltages output from your synths.

Eventually, I had to tear myself away from Voice Controller and try out some of the other plug‑ins. Silent Way Trigger provides trigger, gate, envelope and velocity voltages from a MIDI trigger source. It has a similar range of controls to the corresponding features in Voice Controller, with all of its offset, envelope, smoothing and the ability to set minimum and maximum CVs. Simple and functional.

Silent Way DC is, perhaps, the simplest of the Silent Way plug‑ins. Its job is to produce constant control voltage outputs, and it is actually a two‑channel plug‑in, each channel of which has independent modifiers. It might not seem so exciting until you remember that all of these plug‑ins can be automated from your host DAW, whereupon you'll hopefully begin to see the potential. It wasn't long before I had some hardware controllers assigned to DC's knobs (all of the controller numbers are detailed in the manual) and had recorded some inspired filter and sync'ed oscillator sweeps on a few tracks of my DAW. Offsets and multipliers can be applied to the output voltage.

Silent Way LFO is also a two‑channel plug‑in, enabling sophisticated free‑running or tempo‑sync'ed cyclic control‑voltage waveforms to be created. This is a notch or two above your typical on‑board synth LFO, allowing, as it does, a number of LFO waveforms to be freely mixed: sine, triangle, square (with pulse‑width control), sawtooth, random and noise. One particularly interesting feature of LFO is its ability to add an asymmetric offset to the waveform. This and the other settings are reflected in the waveform display, making everything easier to understand and adjust. There is even a 'quadrature' option, which can lock the two LFOs together with a variable phase offset. LFO is flexible enough that it can be used to create clock pulses for an old analogue drum machine, and there is even a variable 'swing' control, which many old machines lack.

Silent Way CV Input is perhaps the most surprising weapon in Silent Way's armoury. While the rest of the plug‑in suite concentrates on getting CVs out of your computer, CV Input enables you bring CVs from your analogue synths into your computer. It does this by making use of an external VCA to track a controlling (uni‑polar/bi‑polar) voltage, through which is passed a signal from a 'Generator' instance of CV Input, which is subsequently picked up by a 'Decoder' instance of CV Input. This is reminiscent of Waves' EQ cloning plug‑in. Although it may sound slightly confusing, once you understand the principles involved the application is easy enough. Since the plug‑in is stereo‑capable, the sending and receiving of a mono tracked signal may be set up with a single instance of CV Input, which I feel helps to simplify the process. I was able to create a VCA loopback with my Sequential Pro One and use the resulting CV tracking to control my Minimoog's filter. Clever stuff — and, importantly, recordable in your DAW.

Unlimited Control

My time with Silent Way was quite a liberating experience. The fact that someone took the leap of logic to turn audio interfaces into MIDI‑to‑CV converters is innovative in itself, but Expert Sleepers have taken the original concept up a whole series of notches with a range of genuinely useful plug‑ins. These go far beyond simple monosynth control and into the dizzy realms of powerful modular synth systems, where I can imagine a very sophisticated control system with Silent Way at its heart. The only limiting factor is the number of outputs you can muster from your interfaces.

Voice Controller is the star of the show for me. Slick and self‑contained, this has the effect of pulling a much‑loved analogue synth into the heart of your DAW, with a level of control that is usually the reserve of a virtual instrument.

When I was asked to look at Silent Way I wondered if it was merely some technical novelty. Within minutes of beginning to explore the possibilities I realised that I was very much mistaken. This is genuinely useful, musical technology for anyone with even the most humble of CV‑equipped synths — and, perhaps, an answer to the prayers of those with mighty modular systems. The catch, if there is one, is that only a few audio interfaces can be pressed into service — and this may soon be a non-issue, if the forthcoming AC Encoder really works.  


The only serious alternative is MOTU's Volta, which we hope to review in SOS soon. It looks slicker, but is arguably less sophisticated, and lacks Silent Way's CV‑input ability. Volta is also available only on the Mac platform.


Latency becomes an issue with Silent Way technology, since the control voltages it produces are at mercy of the audio interface's drivers. This needs to be pointed out in case you want to work in real time, but have an audio interface that does not currently give you the latency you demand. If the latency you experience using a VST Instrument is not good for you, Silent Way will not improve matters. The good news is that any CV latency will be identical to that for your audio, so at least they will be in sync to sample accuracy.

Open Platform

Silent Way is compatible with the Open Sound Control (OSC) protocol, meaning that it can communicate with other OSC‑friendly applications, though these remain thin on the ground as yet. For the true nuts‑and‑bolts enthusiast, meanwhile, Silent Way's abilities can be expanded by user‑writable scripts utilising the Lua scripting language. Expert Sleepers provide more information and examples for you on their web site.


  • Brings genuine analogue synth control right into the heart of the modern DAW.
  • Sophisticated generation of CVs, with plenty of modulation options.
  • Sample‑accurate timing.
  • Capable of polyphonic use.


  • Currently, only a few audio interfaces are compatible.
  • Available voltage range may not be great enough for some users.
  • Susceptible to the same latency as a virtual instrument.
  • Uses up audio outputs that may be needed for other duties.


Silent Way is a genuine alternative to a MIDI‑to‑CV interface, and capable of a greater level of sophistication. Some of the power in Silent Way may be superfluous for those with simple monosynth requirements, who may find all they need in the Voice Controller plug‑in alone, but for modular die‑hards it could be the answer to a prayer.

Test Spec

  • Silent Way v1.4.3.
  • Laptop PC with 1.8GHz Intel Pentium‑M CPU and 1GB RAM, running Windows XP SP3, and MOTU 828 MkI interface.
Published May 2010