![]() The KA6 could output a range of ☒V, the M4 achieved about ☓V whereas the Babyface Pro could produce the whole ±5V. 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 Eurorack format specifies a control voltage range of ±5V, but many portable and bus-powered interfaces are not capable of such wide voltage swings. However, just because an interface has DC-coupled outputs doesn't necessarily mean it'll be a fully capable source of CV. 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). 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. 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. This is a disaster for CV, as it cannot output the sort of slow-moving or static values we need. Most audio interfaces have what is called AC-coupled outputs, where a capacitor is used to filter out the extreme low frequencies. One is a bit of a fudge, while the other requires that you have the right sort of audio interface. 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. 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. In a system designed to record and play back audio signals, the ability to reproduce signals below 5Hz or so is a mixed blessing. 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. Audio voltages are constantly changing, or otherwise we wouldn't hear the results as audio. The issue here is that although audio and CV share the same method of delivery, their content can be completely different. 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 outputs on computer audio interfaces are, as you'd expect, designed to output audio signals. ![]() 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. 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. ![]() An LFO, for example, generates a cycling control voltage that we use to modulate parameters within a synthesizer. Part of the point of modular synthesis is that there is some crossover between the content of audio and CV signals. The TheoryĪudio signals and CV signals are all variations in electrical potential, and they run along the same sorts of cable. 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 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. Everything you need to know to get your DAW talking to your modular system via your audio interface. ![]()
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