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.
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.
There are two ways of dealing with this. One is a bit of a fudge, while the other requires that you have the...