What are AC coupled and DC coupled audio interfaces?

It’s common for people looking into modular synthesis to believe that everything needs to be done with hardware. But there’s still enormous amounts of creative modulating power to be found in software that can be integrated with your modular rack. The most obvious way to do this is with a MIDI-to-CV converter so that you can take the 128 values of MIDI data and convert it into oscillator pitches and control voltages. But control voltage (CV) has an infinitely higher resolution than that and rather than compromise that with MIDI conversion couldn’t we use audio interfaces to route CV signals directly into and out of our computers? Well yes you can but it’s not necessarily as simple as all that.

What do AC coupled and DC coupled mean?

In these two videos our modular adventurer Robin Vincent explores how to get CV signals through DC and AC coupled audio interfaces using the new CV devices in Bitwig Studio and Expert Sleepers Silent Way. There are other pieces of software, like Reaktor, that support the sending and receiving of control voltage signals. These videos will explain what you need to make use of it. But the most important thing to grasp is that it all depends on whether your audio interface is DC or AC coupled.

What does DC and AC coupled mean?

All inputs and outputs on audio devices are designed to deal with audio signals, usually from around 20Hz to 20kHz – stuff we can hear. Many will have filters built in to remove signals outside that range. High pass filters are very common which are there to prevent very low frequency interference. A control voltage can be a completely flat DC voltage, like when it’s a pitch or a constant value, or very low frequency when using it in an LFO. Audio interfaces with these high pass filters would remove these DC voltages and so prevent you from passing CV signals to and from your computer. These audio interfaces are called “AC coupled” because they only pass AC voltages – alternating signals above 20Hz or so. Some audio interfaces do not have these filters and would be called “DC coupled” because they allow low frequency and flat voltages through. These are perfect for routing CV signals into and out of a DAW, but they are not as common. Chances are your audio interface is AC Coupled. But don’t despair, there are ways around it through a clever use of amplitude modulation.

Whichever way your audio interface is wired Robin will show you how to integrate your modular with your computer to greatly expand your modulating and sequencing possibilities. It’s not as complicated as it sounds and the potential in Bitwig’s new CV and modulation devices is quite awesome.

What is DC coupling?

What is AC coupling?

 

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Robin Vincent spends a lot of time messing about with music on a Microsoft Surface and he writes all about it on his website www.SurfaceProAudio.com

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