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AC vs DC Coupling: What, Where, Why

This is part of a series of tutorials based on user questions that we seem to get a lot. Are you new to modular and have a question about Eurorack? Are you an advanced user and have burning technical questions?  Please submit ideas for this occasional column here.

 

We recently had a user ask CV mixers and if/why some mixers are useful for gates and CVs or sometimes just audio. I realized I’d answered this question a few times over the years, and I realized that perhaps it was makings of yet another user-question post. It turns out it all boils down to a concept called coupling (not that kind of coupling). Circuits are either AC or DC coupled, and which one your module is will define the things it can (or can’t) couple with .  So...

AC/DC Coupling: What are we talking about?

When you patch two modules together, they are being “coupled”. The electricity (current) flows from module to another across a patch cable and into the jack. The signal coming in can either be alternating current (AC) or direct current (DC).

Left: Alternating current (AC): the voltage fluctuates over time. Right: Direct current (DC). The voltage is constant. Notice that the AC has an average voltage of 0.5: In this case, both signals carry a DC of 0.5, so the main difference here is that AC is not constant.

Left: Alternating current (AC): the voltage fluctuates over time. Right: Direct current (DC). The voltage is constant. Notice that the AC has an average voltage of 0.5: In this case, both signals carry a DC of 0.5, so the main difference here is that AC is not constant.

Different types of voltage are handled differently at the input. DC-coupled inputs are the more straightforward: the circuitry just takes the input as it is.  The entire signal passes through, more or less untouched.

A simple DC-coupled circuit. Nothing is filtered out in this circuit.

A simple DC-coupled circuit. Nothing is filtered out in this circuit.

 

AC-coupled inputs are different.  They include a capacitor in the circuit that allow them to filter out very low frequencies.

An AC-coupled circuit schematic. Notice the capacitor (C2). This is the heart of this more complicated circuit design. It allows the circuit to filter out low frequencies.

An AC-coupled circuit schematic. Notice the capacitor (C2). This is the heart of this more complicated circuit design. It allows the circuit to filter out low frequencies.

In practice, removes and DC offset, normalizing the signal mean to zero.  Our AC input from above becomes

This is the same wave as above but now is centered on zero because the DC offset has been filtered out.

This is the same wave as above but now is centered on zero because the DC offset has been filtered out.

In essence, what this means is that a DC-coupled circuit allows everything to pass through it while an AC-coupled circuits are usually designed so that they filter out low---usually subsonic (inaudible)---frequencies.

Ok then, what happens to the DC if you feed it into an AC-coupled circuit?

Look back at the figure of direct current.  The voltage doesn’t change over time; this is one of the defining characteristics of DC. If the voltage isn’t changing, the frequency is effectively zero. So it doesn’t really matter how you define “low frequency”: a frequency of zero is low. Pass DC through an AC-coupled circuit and it will be removed.  Pass AC through an AC-coupled circuit and the DC offset will be removed.

So why do we care? We're modular users!

The properties of these circuits define how modules work and interact. If your mixer is DC coupled, then any signal it receives can be processed by it.  

In modular, lots of things are DC.  These are things that are either static or change so slowly that for our purposes, we just call them static. For example, CVs and gates change over very different time periods than audio does; they are typically considered DC. Audio is generally AC. A somewhat side point: by definition, LFO modules have to be DC coupled for the same reason: the output circuits are built the same way so that a signal can be sent out of the module. If an LFO module were built as an AC-coupled circuit, it would run the risk of filtering out its own output!

 

An AC-coupled mixer will work with audio, but if you are looking for something to mix CVs or LFOs, it’s not the module you’re looking for.  An AC-coupled module, by definition, cannot correctly process DC signals. Look for a DC-coupled mixer for those purposes...but note that some DC-coupled mixers still may not be designed to mix CVs, so YMMV.  Do your due diligence before you purchase!

Bonus: highpass filters

If the mechanics of what I’ve been describing sound familiar, it’s with good reason. AC-coupled circuits are built with a simple highpass filter. The details of how they are executed vary, but the vast majority of AC-coupling circuits are just simple first-order high pass filters.

 

Stay tuned to the Noise Engineering blog for more answers to common modular questions and tips and tricks for modular users. And if you have questions you'd like to see answered here, please drop us a line.

Useful links

http://www.ni.com/white-paper/14753/en/

http://blog.prosig.com/2013/01/15/what-are-ac-and-dc-coupling/

http://digital.ni.com/public.nsf/allkb/0F81C33977056A7D862566F1005CAA71

https://community.plm.automation.siemens.com/t5/Testing-Knowledge-Base/AC-and-DC-Coupling-What-s-the-Difference/ta-p/355024

 

 

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