Hello! I recently got a Behringer 112 dual VCO (inspired by Roland 100M series) and I was surprised to learn that the saw and square waves are unipolar (0-10V), which is unlike all VCOs I’ve ever had in the past.
My question: Will the direct output of this VCO going in to something like Polaris–without any DC offset–become bipolar at the filter’s input? My understanding is most (all?) filters are AC-coupled as they are only designed for audio input, which means they ignore the DC offset in the input signal inherent in a unipolar VCO. Is that right?
I just want to make sure there’s nothing special about the Behringer System 100 filter and that I can use this VCO with any filter, Polaris especially. Thank you!
Follow-up question: What about VCAs? I know many of those are DC-coupled…?
Any AC-coupled input or output will effectively change a unipolar signal to bipolar by filtering out the DC component of the signal. You genenerally don’t need to worry about the signal being compatible. It should work fine. Filters don’t need to be specifically designed to handle unipolar signals.
There’s basically two situations where AC-coupled vs DC-coupled is a concern:
- AC-coupled modules are not good for processing CV because they filter out low frequencies. For example, sending it a slow bipolar square wave through an AC-coupled module may result it a signal that’s 0V most of the time
- You should run the end of your signal chain through an AC-coupled module (or high pass filter with very low cutoff) because a DC component reduces headroom, so you want to remove it from the final mix to improve dynamic range. I’ve also heard a strong DC component can potentially damage low-quality speakers, but I don’t know anyone this has happened to.
With that in mind, you can’t break anything within the modular synth and should feel free to connect any VCO to any VCA or filter. Worst case, if you are in LFO mode your CV might not make it through an AC-coupled module as intended. Sending unipolar audio rate signals to AC-coupled modules have no audible change because we can’t hear the DC they filter out.
Regarding your specific question about Polaris: I just tried it and it seems the input is not AC-coupled, and it depends on the filter circuit whether the output will be AC-coupled or not. I have a Doepfer A-111-3 VCO and its pulse wave oscillator is unipolar like your Behringer. If I run it through Polaris’ normal LP filter with the cutoff all the way up, the output is still unipolar. If I switch to HP and turn the cutoff all the way down, it’s bipolar. If I use the multimode output and cycle through the various LP filters, some output unipolar and some output bipolar. This indicates to me that none of the IO in Polaris is AC-coupled, but some of the individual filter circuits are. However, none of this matters because you can’t hear the DC component. It filters like you’d expect in all cases.
Regarding VCAs: again don’t worry, you can connect any VCO or any module really. In my experience, VCAs tend to be DC-coupled because they can be very useful for maniuplating CV. Usually it’s the end-of-chain mixer modules and output modules that are AC-coupled (and many audio FX modules too).
Thanks a ton for the thoughtful answer! This all makes sense to me. You’re helping me finally crack the code on AC vs DC coupled!