MIDI v CV

First – why do it at all? We’ve got MIDI going in and out of our computer without any trouble and most synths have MIDI and you can get MIDI-to-CV conversion in your Eurorack – so why not use MIDI? Well, you can, of course – do that, that’s super. However, MIDI and CV control can be quite different and each have their pros and cons. MIDI can be very precise, within it’s quite low resolution – hit a note and it will always be that note. CV can be all over the place, but it can also be all sorts of things, infinitely variable and flexible. MIDI is dictatorship – do this, play this, whereas CV is more of a conversation – it interacts with different bits of circuitry and produces all sorts of results – some expected, some not quite so much. Many simple tasks of playing notes and sequencing can perhaps be simpler with MIDI but once you get into manipulating control voltages you’ll find it a lot more creative in ways you weren’t expecting.

MIDI that’s being converted into CV is never going to give you the flexibility of working with CV at source. This type of analogue control is flowing through your Eurorack and so why run that through some kind of conversion – why not keep it analogue into and out of the computer.

So assuming you are convinced that connecting your computer to your rack via CV is a good thing to do – how do you do that?

Well your computer already has analogue signals coming in and out via the audio interface. Could they not be repurposed for control voltage rather than analogue audio? Well yes, sort of. This is where we get into this messy business of DC and AC coupling. Sounds sexy but sadly it’s not.

AC – alternating current – analogue audio is an alternating signal, a wave, a combination of sine waves that wiggle about and create the vibrations of sound waves that drive speakers, rattle our ear drums and enable us to hear.

DC – direct current – control voltages can be completely static, like holding a single note, or a series of values in a sequence. A trigger or a pulse is just moving from zero to another value and back again. There’s no vibration. A control voltage would push your speaker cone out and hold it there. It can of course be a wave using low frequency oscillators and there becomes a point at which those oscillations can reach the hearing range and it starts to become an AC analogue signal. But we’re not trying to capture sound, we want to capture and generate CV.

So what’s the problem?

The problem is that most audio interfaces are designed to filter out DC signals – we call them AC Coupled.  When you design a piece of audio gear you tend to design it so that only the audio you want to hear is present – anything else is noise and distortion. And so to minimise that DC signals are filtered out. As CV signals are rarely moving more than a few hertz (cycles per second) then a high pass filter with a low threshold should do it. And so DC is removed along with any chance of the signal containing useful, static control voltage signals.

But if you can’t hear these static or low frequency DC signals then why bother to filter them out? Well they could have an impact on the audio signal. If you have a DC signal flowing through your audio interface input at a set voltage it’s going to combine with an AC audio signal and artificially boost it. This could result in distortion.

However, there are some audio interfaces that have DC Coupled inputs and/or outputs. Some that are designed for CV and some, like all the MOTU ones, just forgot to put in a DC filter and by happy accident they are great for CV.

So the solution then is to get an audio interface that’s DC Coupled – yes! But which ones are? Unfortunately not very many. MOTU interfaces are good, accidentally, RME tend to have DC coupled headphone outputs. Avid HD interfaces are ready to go and so is the new Presonus high end interface. So manufacturers are starting to get the sense that this is something that people want but it’s very slow moving. Check with your manufacturer to see what the story is. There’s a quite dated list on the Expert Sleepers website. Expert Sleepers are all over this DAW-to-CV thing and in fact their ES-8 USB CV Eurorack module is one of the best purpose built solutions around. It’s a proper USB audio interface with ASIO drivers and everything and gives you 8 audio or CV streams in and out of your computer – awesome! What they don’t have is all the other things you want in an audio interface like mic preamps, headphone sockets and such like, so it’s not an ideal solution for what i’m doing.

They have another option with the ES-3 – this takes an ADAT output and converts that into 8 DC coupled streams of CV into your modular. So if you have an audio interface, with all the mic and headphone parts AND a spare ADAT port then this is more complete solution in my view – and it’s cheaper than the ES-8. So you can use the analogue parts of your audio interface for audio and the ADAT part for CV – fabulous!

Now, I found a much easier and altogether more groovy erm… partial solution to this. Partial because it doesn’t have the regular audio interface features but it does give me a couple of DC Coupled inputs and outputs over USB and is also a pretty darn cool Eurorack module all by itself – and that’s the Roland Aria range of modules. You can pick one up for around £200 and they give you all their features with weird internal modular patching plus it can act as a DC coupled audio interface to your computer – awesome! And more importantly it’s going to give me a way of trying this stuff out before I get an interface with ADAT that i can use more comprehensively.

Right – now you’ve got your DC Coupled interface what now? Well next you need your software to be able to generate and/or interpret control voltage. There’s a few things that do this – some software like Reaktor can do this, so can CV Toolkit and there’s a plug-in from Expert Sleepers called Silent Way – all of these will output CV. However, in this video i’m going to be using the new Hardware CV plug-ins in Bitwig as a continuation of my review of Bitwig 2 – check that out if you haven’t seen it.

The rest is in the video….