The new CV Instrument plug‑in will take your MIDI data and output it as control voltages and gate signals through your DC‑coupled audio interface.
With the introduction of CV Instrument, you can control your modular rig from Studio One!
One of the most interesting and yet slightly under‑the‑radar new features in Studio One 7 is the CV Instrument plug‑in. It sits very unassumingly in the instrument plug‑ins list, and is easily ignored. For modular fanatics such as myself, who enjoy nothing more than complex interconnected workflows, it is a source of exciting Studio One‑to‑Eurorack action.
However, there seems to be very little information out there on how to use it. As I mentioned in my Studio One 7 review, the manual is factual rather than instructional; hopefully, I’ll be able to help with that. So, couple your DCs, grab some patch cables, plug in some mini‑jack adaptors and let’s get Studio One talking to your modular.
CV Instrument takes a regular MIDI note from your MIDI controller or sequence and converts it into a voltage to send to your VCO.
Nuts & Volts
CV Instrument is a virtual instrument plug‑in that can send note pitch and gate information out of Studio One and into your Eurorack synthesizer in a form it will understand. That form is CV, or Control Voltage. The pitch of a Voltage Controlled Oscillator (VCO) is... controlled by voltage, which means that as you change the voltage, the pitch or frequency of the oscillations changes. As standard, a change of 1V equates to an octave shift in pitch. So, CV Instrument takes a standard MIDI note from your MIDI controller or sequence and converts it into a voltage to send to your VCO. Easy, right? The process is easy enough, but the physicality can be a bit of a struggle.
CV happens along the same pathways as audio signals. You’re not sending MIDI or other digital data between machines, but varying voltage signals down jack cables. CV is the same as analogue audio, except it’s usually moving too slowly for us to hear. To get CV out of our DAW, we will need to use an audio interface — more than likely the same audio interface you’re using for audio. However, many audio interfaces have a high‑pass filter built into the signal path to remove exactly the sort of voltages we are trying to generate. This ‘AC coupling’ can have benefits for audio applications, but makes the outputs useless for CV.
Before you can send CV to your modular from CV Instrument, you need to tell it which analogue outputs on your audio interface to use.
So the first piece of the puzzle is that your audio interface must not be AC‑coupled; it must be ‘DC‑coupled’ so that it can let slow‑moving or stable voltage signals through. We’re only going in one direction, from Studio One to our modular, so our audio interface only needs to have DC‑coupled outputs. Fortunately, this is becoming more common in modern audio interfaces. Most interfaces from PreSonus, MOTU and RME are DC‑coupled, and you should find it mentioned in the specifications of other brands and models. However, not all interfaces are the same, and you may find that some will work better than others. The Komplete Audio 6 from Native Instruments, for instance, is DC‑coupled but can only output a range of 2V, and so won’t give you more than two octaves of notes.
My audio interface is the excellent SSL 12, which has DC‑coupled outputs, but for reasons I can’t yet explain, it refused to work properly with CV Instrument, whereas it works fine with the very similar HW CV Instrument device in Bitwig Studio. [UPDATE: this SSL 12 bug was fixed by PreSonus in the Version 7.1 update, just after this column was written - Ed.] So I swapped the interface for the MOTU M4, and that’s working fine. My point is that if you encounter problems getting CV Instrument to work properly, it’s likely to be a problem with your audio interface.
Connections
Usually, you’d need two DC‑coupled outputs for each instance of CV Instrument, although it depends on what you’re doing. So, sticking to the most likely scenario of running a sequence to a VCO while triggering an envelope patched to a VCA or filter, you need one output to carry CV pitch and one to carry gates or triggers.
It’s vital that you remove these outputs from your mix, as DC voltages are not going to do your speakers any good. Check any monitoring settings in your audio interface’s software mixer if it has one.
In Studio One, you can set up the outputs in the Song Setup page and label them up nicely. However, CV Instrument will do this for you, so you can hold off on this little bit of studio routing management for now.
Once you’ve assigned your CV I/O in CV Instrument, the Song Setup page will update to reflect that those I/O are not to be used for audio.
To make the right connections, I’m taking two nice long Eurorack patch cables, putting small‑to‑big jack adaptors on one end, and plugging them into outputs 3+4 on the back of the MOTU M4. I’m plugging the other end of the cable from output 3 into the 1V/oct input on my VCO to control the pitch. The cable from output 4 is being patched into the trigger or gate input of an envelope module, which is patched to open the VCA that the VCO is going through.
The idea is that when a note is pressed or sequenced in Studio One, it will send pitch to the VCO and trigger the envelope so that we’ll hear our modular play that note.
CV Instrument
With the connections in place, we need to set up CV Instrument to use them. Drag the CV Instrument plug‑in into space in the arrangement window and it will create an instrument track, just like with any other VST instrument. If you check the Song Setup options, you should see that CV Instrument has automatically created a couple of output busses for CV Pitch and CV Gate. It doesn’t allocate them an output until you’ve selected them in the instrument itself.
For pitch to track correctly, you must calibrate CV Instrument using a steady tone (a VCO in your modular is an excellent choice). Connect it to an input on your interface, and select that input in the Audio In menu within CV Instrument.
So, from the drop‑down menu for CV Pitch Out and CV Gate Out, select the appropriate outputs. These choices are reflected in the Song Setup page. On the CV Pitch Out panel you have a choice between 1V/oct and Hz/V. Eurorack and most modern modular synthesizers use 1V/oct. Hz/V is used by some vintage synths, such as the Korg MS‑20.
On the CV Gate Out side you have a couple of choices. Choose Gate, and a high voltage will be sent for as long as you hold a key, or for the length of a sequenced note. Choose Trigger, and CV Instrument will send out a short pulse on every key press or note. You can manually set how long you’d like that pulse to be. V‑Trig is most commonly used by almost everything; it maintains 0V and then puts out a high voltage when activated. S‑Trig is the opposite and was used in vintage Moog Modular systems. You’ll also find it on the Behringer Eurorack Moog Modular clones.
If you’ve made the right connections, you should be able to play your MIDI keyboard and CV Instrument will change the pitch of the VCO and open the VCA. The sound will, of course, be coming out of your modular, not Studio One or your interface. You also don’t need to use the CV Gate output and a VCA; you could have the VCO droning and simply use CV Pitch Out to change the pitch. Modular is nothing if not versatile.
Calibration
You’ve probably noticed that it doesn’t sound very good. The pitch is all over the place and the intervals are nothing like semitones. This is because CV Instrument needs to be calibrated to the range of CV levels available through the audio interface.
CV Instrument needs to be calibrated to the range of CV levels available through the audio interface.
To calibrate, CV Instrument needs a reference pitch to measure voltage against. Handily, we have a VCO sitting right there, generating pitch. So, we need to take the audio output of the VCO and plug it into Studio One via an input on your audio interface. You need to select that input on the left under Audio In on the CV Instrument panel. This is only needed for calibration, so you don’t need to set up a special input or anything. I’d recommend using an input that has a gain control, as you might need to adjust the input level to get the calibration to work. The Audio In has a useful level meter that shows us that we’re getting an input. If you are triggering an envelope, then you’ll need to play some notes to see it. I find it useful to monitor the VCO during calibration and for testing to make sure it’s done it correctly. There’s a little Monitor knob to help you with that.
Hit Calibrate, and you should hear a fairly random range of pitches as it tries to home in on a usable voltage range. After 5‑10 seconds, the Calibrate button will either be bordered in red to indicate failure or get a nice green tick that tells you everything is fabulous and that you can now play your modular from your keyboard.
For the best chance of success, here are my calibration tips:
- Use a sine wave as the source against which CV Instrument is calibrating. If you only have a sawtooth and square, try the square. Failing that, try a square through a filter to get as pure a sound as possible.
- Input level should be just above ‑12dBFS on the Audio In meters.
- Your VCO should be set to around C3. If it has an octave switch, try octave 2.
If the calibration fails, try the following:
- Use different octaves or adjust the tuning up or down. If you’re not hearing the pitch change during calibration then you are not starting at the right pitch.
- Try it without the gate so the VCO is droning.
- Adjust the input level to ensure it’s not distorting.
If it calibrates correctly, then the range available to you will be highlighted on the CV Instrument keyboard. The MOTU M4 can output about 3.7 Volts, so I get three octaves and eight semitones’ worth of usable melodic range. That’s certainly enough for a bass line.
CV Instrument In Use
The new clip launcher is perfect for sending sequences or patterns to your modular. If you right‑click an empty scene for the CV Instrument track, you can select Insert Pattern and the editor will open with an empty pattern ready for your sequence. You can change gate times, and add probability and delay to the notes to craft the grooviest of bass lines.
If percussion is more your thing, then you can use the gate output of CV Instrument to trigger percussion modules. You’ll need a separate CV Instrument with its own physical output for each percussion module you intend to play. That may make generating patterns a bit more challenging than using MIDI, as each kick, snare, and cymbal will be on a different track.
Not just for notes: you can also use CV Instrument to generate modulation signals from your MIDI data.
Modulation possibilities are quite good fun. You can use MIDI notes to send out stepped voltages to control filter cutoffs, wavefolding, or anything you like. To get smoother steps or more LFO‑like behaviour, you can use the Glide function on the right of the CV Instrument. To get Glide to work, the MIDI notes need to overlap so that they become legato. With a pattern of overlapping notes and the right Glide time, you can create quite a convincing LFO.
If you have lots of DC‑coupled outputs, then there’s no telling how much modular you could control from Studio One. Even with the MOTU M4, I have potentially four channels of CV, which could be running a pair of sequences or one sequence and some modulation, or I could use it as an elaborate MIDI‑to‑CV converter.