Become comfortable with the concept that voltage levels (“CVs”) are the external “parameter settings” for a module, and that gates are used to signal note ons/offs.
- A modular synthesizer is by definition built out of individual modules. One module may be a tone generator, an oscillator. A different one may be a tone modifier, a filter. Another may be a loudness shaper, an amplifier. And others may be modulation sources, such as low frequency oscillators and envelope generators. But if each module is independent, how do they talk to each other? The answer is these, patch cables, and these cables carry voltage. That voltage, in turn, tells another module downstream what to do.
How to change a parameter, or maybe sends it some audio that it's supposed to process. For example, this simple module is a control voltage processor. Among other things, you can put out in negative or positive voltage on demand. Let's connect to one of its outputs and look at that on a multimeter. I connect the negative and the positive, and as I turn this offset control, you can see the voltage changes. In this case from almost +5 volts to almost -5 volts.
Okay, what can we do with that? Well, we can use it to control the pitch of a voltage controlled oscillator. I'm gonna grab another cable, take a copy of that same output and put it to the control voltage input on this oscillator. Turn up its volume, (steady vibrant tone) Then turn this knob, (pitch slides up) You'll hear the pitch of the oscillator change in response. (pitch slides down) I can even go negative to tune it so low you can't even hear it anymore.
(low tone slowly slides up in pitch) And it so happens that the devices we can connect to control our modular, such as this keyboard behind me, also send out voltages. So let's connect that to our voltage control oscillator. This black line carries the control voltage out from this keyboard. I'm gonna connect it to a buffered multiple. It's a module that makes exact copies of the voltage being sent into it. Now I've got one copy of it on our multimeter, and...
send another copy of it to our one volt per octave input on our oscillator. (Higher steady tone) Now as I play middle C, I get this pitch. If I play one octave up, (pitch jumps up one octave) you'll see the voltage jump from one to two volts, and the oscillator is playing a note one octave higher. I play one octave lower, I go down to zero volts, and I go to a low pitch. (tone slides up by half steps) As I play each semi tone in between, the voltage increases one twelfth of a volt, and the pitch goes up one semitone.
Voltage can be used to control other modules as well, such as our VCA, a voltage control amplifier. So let's take the output of our voltage control amp, connect our oscillator output to one of its inputs, I've turned up its input level, turned up one of its modulation input voltages, and let's go look at the output of our control voltage processor again. This time I'll send it off to an input on our voltage controlled amp.
Now as I turn up the offset, the sound becomes louder. (steady tone increases volume) As I turn down the offset,... the sound disappears. The loudness of the amplifier is directly proportional to the voltage I'm sending it. That's the principle of voltage control. Now there's another type of voltage that also gets passed around inside a modular synth. That's gates and triggers. It's a voltage that stays at zero when nothing's happening, when you're not playing a note, then jumps up to high level, like five or 10 volts when you are playing a note.
Let's take a look at that. This red connector is the gate output for my keyboard. So I'm gonna plug it into another section of my buffered molt. Look at that on my digital multimeter, and now when I play a key, you'll see that it jumps up to five volts. And as soon as I release that key, it disappears. Well, we can take that voltage, send it to our amplifier, and as soon as I play a note, you can hear a sound come out of the amplifier that originated at the oscillator.
(high-pitched steady tone) Now these gates are not normally used just to control the loudness of an amplifier. They're usually used to trigger another device, such as an envelope generator, which produces a varying voltage during the course of a note. So let me pull this cable out, send this gate down to the external input on this envelope generator to trigger it, get this out of our way, then take an output of that envelope generator and plug it into the modulation input, the voltage controlled input on our VCA.
I'm going to set our envelope to a slow attack, slow decay, a medium level sustain, and a very slow release. And just for fun, let's watch that voltage during the course of a note. When I play this note, listen to how the loudness changes, and also watch how the voltage changes. (high-pitched tone increases in volume) (high-pitched tone decreases in volume) The note got louder and then softer. It's being held at this intermediate level, and when I release this key, it fades to silence, as does the voltage that's coming out of the envelope generator.
Let's slow this down even more so you can hear the difference. I'll change the level of the sustain, play a note, (high-pitched tone decreases in volume) Release the note, and it fades away to silence. So that's the secret of how modules talk to each other. They use voltages passed through patch cables. Some may control pitch, some may control loudness, how much a filter is being opened up, et cetera. Some may be triggers to say, hey, a note has started playing, do something in response.
But nonetheless, voltage is the language that these machines talk. Now, a module may be digital or analog. It doesn't matter. All of these jacks are analog control voltage inputs and outputs so that they speak the same language no matter what's going on inside of them. I know that all these cables can become confusing to understand. That's why I've made a point of color coding my cables for all the patches that I use in this course. White cables equal a special one volt per octave signal that's used for pitch, blue cables are for other modulation sources that may control loudness, filter cutoff, et cetera, yellow cables carry the audio signals from one module to another, red cables are our gates and triggers, and green cables are for clock signals, tempo information that runs between modules.
Finally I use black cables to connect to the outside board so you won't be distracted by them. For example, my demo system here, these two black cables happen to go off to my soloscope, this black cable is an audio in for my audio interface, this one's a headphone connection, and this one goes to my instrument tuner. You'll notice I am using a buffered multiple to copy my main audio signal to my output module and also to my tuner and to my soloscope. So if you understand how sound is made out of harmonics and how voltage is used to control modules inside of synth, now you know the basics of how these machines work.
This course will be based around the widely available Roland SYSTEM-500 Eurorack-standard synthesizer modules, complemented with modules from several other manufacturers. As most modules, synthesizer keyboards, and plugins are designed around the same principles, this course lets you dig deeper into synthesis and programming your sounds regardless of the equipment you own.
- Putting a system together
- Getting sound in and out
- Patching a typical synthesizer "voice"
- Tuning voltage-controlled oscillators (VCOs)
- Working with waveforms
- Using filters
- Exploring frequency modulation, waveshaping, oscillator sync, effects, and more