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Learn about the basics of subtractive synthesis with the help of VCV Rack.


In this video, we're going to learn how a subtractive synth actually works. 

We're going to be working with VCV Rack and go through the different modules that are needed in a subtractive synth. We're also going to be using a spectrum analyzer and get a grasp over the fundamentals that are required before we actually start to program our microcontroller. 

Let's get to it. Let's start by looking at the instruments we have here and see what functionalities it actually has. You might have recognized already, this is an oscillator. Control the frequency of it. There's also a filter that's controlled by a low frequency oscillator. When the oscillator opens, we can actually see the filter being opened. And it can change the LFO speed. We can also control the volume here. And we can control basically turning it on and off. And this is all done in this little piece of code here. We're going to go through this and explain every each of these steps to understand what is actually happening. How are we mapping our knobs, how are we running the synth engine and so on and so forth. 

Before we get started, I want you to go to and install VCV Rack. You simply click on this large image here and then you scroll all the way down or 'Get Rack 2' and you get your license here. This is free. You don't need to download the pro version, unless you want to support them or you want to use it inside of your door. But other than that, for what we do here, this is just for experimentation, and it's just easier to explain some of the fundamentals of synthesis with this lovely, lovely program. 

So VCV Rack, when you open it, you're going to see this patch. And if you press here, you should be able to hear some sound. If you're not hearing sound, you might need to change here in the 'AUDIO' there's like an audio module. This is the output. You might want to change this to your headphones or speakers. I have many of them here because I'm recording audio and video in a bit of a complex setup because of this video recording. But for you, you're probably just going to have a few based on your sound card. 

Okay, so let's delete everything here. I'm just going to leave this one. And when I right click anywhere here, I am able to add modules and I have lots of modules. You might have less, but that again doesn't really matter. Maybe to give you an indication you go to VCV Rack, you can go to the library, and in the library you can download a lot of modules. Some of them you need to pay for. Some of them are completely free. Most of them are actually completely free, which is quite amazing. 

And what we're going to be doing is looking for an oscillator. Let's pick the one that you also have. VCO, this guy. And let's also take a filter, so just typing 'filter' and this is a VCF - voltage controlled filter. And before we use the filter, let's just use the oscillator and see what happens when we connect our sine wave from here. So this is a sine output. And we put the sine output into here, and we can control the volume and just hear this oscillator play. Now, if we would take a sawtooth, let's maybe lower the volume first and connect a sawtooth to the output. We can hear now a sawtooth. There's a difference in timbre in these. And I want to show you what this means. So when I right click and I search for a spectrum analyzer, I get this module and you do not have this. So you can quickly go to the library here and search for an analyzer and then you'll find it. This is from Bogaudio. You can download it by pressing basically '+ Add'. I have it so I can only remove it, but you can download it by pressing '+ Add' and then simply go to library and update - 'Update all'. Back in our patch, we're going to connect our sine wave into here. And then we're going to also connect our output. And we can see, that this is our sine signal and it has some additional frequencies here. There is actually one frequency that's very dominant, and then there are a few additional harmonics, and we can change the frequency here and we can see how the frequency changes. And we can level up and we can really hear. How the frequency is changing. 

But what's interesting to see is that if we use a different waveform, we get more of these harmonics. And these harmonics basically change the sound, the change the timbre of the sound. Now what a filter does is simply cut off some of these frequencies, so then we have control over our sound. Let's see - so I have a sawtooth and I put it into the input here. Let's put the sawtooth into here as a raw sawtooth, but we also put it into the filter, and then the LPF - so low pass filter. Let's put here and see what happens now. All of this harmonic information that we have in the green signal, which is the raw signal coming from the VCO, is being controlled by the filter knob. Let's have a listen. So I'm going to connect my output. Let's, first of all, lower the volume. Connect the output into our low pass filter out. And we can hear that it's actually controlling the volume, but it's not controlling the volume of the whole thing. It's very different than controlling the volume here. This is just controlling all the frequencies at the same time, lowering all of them equally while. In here. We hear just a few frequencies. Basically the fundamental frequency here, which is this. And the harmonics are being cut off. If we take our sine wave and put it here. Sine wave is orange. And let's take the saw and so we just have the sine now and we can see this is our fundamental frequency and we have some additional harmonics here. But when we have a saw we get more harmonics. And then if we take this saw and we put it into a filter, we can control the amount of harmonics. 

Okay, so this is one principle of synthesis that we wanted to talk about. Now, let's talk about modulation. Modulation is basically when you take one signal and you send it to another signal to modulate it. So if, for example, I have here my filter and I want to have this happen automatically, the way to do this is to modulate this knob. 

So let's let's see how we do this. I can go and take so right click and go to search for an LFO. So a low frequency oscillator L-F-O. And we can take this one. You also have this one even if you don't download it, it comes with the VCV Rack install. And I have here a sine wave and we can see that if I take this sine wave and I put it into the cutoff frequency which is here, you can see 'CUTOFF'. I get now this light that's coming from here to here indicating that there is a frequency that's animating in this speed in this shape and it's going into the filter. But there is a valve here and it's closed. You can see that it's actually saying 'Cutoff frequency CV: 0%' and we can add in and see what happens. Every time that this light opens up, the filter is opening. This knob is opening because this signal is sent to it. And with this valve, we can control how much of this signal is going to be sent. Say, double click on this. I can turn it off. And I can also invert this. I could go to the other side, which won't do anything now but if I invert this we would be able to get the other signal. 

Now why inverting is interesting because if I take, for example, a sawtooth here. And I open this up. This is the shape of a sawtooth. And if I want to invert that, to get this punch, I could simply bring this up. Take this and invert it. All of a sudden I get something that sounds like a plunking of of a string, for example. Cool. I can also add resonance here. And I can change the speed of my oscillator. You could really see how the frequency is being cut. Really cool. Now, just like we have a low pass filter, we also have a high pass filter. So then. We can play with that for hours and make lots of sound. And let's turn this off. And turn this one down. 

And the point here is that we can start modulating different controls. So here I have the cut off. I also have the resonance, I have the drive. I can modulate any of these. And with these little valves I can decide how much of the signal is being sent to control the signal. 

Okay, now that we understand the fundamentals of subtractive synthesis and subtractive basically means that we are subtracting some of these harmonics from the signal. So, now that we understand this concept, we can continue to our program and see how we do it in programing inside of our DaisyDuino environment. Let's do this in the next video.