Analog synthesizer basics

This article is a draft, and under active development. If you see a paragraph that just ends or other weirdness, I'm probably in the middle of something.

Let's talk about analog synthesizers.

When you are first learning about analog synthesizers it can be very intimidating. There is a lot to learn! VCO, VCA, VCF, LFO, there are so many acronyms, specialized terms and concepts, and even the equipment itself can be bristling with complexity:

An analog modular synthesizer created by Dr. Joseph Paradiso
Dr. Joseph Paradiso's custom designed modular synthesizer. (Photo used with permission.)

It seems daunting, but once you learn a few basic concepts and how they relate to each other, the way forward becomes a lot clearer. Once you master the simple concepts, it gets much easier to learn advanced concepts, because you will know how to ask questions the right way.

Even if you are firmly in the digital camp and you don't have the slightest desire to get your hands dirty with soldering or building your own analog synth module, a working knowledge of analog synthesizers will help you understand a lot of the analog synthesizer simulator apps out on the market today.

In fact, let's start with a digital synthesizer. How does a digital synthesizer work? Well, it just does. There is a little computer in there somewhere, and someone has programmed the computer to make the sounds when you press the buttons. That's it! If you want to know much more about the "how" than that, or if you want to make your own digital synth, you have to understand how to program a computer.

Okay, maybe that is a little bit too reductionist. Don't get me wrong, I don't mean to slight the digital guys, I know there is a ton of work that goes into a digital synth, even the simple ones. But when you get down to the fundamentals of digital synthesis, someone is programming a little computer somewhere and that computer is what is making the sound.

But an analog synth, well, there is no computer. In an analog synthesizer, a circuit creates a fluctuating voltage which is then amplified. Really simple analog synthesizers are single, simple circuits. For instance, with a handful of components you can make a simple light dependent theremin.

[insert light dependent theremin circuit] [picture of get lo fi circuit?]

How cool is that?

There are a ton of small circuits out there that each have a specialized purpose and a unique sound. Some of my favorites are the Atari punk console and the drawdio (which is really more of an interesting input method connected to a simple oscillator.)

Unfortunately, the more complex the sound you want, and the more control you want over your sound, the more complex the required circuit becomes. Once the complexity of the circuit increases, it becomes more and more difficult to design, debug, and build. Think of how impractical it would be to design, prototype, debug, and build a new analog circuit every time you wanted to create a new sound!

There is a better way, and it is called modular synthesis.

In a typical practical, large-scale analog synthesizer, there are a series of circuits, (called modules). Each module is custom designed for a specific purpose. Some modules create sounds. These sounds can then be sent to other modules that can shape or filter a sound in interesting and unusual ways. There are modules that can mix sounds together, modules to pan the source to the left or right of the stereo field, and modules to change the volume of an incoming sound. Some modules, such as keyboards or sequencers, exist just to control, alter, or communicate with other modules. And of course, you can have hybrid modules that can combine any number of these functions.

The order these modules are in determines the sound you ultimately get. And, of course, it is all entirely up to you. This modularity and configurability is why the whole topic of modular synthesis is really big and open-ended.

In the following pages I'm going to break down the various components of an analog synthesizer.

VCO: Voltage Controlled Oscillator

The oscillator is the thing that goes beep.

"Oscillator" is a fancy word that means "something that goes back and forth", and if you look at a picture of a sound wave, it sure looks like something that is moving back and forth:

If the sound wave is too abstract for you, think of a speaker making a beep. That speaker is going back and forth and back and forth. If the beep is a low enough bass note, you can see and feel the speaker going back and forth.


What is making the speaker go back and forth? Well, the amplifier, technically. But what is the amplifier amplifying? The amplifier is amplifying the beep. Where does the beep come from? The oscillator.

Okay, so oscillators go beep. A VCO is a voltage controlled oscillator, which means it is a "voltage controlled thing that goes beep." What does "voltage controlled" mean?

It turns out that it's really easy to design a simple oscillator. REALLY easy. (In fact, many Electrical Engineers spend their entire professional careers trying to make their circuits STOP oscillating. ;-)

The problem is, a simple oscillator will only oscillate at one frequency. A simple oscillator only goes beep. It won't go boop. You can't play notes with a simple oscillator, you can only play one note, and that is the note that the oscillator oscillates at. Beep. Beep. Beep. Beep. Beeeeeeeeeeeeeeep. Hard to have a melody with only one note.

Ah! But a piano, now, a piano has a whole bunch of strings. What if we designed an electronic piano which has a whole bunch of simple oscillators at different pitches? Yes! You can make a synthesizer with a whole bunch of simple oscillators, other people have done it, and you can do it too. But designing, tuning and building the oscillators is time consuming, tedious, and can be expensive, especially if you want an instrument with more than an octave or two of range.

No, it would be far better to have one circuit that goes beep, and then figure out a method of controlling the pitch of the beep. Then we could have one circuit to make ALL the beeps.

Our one circuit that makes all of the beeps is the Voltage Controlled Oscillator, or VCO. And we control which beep we want (the pitch) with a control voltage.

VCA: Voltage Controlled Amplifier

The VCA, or Voltage Controlled Amplifier, is exactly what it says on the tin. You put in a signal you want to amplify, and a control voltage. If the control voltage is zero volts, the VCA spits out nothing. As you increase the control voltage, you amplify the incoming signal more and more.

Making a note

Hit a key on a piano. The movement of the key causes a lever to move a hammer, which hits some strings, which are tuned to the correct note. Conceptually easy: one note, one hammer.

With an analog synthesizer, your keyboard will output one voltage, that's the pitch. But how do you make a note start and stop when the key is pressed and released?

The keyboard will also put out two additional signals. One is called the trigger, which is a little pulse (usually at 5v) that signals the start of a note. The second signal is called the gate, and the gate starts low and is held high for as long as a key on the keyboard is pressed.

So you can imagine, just ignore the trigger for a moment, and hook the pitch output of your keyboard to the pitch input of your VCO. Hook up the output of the VCO to the input of the VCA, and the output of the VCA up to your speaker amplifier. Finally, hook up the gate of the keyboard up to the control voltage in on the VCA.

You would have a keyboard where each note instantly turns on and off. And this indeed is how many electric pianos operate. But this is not suitable for dramatic slow building swells, or notes that decay over time. How do we create those?

Envelope Generators

So, a voltage controlled oscillator is one in which an incoming voltage controls the pitch at which an oscillator will oscillate.

LFO: Low Frequency Oscillator

VCF: Voltage Controlled Filter

And how do all of these parts fit together? Without a synth in hand while reading the text,