Synth: Subtractive synthesis and VCO's

A few months ago, I promised Mike to talk about FM synthesis and subtractive synthesis. I ran into technical difficulties with that... so it's taken me this long to give up and take another approach. [1]

Most analog synthesizers take an approach called subtractive synthesis. This is where you take a strong starting signal (sound) and you remove parts of the sound to get the sound that you want. A very simple example of this is volume pedal. You take a strong input signal (like a guitar) and plug it into the volume pedal. When the pedal is all the way up, the signal should just pass through (unchanged). When you turn the pedal down, it subtracts from the original signal to make it quieter. This is actually one way to get synthesizer-like sounds out of your guitar (by doing volume swells or emulating an envelope generator).

Everything starts with a signal source. The source can be:

• An oscillator — an electronic device that creates basic wave-forms like a sine wave, triangle wave, sawtooth wave, etc.


• A sample — an actual recording of an instrument or sound. It's usually relatively short, and just a single note.


• An instrument (like a guitar, voice, electric piano)

But, let's start by talking about the voltage controlled oscillator (VCO).

A basic VCO has an input (a DC voltage from 0 to 10V) and an output (the audio signal with the desired osciallation). Most VCO's will let you select between several types of waves (click for Wikipedia links, some with audio clips): a sine wave (flute-like sound -- like a hearing test), a square wave (computer beep), a triangle wave (a harsh, electric-organ-like sound), and a sawtooth wave (a very harsh, annoying alarm-clock-like sound). As you turn up the voltage on the input, the frequency of the output signal will also go up. The standard is that the frequency will double for every 1V that you add. (Thus, they say "one volt per octave.")

The VCO does not have a volume knob, nor an on/off switch, nor keyboard, nor any other sort of nice things. The VCO is always on. If you plug the VCO directly to the power amplifier... you will always hear noise.

To control the voltage, you will typically feed it a signal from a keyboard controller that has a voltage output. Since you can only send one voltage to the VCO (and the VCO can only take one), this is a monophonic setup. No matter how many buttons you push, the controller will only send one voltage to the VCO (so you only get one note out).

So, what would happen if I put something between the keyboard controller and the VCO? Something that changed the voltage a little... like a slow-moving oscillator or something? What would happen? Answer: Vibrato.

Or... what would happen if I put something like an envelope generator between the keyboard and the VCO, and I made the pitch go up slightly after striking the note, and then have it go back down gently? Answer: It will sound like a string being plucked, or like a gong.

These two ideas are actually the start of a concept called FM Synthesis.

Now, as we said, the the output side of the VCO is always on. So, next time we'll talk about how to use an envelope generator and a voltage-controlled amplifier (VCA) to create attack, sustain, decay, etc.

[1] See the comments for a description of what I tried to do and the problems that I had.