What All Those Knobs on Your Synthesizer Do

Synthesizers (or software simulations) often have a bunch of knobs and sliders labeled with acronyms and jargon. Without knowing what these settings do it’s hard to make sounds other than by trial and error. The goal of this article is to demystify your synth so that it makes more sense.

The Big Picture

Many synthesizers use subtractive synthesis. This means that a tone containing many frequencies is produced, and then some of the frequencies are removed (filtered) to create a particular sound. This is similar to how selectively removing parts of a block of marble can reveal a sculpture. The sound can then be further modified by changing various parameters over time while a note is played.

A synthesizer that uses subtractive synthesis usually has these main components:

Voltage Controlled Oscillator (VCO)

Creates a base tone.

Voltage Controlled Amplifier (VCA)

Controls the tone’s loudness.

Voltage Controlled Filter (VCF)

Controls whether the tone sounds bright or dull.

Attack/Decay/Sustain/Release (ADSR) Envelope

Allows changing the VCO, VCA, or VCF’s settings in a custom pattern each time a note is played.

Low Frequency Oscillator (LFO)

Allows changing the VCO, VCA, or VCF’s settings over time in a repeating pattern.

With these 5 components you can create a large number of sounds!

Some synthesizers use other kinds of synthesis, such as FM synthesis, sample-based synthesis, or wavetable synthesis. This article only covers synthesizers that use subtractive synthesis, but some of the concepts are more broadly applicable.

VCO – Voltage Controlled Oscillator

This is where everything begins. An oscillator creates an electrical signal such as a sawtooth wave, square wave, or triangle wave. This electrical signal is eventually sent to speakers where it turns into a sound wave that you can hear, but not until being modified by a VCA, VCF, ADSR envelope, or LFO. By itself the sound created by an oscillator might seem simple, but it provides the raw material for all of the noises a synthesizer can make.

Often this control will be labeled with the acronym VCO, for voltage controlled oscillator. The term voltage controlled is a fancy way of saying that the oscillator’s output frequency can be changed. Inside a synth, a VCO has a voltage coming in which determines the frequency (i.e. pitch) of the sound it produces. Change the voltage, and you change the output frequency. Pressing a key on the synthesizer keyboard causes a particular voltage to be fed into the oscillator, causing the pitch you hear to match the key you pressed.

Here’s an example of an oscillator in action. Notice how when you move the frequency slider the pitch changes:

In this next example the VCO is controlled by a piano keyboard instead of a slider. Each time you press a key, it changes the frequency that the oscillator emits:

Notice that after playing a note the sound never stops until you turn the synth off. We’ll see how to improve that in the next section.

Often a synth will have multiple oscillators that can produce sound at the same time, which allows creating more interesting sounds. You can sometimes also change the octave the oscillator emits. This will often be labeled with numbers like 2’, 4’, 8’, 16’, etc. The lower the number, the higher the pitch.

VCA – Voltage Controlled Amplifier

Think of this as a volume knob. The electrical signal that comes from an oscillator is fed into a VCA to be amplified, before being sent out of the synth to some sort of speaker.

Since it is a voltage controlled amplifier, the amount of amplification it performs can be changed. Typically this is not controlled directly by knob or slider on the synth, but by another component such as an ADSR envelope or LFO (both of which we’ll talk about below).

Another important use of a VCA is to act as a gate. Notice how in the oscillator examples above notes played forever (until you turned the synth off). A VCA lets you stop the sound when you let go of a key. Each time you press a key, the synthesizer will send a gate signal turning the VCA on (and letting sound from the oscillator through), and when you let go of the key, another gate signal is sent to turn the VCA off.

In the example below you can control the VCA volume by moving the slider^[1]. Also, notice how each time you let go of a key the sound now stops, due to a gate signal being sent to the VCA.

VCF – Voltage Controlled Filter

Sitting between a VCO and a VCA is a voltage controlled filter. A filter removes frequencies that occur above or below a certain frequency, or within a certain range. Since it is voltage controlled, you can change which frequencies the filter removes, either with a knob or slider, or indirectly via a different component such as an ADSR envelope or LFO.

Although the typical base sounds generated by a VCO ( sawtooth wave, square wave, triangle wave) might seem simple, they actually consist of many different frequencies combined together. Think of the sound created by a VCO like a block of marble. Similar to how selectively removing parts of a marble block will reveal a sculpture underneath, selectively filtering some of the frequencies from a VCO’s output lets you create many different types of sound.

A synth will typically have a knob or slider that controls the filter’s cutoff frequency. Any frequencies above this will be removed^[2]. When this is set to a low value, the sound will be muffled like it is coming from the next room. A separate knob or slider will usually control the amount of resonance. Resonance means that frequencies near the cutoff frequency will be amplified, and this slider controls how pronounced they are.

In this example, you can see how changing the cutoff frequency and resonance affects sound coming from an oscillator:

ADSR Envelope

Stands for Attack, Decay, Sustain, and Release Envelope. An envelope allows changing a parameter over time while a key is pressed. Most commonly an envelope can be connected to the VCA volume or VCF cutoff frequency. For example, when connected to the VCA volume the 4 parameters have this meaning:

Attack

After a key is pressed the rate at which the volume goes from silence to full volume.

Decay

The rate at which the full attack volume fades to the sustain volume after the attack is complete.

Sustain

The volume that is held after decay ends, until the key is released.

Release

The rate at which the sustain volume fades to silence after the key is released.

When you press a key, the attack and then decay portions of the envelope will take effect, until the sustain volume is reached. The sustain volume will then be held as long as the key is pressed. When the key is released, the release portion of the envelope will take effect until the volume drops down to zero.

Different ADSR envelopes allow creating sounds reminiscent of traditional musical instruments. For example, when simulating a struck instrument such as a piano or bell you’d want a quick attack, whereas a bowed instrument such as a violin would have a slower attack.

An envelope can also often be connected to the VCF’s cutoff frequency. Typically, the cutoff frequency control will represent the initial cutoff frequency that the note begins at. A separate control will indicate a separate frequency above the cutoff frequency that is reached at the end of the attack phase. This control can be labeled in various ways but on the synth below is called “Env. Amount”. The sustain value will be between the initial and end-of-attack cutoff frequencies. When the note is released, the cutoff frequency will sweep back down to the initial amount.

Here is an example of an envelope connected to a VCF:

LFO – Low Frequency Oscillator

A low frequency oscillator is like a regular VCO, except that it’s normally used to modulate another parameter, instead of directly producing sound itself. In fact, it wouldn’t be very useful for producing sound, because often the frequencies it outputs are lower than what humans can hear (hence why it is called a low frequency oscillator). Along with an ADSR envelope, an LFO allows for parameter values to change over time while a key is pressed.

For example, if a LFO produces a signal whose amplitude goes up and down, like a triangle wave, and is connected to a VCO, the frequency of the VCO will go up and down in a triangle wave. This sounds like vibrato. When connected to a VCA, it will cause the sound to become louder and quieter in a regular pattern, like tremolo.

In the example below an LFO is connected to a VCO, causing the VCO’s pitch to change in a repeating pattern. Changing the LFO’s waveform changes the pattern. The LFO’s frequency slider changes how fast the pattern repeats, and the LFO’s amount slider changes how much the pitch changes. If either slider is all the way to the left the LFO is effectively turned off.

An LFO can often be connected to a VCF’s cutoff frequency, as shown in the example below. This causes the cutoff frequency to be modified in a repeating pattern.

Although it’s called a low frequency oscillator, a synth will sometimes allow an LFO to output higher frequencies as well. When frequencies get high enough and are connected to the VCO pitch, they stop causing a vibrato effect and instead start changing the timbre of the sound. This is the basis of FM Synthesis (which is beyond the scope of this article).

Footnotes

1. In real life, there won't necessarily be a knob or slider that directly lets you change the VCA volume. ↩
2. The fact that the knob/slider controls which high frequencies are removed is not set in stone. Some synths might give you more options, such as removing frequencies below the cutoff frequency, or setting a "band-pass", which keeps frequencies that are in a certain range but removes everything above or below. ↩