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What All Those Knobs on Your Synthesizer Do

A synthesizer (or software designed to look like a physical synthesizer) often has a wall of knobs and sliders with weird, arcane labels. Without knowing what these do, it’s hard to make sounds other than by trial and error. Those names of those controls are sometimes obscure and come from details of how traditional analog synthesizers work internally. Once you know what they mean your synthesizer will make a lot more sense.

VCO – Voltage Controlled Oscillator

This is where everything begins. An oscillator generates a basic electrical wave, such as a sine wave, square wave, saw wave, or triangle wave. This electrical wave is eventually sent to speakers where it turns into a sound wave that you can hear. By itself, it will produce a very simple sound, but when combined with the other controls such as a VCA, VCF, LFO, or ADSR envelope you can get more interesting noises.

Sometimes these controls will be label “Oscillator” or “osc”, but you also might see the abbreviation VCO, for voltage controlled oscillator. The term voltage controlled refers to how the oscillator inside an analog synthesizer works. It has a voltage coming into it which determines the frequency (i.e. pitch) it produces. Change the voltage, and you change the frequency. Pressing a key on the synthesizer keyboard (or triggering a MIDI note) causes a different voltage to be fed into the oscillator, causing the pitch you hear to change.

So, the term voltage controlled is really a fancy way of saying that it’s possible for the oscillator (i.e. sound generator) to generate different pitches. A bit redundant when thinking of a synthesizer as a musical device, since that’s sort of the whole point. But, the term VCO has stuck as a historical artifact. You’ll sometimes see it used in other contexts where it’s not technically accurate, such as in digital software that simulates an analog synthesizer.

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

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Power

VCO

Wave Form
Frequency

Pro Tip Make sure to click the “Power On” button first.

Now, instead of a frequency slider, here’s a more realistic example with a piano keyboard. Each time you press a key, it changes the frequency that the oscillator emits:

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Power

VCO

Wave Form
F1 F♯1 G1 G♯1 A2 A♯2 B2 C2 C♯2 D2 D♯2 E2 F2 F♯2 G2 G♯2 A3 A♯3 B3 C3 C♯3 D3 D♯3 E3 F3 F♯3 G3 G♯3 A4 A♯4 B4 C4 C♯4 D4 D♯4 E4 F4 F♯4 G4 G♯4 A5 A♯5 B5 C5 C♯5 D5 D♯5 E5 F5

Sometimes a synth will have multiple oscillators that can produce sound at the same time. You can sometimes 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 signals that come from an oscillator are then 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 LFO or ADSR envelope (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 the gate signal being sent to the VCA.

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VCO

Wave

VCA

Amplitude
F1 F♯1 G1 G♯1 A2 A♯2 B2 C2 C♯2 D2 D♯2 E2 F2 F♯2 G2 G♯2 A3 A♯3 B3 C3 C♯3 D3 D♯3 E3 F3 F♯3 G3 G♯3 A4 A♯4 B4 C4 C♯4 D4 D♯4 E4 F4 F♯4 G4 G♯4 A5 A♯5 B5 C5 C♯5 D5 D♯5 E5 F5

VCF – Voltage Controlled Filter

A filter removes all 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 via a different component such as an LFO or ADSR envelope.

A synth will typically have a knob or slider that controls the 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.

Sound is first generated in a VCO, is then fed into a VCF where it is altered, then into a VCA, and then finally out into your speakers.

The different types of waves generated by an oscillator (i.e. sine, square, saw, etc.) will respond to the cutoff frequency a bit differently. In particular, the filter will have more of a pronounced effect on sounds with a lot of harmonics. Square, sawtooth, and triangle waves have a lot of harmonics, while sine waves have no harmonics.

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

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Power

VCO

Wave

VCF

Cutoff Frequency
Resonance

Pro Tip For a wah-wah effect, jack up the resonance and move the cutoff slider up and down quickly.

ADSR Envelope

Stands for Attack, Decay, Sustain, and Release Envelope. Most typically, an envelope controls the VCA volume over time. Each time a note is played (either by a key press or a MIDI event), the envelope will change the amount of amplification over time, using four parameters:

When you press or key (or start a MIDI note), the attack and then decay portions of the envelope will take effect, until the sustain volume is reached. The sustain volume will hold as long as the note continues. When the key is released, the release portion of the envelope will take effect until the volume drops down to zero.

Different ADSR envelopes are 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.

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Envelope

Attack
Sustain
Decay
Release

VCA

Amplitude
F1 F♯1 G1 G♯1 A2 A♯2 B2 C2 C♯2 D2 D♯2 E2 F2 F♯2 G2 G♯2 A3 A♯3 B3 C3 C♯3 D3 D♯3 E3 F3 F♯3 G3 G♯3 A4 A♯4 B4 C4 C♯4 D4 D♯4 E4 F4 F♯4 G4 G♯4 A5 A♯5 B5 C5 C♯5 D5 D♯5 E5 F5

Although an envelope most typically controls the VCA, it is sometimes possible for an envelope to modify another parameter, such as the filter cutoff frequency. The envelope can also sometimes be turned on or off by a toggle under the “VCA” header: gate or hold (for off), and “env” for on.

LFO – Low Frequency Oscillator

It is just what the name says it is - an oscillator that normally produces very low frequencies (usually below 20Hz). These frequencies are actually so low that humans can’t hear them. Pointless, right?

The real use of an LFO to modulate other components, such as a VCO, VCA, or VCF. For example, if the LFO produces a signal whose amplitude goes up and down, like a sine wave, and is connected to an oscillator, that will cause the frequency of the oscillator to go and down in a regular pattern. This sounds like vibrato. When hooked up to a VCA, it will cause the sound to become louder and quieter in a regular pattern, like tremolo.

In this example, notice how changing the wave type from the LFO changes the sound coming out of the VCO:

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LFO

Wave
Frequency
Amount

VCO

Wave
F1 F♯1 G1 G♯1 A2 A♯2 B2 C2 C♯2 D2 D♯2 E2 F2 F♯2 G2 G♯2 A3 A♯3 B3 C3 C♯3 D3 D♯3 E3 F3 F♯3 G3 G♯3 A4 A♯4 B4 C4 C♯4 D4 D♯4 E4 F4 F♯4 G4 G♯4 A5 A♯5 B5 C5 C♯5 D5 D♯5 E5 F5

An LFO can sometimes be connected to a VCA or VCF. When connected to a filter’s cutoff frequency you can get some interesting effects. Try playing around with the cutoff frequency and resonance sliders:

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LFO

Wave
Frequency
Amount

VCF

Cutoff Frequency
Resonance
F1 F♯1 G1 G♯1 A2 A♯2 B2 C2 C♯2 D2 D♯2 E2 F2 F♯2 G2 G♯2 A3 A♯3 B3 C3 C♯3 D3 D♯3 E3 F3 F♯3 G3 G♯3 A4 A♯4 B4 C4 C♯4 D4 D♯4 E4 F4 F♯4 G4 G♯4 A5 A♯5 B5 C5 C♯5 D5 D♯5 E5 F5

Pro Tip Use this with low notes to get the signature “wobbly” dubstep bass sound.

Although it’s called a low frequency oscillator, you can use higher frequencies as well (assuming the synth allows you to). 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).

Conclusion

Everything makes sense now!

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.
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