Synthesizer Basics

This page will give you the very basics of a synthesizer patch
The type of synthesis I use, and my modules use is called ‘subtractive synthesis’ which means the sound is altered by using a filter.

On this page I am concentrating on voltage controlled analog synths.

A Simple synthesizer can look like this:

This is a simple patch with a single oscillator, the gate is in parallel and feeds both envelope generators (ADSR)

Example above we have now got 2 oscillators and a voltage controlled mixer (VCM)

** The cutoff control is mixed with the ADSR as there is only one cutoff control on the filter, a synth module will do this and give you the CV input as well as the pot on the front panel.

Acronyms you will come across on this page are:
VCO – Voltage Controlled Oscillator
VCM – Voltage Controlled Mixer
VCA  РVoltage Controlled Amplifier
VCF – Voltage Controlled Filter
ADSR – Attack / Decay / Sustain / Release

Oscillators

The first thing we have is an oscillator, this produces various waveforms usually Sawtooth, Square (sometimes called Pulse) and Triangle, some have sine wave and most have adjustable pulse width (which is basically how long the square wave is switched on for).

The oscillator pitch is controlled using a voltage at a level of 1 volt per octave, tuned to the lowest note what represents 0 volts, then 1 volt is the next octave up etc.
Oscillators are always buzzing away in the synth, they never stop just change frequency. To control the sound we have to add extra stages.

Filter

The next stage commonly in a single oscillator is the Filter, The filter can have a mixer input where we can connect multiple oscillators or mix different waveforms.
The Low pass filter is the most common used, some synthesizers have the option of different types but some just have a low pass and no others. This type of filter allows frequecies lower than the cutoff to pass through, this is controlled with a voltage.
The resonance is also voltage controlled, this is how much of the output is fed back to the input.

The cutoff and resonance can be controlled via knobs on the filter or via external sources such as an envelope generator

Amplifier

The amplifier is usually the final stage, this is where we can turn off the sound until we press a key or trigger with some sort of voltage input, this is known as the gate. Remember the oscillator is always running.
This can be a control voltage from the keys velocity, a trigger pad or an envelope generator.

The Voltage controlled amplifier (VCA) is a very useful device, they can be used to control the level of any analog signal in a synth, such as the envelope amount from the ADSR to the filter cutoff for example.

Mixer

The voltage controlled mixer (VCM) is simply just 2 or 3 VCAs that take 2 to 3 oscillators or a noise source allowing you to change the level of each and then mixing them together before we enter the filter stage.

Envelope Generators

The envelope generator, seen by the controls ADSR on a synth is a voltage that changes over time. There are normally 2 of these, sometimes 3 on more expensive synths. They’re used to control the Amplifier, Filter cutoff or in the case of a 3rd allow you to control other parameters.

Let’s break down the ADSR, for the volume of the output for example:
A = Attack – How long before the volume is at the top level.
D = Decay – How long before the volume reaches the sustain level
S = Sustain – The level that the volume is at while the key is still held
R = Release – The time after the key is released before the volume level is 0.

For example you would use the attack to create a string style sound, and if you don’t use the sustain level the sound will stop even if you’re still holding the key.
The Attack, Decay and Release are all time values. The sustain is the output level.

They’re triggered by a gate pulse, so when you press a key the envelope is triggered. This is usually a 5 volt digital signal on or off.