Modular primarily creates sound with subtractive synthesis. Subtractive synthesis is the method of taking a harmonically rich sound source and modifying it by subtracting unwanted harmonics to create the desired sound.
A typical subtractive synth has SOURCE, MODIFIER, AMPLIFIER, and CONTROLLERS sections. In addition, modular has a ROUTING section of modules.
A SOURCE module creates sound.
A MODIFIER module changes the sound.
An AMPLIFIER module changes the sound loudness.
A CONTROLLER module alters a parameter over time.
A ROUTING module directs a signal.
Source modules consist of oscillators and noise.
An oscillator creates a repeating waveform at a specific frequency that is usually directly related to the keyboard note you are pressing. Waveforms can range from sine, triangle, pulse (square), and sawtooth.
Noise creates a random waveform with a specific energy. For example, white noise has equal energy over all frequencies while pink noise has equal energy over all octaves.
Modifier modules consist of filters and effects.
Filters modify the sound by removing harmonic content. The most common filter is a low pass filter. The low pass filter removes higher frequencies and only lets the low frequencies pass.
Effect modules take the sound and change it. For example delay effects repeat the sound over time.
Amplifier modules consist of amplifiers.
Amplifiers modify the sound by changing the loudness of the sound over time. The amplifier is typically controlled by an LFO or envelope generator to make the changes in loudness. Another function of an amplifier is to boost or attenuate the signal.
Controller modules consist of envelope generators and low frequency oscillators (LFO).
Since modular is purely digital, the ability to route any signal into any input allows you to use audio modules as a controller.
Routing modules are not found in traditional style synthesizers because all of the sections above are permanently connected together inside the synthesizer. On a modular synthesizer, routing modules are required so that you can split, combine, fade, or invert signals. These modules can process both controller and audio signals.