6. Filter

A filter can remove a certain group of overtones from an overtone spectrum. Most common are 3 types of filter:
  • A low pass filter (LP) lets pass the low frequencies and blocks frequencies that are higher than the cut off frequency (F cut). It is the most common filter and is used to soften acid waves like square- or saw-waves. It makes the waves “rounder”.

  • High pass filter (HP) lets pass higher frequencies and blocks lower ones.

  • A serial sequence of LP and HP results in a band pass filter (BP) that lets only pass frequencies in a certain range.

It is also possible to enhance frequencies in a certain range with the resonance parameter (Q). It makes a sound more brilliant.

Filter Modulation

The frequency that is at the edge of the filter curve is called the cut off frequency. By moving this frequency you can change the sound from acid to dull. This moving can be done with a modulation curve so that you can dynamic change the sound during the lifetime of a tone.

The modulation can be done with an ADSR curve or better with a filter curve that starts at a certain level and move exponential to the end level, because a filter cut off frequency should never be zero like an ADSR. As a result the sound will shift from bright to dark or vice versa.

A “Shift Effect” start bright and ends dull. It is often used for percussion instruments because natural higher overtones decrease faster than lower.

Sweep Effects” starts soft and ends bright. It is often used for synthesizer PADs.

Modulation with a LFO results in a “Wah-Wah Effect”.