19. Filters

The filter is a modifier module that is a basic part of almost all synthesis systems. A filter is a module that allows only certain frequencies it receives to pass through, and at the same time is a barrier to others. In this way a filter is used to screen, or filter out, unwanted frequencies. Compare this to a pair of coated glasses that block ultraviolet light while allowing all other light to pass through the lenses. The filter in a synthesis system works in the same fashion.

The way a filter is used in a synthesis system is straightforward. The output of the oscillator or other generator module is sent to the filter. The filter then allows certain frequencies to pass, and simultaneously "filters out" others. The use of this technique is called subtractive synthesis.

Example 19-1: A filter is a barrier that is designed to screen out certain frequencies sent through it.