20. Low-pass Filters

There are several types of filters commonly used in electronic music synthesis. Perhaps the most prevalent is the low-pass filter, which permits frequencies below a specified frequency to pass while removing frequencies above this frequency. The point above which the higher frequencies are removed is called the cutoff frequency (abbreviated fc) and is adjustable so that all, some or no frequencies can pass.

Example 20-1: A low-pass filter attenuates frequencies above the cutoff frequency.

The ability to determine the cutoff frequency is the basic control that all filters possess. The cutoff frequency is often adjusted with a control knob or by entering a number that sets a particular cutoff frequency.

Low-pass filters do not completely remove all frequencies above the cutoff frequency. Instead, low-pass filters gradually attenuate (reduce in amplitude) the frequencies above the cutoff frequency.

A filter's response curve refers not only to the cutoff frequency but also to the rate at which the frequencies above the cutoff frequency are attenuated. Frequencies above the cutoff are gradually attenuated; some filters attenuate frequencies more precipitously than others. The steepness of this attenuation is the slope.

Diagram 20-2: Response curve.

The slope of the filter in some systems is adjustable so that one of several slopes can be selected. In many cases, however, the slope is not controllable by the user.