Filters

Most sounds contain vibrations at a number of different frequencies. For instance, a sawtooth wave whose fundamental frequency is 100Hz (100 cycles per second) also contains energy at 200Hz, 300Hz, 400Hz, 500Hz, and so on. The frequencies where the sound has energy are called partials. All of the partials together make up what we hear as the composite tone.

A filter changes the tone coming from a synthesizer by reducing the levels of certain partials. For example, it might allow the partials at 100Hz and 200Hz to pass through while reducing the level of the 300Hz partial slightly and reducing the level of partials at 400Hz and above by a lot.

TYPES OF FILTERS

Many filters offer several modes of operation. The names of the modes give you information about the type of filtering. A lowpass filter allows the lower-frequency partials to pass through, while attenuating (reducing the level of) the higher-frequency partials. A highpass filter does just the opposite: It allows higher partials to pass through while attenuating the lower partials. Depending on how it’s set up, the output of a lowpass filter typically sounds muted, with a strong bass and not much treble. The output of a highpass filter is just the opposite — a brittle treble sound with little or no bass.

Other common types of filters include bandpass and notch. A bandpass filter attenuates both the low and high ends of the frequency spectrum, while allowing the partials in a midrange band to pass through. A notch filter (also called a band-reject filter) does the opposite: It filters out partials in the midrange, while allowing both low and high frequencies to pass through.

FILTER PARAMETERS

The most important knob or slider on a filter is the cutoff frequency control. Sometimes it’s called simply “cutoff,” or, if panel space is tight, “cut.” This control determines which part of the frequency spectrum the filter operates on.

The best way to understand what the cutoff knob does is to play a synth while moving the knob. With a lowpass filter, for instance, lowering the cutoff frequency (turning the knob to the left) causes more of the high partials to be removed. If a lowpass filter’s cutoff is raised all the way, the filter will do nothing, because all of the partials will be below the cutoff point, so they’ll pass through the filter unchanged. As you raise the cutoff frequency of a highpass filter, more of the lower partials will disappear, until all you’re left with is the extreme highs.

A filter can’t add partials that aren’t in the tone to begin with. If you send a low-pitched tone that has no significant high partials (such as a 100Hz triangle wave) through a lowpass filter, moving the cutoff frequency from 2,000Hz down to 1,000Hz will have no effect on the tone.

The filter’s resonance parameter produces a resonant peak at the cutoff frequency. The resonant peak will boost the level of any partials that are near the cutoff frequency. If you’ve turned up the resonance, moving the cutoff frequency knob will produce an effect called a filter sweep.

Other filter parameters include overdrive, which adds warmth or thickness to the tone, and rolloff slope. Rolloff slopes of 12dB and 24dB per octave are typical. The higher the number, the more the filter will attenuate partials that lie beyond the cutoff frequency.

FILTER MODULATION

While you can move the cutoff frequency by hand, in many musical situations it’s better to control it automatically using a modulation source. The most important sources for filter cutoff modulation are an envelope generator, velocity, the keyboard, and a low-frequency oscillator (LFO).

By modulating the cutoff frequency of a lowpass filter from velocity, for instance, you can cause notes to be brighter when you hit the keys harder. An envelope will give a shape to each note by moving the cutoff up or down while the note sounds. An LFO will cause a wah-wah type of sound.