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Synth Filtering Tips From Novation

Filtering is a subtractive process. Subtractive implies that part of the sound is subtracted somewhere in synthesis. The Oscillators provide the raw waveforms with plenty of harmonic content and the Filter section subtracts some of the harmonics in a controlled manner. The type of Filter most commonly found on synthesizers is the Low Pass type.

With a Low Pass Filter, a cut-off point (or cut-off frequency) is chosen and any frequencies below the point are passed, and frequencies above are filtered out. The setting of the Filter Frequency parameter dictates the point below which frequencies are removed. This process of removing harmonics from the waveforms has the effect of changing the sound’s character or timbre. When the Frequency parameter is at maximum, the filter is completely “open” and no frequencies are removed from the raw Oscillator waveforms.

In practice, there is a gradual (rather than a sudden) reduction in the volume of the harmonics above the cut-off point of a Low Pass Filter. How rapidly these harmonics reduce in volume as frequency increases above the cut-off point is determined by the Filter’s slope. The slope is measured in ‘volume units per octave’. Since Volume is measured in decibels, this slope is usually quoted as so many decibels per octave (dB/oct). Typical values are 12 dB/oct and 24 dB/oct. The higher the number, the greater the rejection of harmonics above the cut-off point, and the more pronounced the filtering effect. A further important parameter of the Filter is its Resonance. Frequencies at the cut-off point may be increased in volume by the Filter Resonance control. This is useful for emphasizing certain harmonics of the sound. As Resonance is increased, a whistling-like quality will be introduced to the sound passing through the filter.

When set to very high levels, Resonance actually causes the filter to self - oscillate whenever a signal is being passed through it. The resulting whistling tone being produced is actually a pure sine wave, the pitch of which depends on the setting of the Frequency knob (the filter’s cut-off point). This resonance-produced sine wave can actually be used for some sounds as an additional sound source if wished. The diagram below shows the response of a typical low pass filter. Frequencies above the cut-off point are reduced in volume.

When resonance is added, frequencies at the cut off point are boosted in volume.

 

In addition to the traditional Low Pass Filter type, there are also High Pass and Band Pass types. The type of Filter used is selected with the Filter Type parameter. A High Pass Filter is similar to a Low Pass Filter, but works in the “opposite sense”, so that frequencies below the cut-off point are removed. Frequencies above the cut-off point are passed. When the Filter Frequency parameter is set to zero, the filter is completely open and no frequencies are removed from the raw Oscillator waveforms.

 

When a Band Pass Filter is used, only a narrow band of frequencies centered around the cut- off point are passed. Frequencies above and below the band are removed. It is not possible to fully open this type of Filter, and allow all frequencies to pass.