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A filter is the most common tone-altering module in a modular synth. It typically reduces the strength of select harmonics or overtones, changing the tonal balance of a sound. Filters have a few main characteristics:

Mode or Type:      This defines which group of harmonics is going to be affected by the filter. A low pass filter (LPF) leaves lower harmonics alone, and filters out higher harmonics; a high pass filter (HPF) leaves upper harmonics alone and filters out lower harmonics; a bandpass (BP) filter does not touch harmonics roughly in the middle of the spectrum (more precisely, those centered around its center or corner frequency, defined below) and filter out both higher and lower harmonics; a notch filter reduces harmonics around its center frequency and leaves the upper and lower harmonics alone. There are other configurations as well, such as “twin peak” filters that are essentially two bandpass filters in one module. A multimode filter supports several modes or types inside one package; some give you access to each mode individually, while other ones have you select which one mode you want to use.

Slope:         This defines how strongly the filter reduces the level of harmonics. It is usually defined in terms of poles or decibels/octave (dB/oct), with one pole equaling 6 dB/oct of strength. As an example, a 4-pole low pass filter does not weaken harmonics at or below its cutoff frequency (see below); harmonics that are one octave above the cutoff are reduced in strength by 24 decibels (4 poles x 6 dB/oct); harmonics that are two octaves above are reduced by 48 decibels, and so forth. As you can quickly extrapolate, filters with more poles have a more drastic effect on the sound.

Filters then a few common parameters that control how they act:

Cutoff, Center, or Corner Frequency: This frequency is the pivot point around which a filter works. For example, a low pass filter (LPF) will not touch harmonics below the cutoff frequency, and will start reducing the level above this frequency.

Resonance, Feedback, Regeneration, or Q: Most filters allow you to send some of the output signal back into the input. This feedback results in the strength of the harmonic at the cutoff frequency being boosted, rather than left alone or cut. This provides a “resonant peak” similar to a resonance inside a room, etc. Some filters allow you to increase the feedback to the point where they start to oscillate on their own, usually producing a sine wave at the cutoff frequency. (You might need to feed an initial signal into the filter to “excite” it and start the self-sustaining feedback.) Some filter designs reduce the overall loudness of their output as resonance increases; others allow the overall signal to grow louder.

Most filters allow you to control one or both of the above parameters (and sometimes more, like filter slope) with not just a knob, but also a control voltage – thus the name Voltage Controlled Filter.

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