A VCF shapes timbre by removing frequencies above (LPF), below (HPF), or outside (BPF) the cutoff

A Voltage Controlled Filter (VCF) selectively attenuates portions of a signal’s frequency spectrum. The most common type is the low-pass filter (LPF), which passes frequencies below the cutoff and attenuates those above — as cutoff rises, more harmonics come through, opening the sound. A high-pass filter (HPF) does the opposite: it removes low frequencies, leaving only the upper content. A band-pass filter (BPF) attenuates both above and below a center frequency band. The cutoff frequency of a VCF responds to control voltage, allowing envelopes or LFOs to sweep the filter over time. Filter slope (6, 12, 18, or 24 dB/octave) determines how sharply frequencies above the cutoff are attenuated.

Examples

A VCF in LPF mode with cutoff swept from low to high by an ADSR envelope: the sound opens up on attack then closes on release — classic subtractive synthesis. Setting cutoff to HPF removes kick drum boom to thin a loop.

Assessment

Given a LPF with cutoff at 1 kHz and a 24 dB/oct slope, at what frequency would a harmonic be attenuated by approximately 24 dB? Describe the tonal difference between a 6 dB/oct and a 24 dB/oct low-pass filter set to the same cutoff.