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Low-pass Filter (LPF)

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A low-pass filter (LPF) is a type of voltage-controlled filter module that selectively attenuates or removes the high-frequency components of an incoming audio signal while allowing the low-frequency components to pass through. Low-pass filters are used to shape the harmonic content and timbre of a sound by reducing or eliminating higher frequencies.

A low-pass filter typically has a few key components:

  1. Cutoff frequency: The point at which the filter begins to attenuate the frequencies in the input signal. The cutoff frequency is often controllable via a knob or a control voltage input, allowing for dynamic modulation of the filter’s effect on the input signal.
  2. Resonance (also known as Q or emphasis): A parameter that controls the amount of feedback or resonance at the cutoff frequency, emphasizing or boosting the frequencies around the cutoff point. Higher resonance values can create a more pronounced, resonant sound, and in some cases, self-oscillation, where the filter produces a sine wave at the cutoff frequency.

A low-pass filter module can be used to process and sculpt the timbre of oscillators, noise generators, or any other audio source. By patching the output of an oscillator into the input of a low-pass filter and modulating the cutoff frequency with an envelope, LFO, or other control voltage source, you can create a wide variety of evolving and expressive sounds.

Low-pass filters are a fundamental building block in subtractive synthesis and are essential in shaping the harmonic content and character of synthesized sounds.