Voltage controlled filter

An audio filter is a frequency dependent amplifier circuit, working in the audio frequency range, 0 Hz to beyond 20 kHz. Audio filters can amplify (boost), pass or attenuate (cut) some frequency ranges.

Common filter types used in audio synthesis include:

• Low-pass (high-cut) filter: removes higher frequencies
• High-pass (low-cut) filter: removes lower frequencies
• Band-pass filter: removes frequencies outside a given band
• Notch filter: removes frequencies within a given band
• Shelf filters: raise or lower frequencies above or below a cut off point.
• Peak filters: Raise frequencies within a given band
• Formant filters: raise multiple peaks, often in such a way that they mimic the human voice.

There are many ways to implement most filters, and each has its own specific audio characteristics or flavour. Some common features across most filter types include:

• A cut-off frequency - the frequency at which the filter begins to remove frequencies. This is often voltage-controllable, and can changes over time (e.g. controlled by an envelope, or an LFO).
• resonance - how much the filter boosts the frequency at the cut-off point. This may also be voltage-controllable.
• frequencies beyond the cut-off often have their phase affected.