Envelope filters?

Originally introduced as a key component of analog synthesizers, envelope controlled filters came into prominence in the early 1970s with the release of the legendary Mu-tron III. Envelope filters are wah-wah type sliding filters that are triggered by an input voltage. Unlike standard wah pedals whose center frequency is controlled by moving a treadle, an envelope filter's center frequency is controlled by playing dynamics, allowing for an almost unlimited variety of effects based on expressive nuances in a player's performance.

The filter can be:
  • Lowpass filter: cuts all frequencies above the filter cutoff frequency.
    Only the lower frequencies pass (hence the name).
  • Bandpass filter: cuts both frequencies higher and lower than the filter cutoff frequency.
    Only a certain band of frequencies pass (hence the name).
  • Highpass filter: cuts all frequencies below the filter cutoff frequency.
    Only the higher frequencies pass (hence the name).
  • Notch filter: cuts frequencies in a range around the filter cutoff frequency and passes all frequencies lower AND higher than that range.
  • Tone control: boosts a small range of frequencies, but doesn't block the other frequencies.

These are the most common controls on envelope filter effects:
  • Sensitivity: controls how the filter responds to the input signal, at which input level the effect starts responding and how much the effect responds to the envelope of a certain input.
  • Range: defines the frequency range in which the filter moves, this can be a low/high switch or a rotary pot.
  • Depth: sometimes called range, defines the amplitude of the filter sweep.
  • Up/Down Mode: selects if the filter moves up- or downwards in response to the input signal.
Some interesting links:
autowah, auto-wah, touch wah, wah-wah, wha-wha dynamic filter, envelope filter

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