Envelope Follower

$7.50 $10.00

Envelope Follower


Envelope Follower implements the most common type of such a device, in which an envelope detector extracts the envelope of an input signal and uses it to control the cutoff frequency of a filter that in turn, processes the same input signal. Here, the detector performs RMS detection converted to dB, with the ability to detect values above 0 dB Full Scale (dBFS). The filter is a second-order Zavalishin state-variable LPF with gain compensation, matched analog Nyquist frequency, and soft saturation clipping. The unit is full stereo, with both channels using the same control settings. The gain adjustment prevents increasing Q from overloading the signal.

There are a couple of additional features that take the module away from a traditional Filter Envelope Follower. First, since the filter is state-variable, it seemed only polite to expose some of its other modes; while technically still FEFs, these other modes can sound very different, allowing a number of different uses, especially since you can select different modes for the two stereo channels. Second, you can put the unit into "predictive" mode, at the cost of extra latency; in this mode, the filter modulation is lagged by the number of milliseconds set on the Attack knob, so that the envelope detection has time to "catch up," as it were.


How open the filter is at any given instant is determined by the amplitude of the input signal, and six knob-set front panel control values. So long as the signal is below the threshold, it's just passed through the filter unchanged. Once it exceeds threshold, though, it starts to be filtered differently from the filter's base settings (Cutoff and Resonance, below), with an aggressiveness determined by the Sensitivity knob.

  • Attack Time (mSec) sets the time in milliseconds it takes for the filter to respond once threshold is exceeded.

  • Release Time (mSec) sets the time in milliseconds it takes for the filter response to damp down once the signal drops below threshold after having been above it.

  • Threshold (dB) sets the level in dB that an input signal must reach before being modified by the Envelope Follower.

  • Sensitivity sets the slope of the response curve above threshold; lower sensitivity flattens the curve, keeping the cutoff lower, while higher sensitivity makes the curve steeper, opening the filter more for passages above threshold.

  • Filter Cutoff (Hz) adjusts the filter's base cutoff before modulation.

  • Filter Resonance adjusts the filter's resonance, with gain compensation to prevent overloading.

  • The Predictive Mode Toggle turns the mode on or off. Predictive mode gives a better time response, but introduces as much extra latency between the input and output as the Attack is currently set to. This affects both channels equally, so at least, they'll stay in sync with each other.

  • The Left and Right Input and Output Meters display the values appearing at the stereo input and output jacks.

  • The Left- and Right-channel Filter Mode Toggles change the filter modes for their respective channels. Note that these toggles are multi-state; just keep pressing to cycle through all the possibilities. Holding down Alt (Opt) when clicking cycles in reverse, and holding down Ctrl (Cmd) displays a menu with all available choices.

  • The Stereo Input and Output Jacks receive the stereo input and send the stereo output.

  • The Left- and Right- channel Input and Output Level Knobs adjust the levels appearing at their associated jacks, from 0 to 100%.