FXAlg #900: Env Follow Filt
Algorithm Reference-116
FXAlg #900: Env Follow Filt
Envelope-following stereo 2-pole resonant filter
Allocation Units:
2
The envelope-following filter is a stereo resonant filter with the resonant frequency controlled by the envelope of
the input signal (the maximum of left or right). The filter type is selectable and may be one of low pass (i), highpass
(ii), band pass (iii), or notch (iv).
Resonant Filter Types: (i) lowpass, (ii) highpass, (iii) bandpass, and (iv) notch.
The resonant frequency of the filter will remain at the minimum frequency (Min Freq) as long as the signal envelope
is below the Threshold. The Freq Sweep parameter controls how much the frequency will change with changes in
envelope amplitude. The frequency range is 0 to 8372 Hz, though the minimum setting for Min Freq is 16 Hz. Note
that the term minimum frequency is a reference to the resonant frequency at the minimum envelope level; with a
negative Freq Sweep, the filter frequency will sweep below the Min Freq. A meter is provided to show the current
resonance frequency of the filter.
Block diagram of envelope-following filter
The filter Resonance level may be adjusted. The resonance is expressed in decibels (dB) of gain at the resonant
frequency. Since 50 dB of gain is available, you will have to be careful with your gain stages to avoid clipping.
(i)
(ii)
(iii)
(iv)
R Input
L Input
Envelope
Follower
Resonant Filter
R Input
L Input