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KDFX Reference
KDFX Algorithm Specifications
900 Env Follow Filt
Envelope following stereo 2 pole resonant filter
PAUs: 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), high pass (ii), band pass (iii), or notch (iv).
Figure 10-65 Resonant Filters: (i) lowpass; (ii) highpass; (iii) bandpass; (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 58 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.
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.
The attack and release rates of the envelope follower are adjustable. The rates are expressed in decibels per
second (dB/s). The envelope may be smoothed by a low pass filter which can extend the attack and release
times of the envelope follower. A level meter with a threshold marker is provided.
(i) (ii)
(iii) (iv)
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