FXAlg #901: TrigEnvelopeFilt
Algorithm Reference-118
FXAlg #901: TrigEnvelopeFilt
Triggered envelope-following stereo 2-pole resonant filter
Allocation Units:
2
The triggered envelope-following filter is used to produce a filter sweep when the input rises above a trigger level.
The triggered envelope-following filter is a stereo resonant filter with the resonant frequency controlled by a
triggered envelope follower. The filter type is selectable and may be one of low pass (i), high pass (ii), band pass
(iii), or notch (iv). See the previous section of this book, FXAlg #900, for diagrams of the filter actions.
Block diagram of Triggered Envelope Filter
The resonant frequency of the filter will remain at the minimum frequency (Min Freq) prior to being triggered. On
a trigger, the resonant frequency will sweep to the maximum frequency (Max Freq). The minimum and maximum
frequencies may be set to any combination of frequencies between 16 and 8372 Hz. Note that the terms minimum
and maximum frequency are a reference to the resonant frequencies at the minimum and maximum envelope
levels; you may set either of the frequencies to be larger than the other. 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.
When the input signal envelope rises above the trigger level, an envelope generator is started which has an instant
attack and exponential decay. The generated attack may be lengthened with the smoothing parameter. The
smoothing parameter can also lengthen the generated decay if the smoothing rate is lower than the decay. The
generated envelope is then used to control the resonant frequency of the filter.
The time constant of the envelope follower may be set (Env Rate) as well as the decay rate of the generated envelope
(Rel Rate). After the detected envelope rises above the Trigger level, a trigger event cannot occur again until the
signal drops below the Retrigger level. In general, Retrigger should be set lower than the Trigger level. A level meter
with a trigger marker is provided.
Parameters:
PAGE 1
Wet/Dry
0 to 100%wet
Out Gain
Off, -79.0 to 24.0 dB
FilterType
Lowpass
Min Freq
16 to 8372 Hz
F
Max Freq
16 to 8372 Hz
0Hz 2k 4k 6k
Resonance
0 to 50 dB
Resonant Filter
Envelope
Follower
Trigger
Generator
Triggered
Envelope
Generator
L Input
R Input
L Input
R Input
