4-Pole filter with 2 VCFQs
2007-05-13 by simulacreant
Hi,
I'm having a bit of trouble recreating this patch. Basically, I don't
understand the instructions for implementing the amplitude control.
They say to 'monitor the VCA output by patching to an envelope
detector(I'm using a DTG with signal in) but they don't say where to
patch the inverted output of the envelope detector.
Does anyone have an alternate patch or a differently worded
description of this patch? Try following the directions and let me
know if you figure it out.
Thanks!
Here's the full text:
4-Pole filter, by Harvey Devoe Thornburg
How to get a *killer* 4-pole filter sound from two VCQF's
Required: 2xVCQF, 1 VCA, 1 envelope detector, 1 dual
processor/equivalent (used for offset inversion)
The VCQF is a state-variable 2-pole. Although it has a very nice,
clean sound (great for processing drum loops) it falls short if you
expect that Minimoog/VCS3/TB-303 type sound. By using controlled
feedback around 2 VCQF's in series, you can get a nonlinear filter
that IMHO sounds even better than any of the aforementioned filters.
The patch may seem complicated so it's good to start with an overview.
Clearly, two two-poles in series gives us a four-pole. The problem
with this open-loop series configuration is that you'll get large
amplitude swings when sweeping over a harmonic signal at high Q.
Unless you like this sound, you'd remedy this by using a compressor.
But what can you do about self-oscillation? The trick is to use
feedback to generate Q externally, but control the amplitude of this
feedback by monitoring the overall output amplitude. How is feedback
used to generate Q? We use Moog's old trick of feeding back the
inverted output of the open-loop 4-pole combination. By themselves,
the filters provide 180 degrees of phase shift at the cutoff
frequency. Inverting this generates another 180 degrees of shift,
which regenerates the signal at the cutoff frequency, providing a
resonant peak at that frequency. We get a screeching self-oscillation
at uncontrollable high amplitude. This establishes the need for
external amplitude control. If we can monitor the signal amplitude
somewhere in the feedback path, then we can use a VCA to attenuate the
feedback gain, keeping the "screeching" well-behaved. Now when we
play with If the feedback gain gets too large (>4), this configuration
will go unstable.
Patch:
The input for this configuration will be the regular input
(the "IN") for the first filter. The output will be the LO output of
the second filter. Track the cutoffs of both filters in parallel
(*make them equal at all times*), and set the Q to minimum, since
we're generating the resonance externally. Also if there is a "gain"
knob set this to maximum. (on the VCFX the gain is always set to
maximum). Patch the LO output of the first filter to the input of the
second. Now create the feedback loop. Patch the BAND output of the
second filter to an inverter (use one of the dual processor sections,
with the scaling knob full left). Take the output of the inverter and
run it through a VCA. Patch the output of the VCA to the AGC input of
the first filter, thus closing the loop. It is highly recommended to
use a UAP as the VCA, because we want something capable of large gains
(4x is necessary for self-oscillation). Offset the output of the
second processor section to +5 volts, and patch this to the VC input
of the VCA.
At this point, the only thing left to do is to implement
amplitude control. Monitor the VCA output by patching it to the input
of an envelope detector. Invert the output of the envelope detector
using the processor controlling the VCA. You won't need to do a full
inversion: turn the knob only partially left. Now you should have a
great sounding four-pole filter!!! You can play with the Q control on
the first filter to vary further nuances in the sound. It's best that
the input to this filter be full-amplitude, like a sawtooth or square
wave from NTO.
I'm having a bit of trouble recreating this patch. Basically, I don't
understand the instructions for implementing the amplitude control.
They say to 'monitor the VCA output by patching to an envelope
detector(I'm using a DTG with signal in) but they don't say where to
patch the inverted output of the envelope detector.
Does anyone have an alternate patch or a differently worded
description of this patch? Try following the directions and let me
know if you figure it out.
Thanks!
Here's the full text:
4-Pole filter, by Harvey Devoe Thornburg
How to get a *killer* 4-pole filter sound from two VCQF's
Required: 2xVCQF, 1 VCA, 1 envelope detector, 1 dual
processor/equivalent (used for offset inversion)
The VCQF is a state-variable 2-pole. Although it has a very nice,
clean sound (great for processing drum loops) it falls short if you
expect that Minimoog/VCS3/TB-303 type sound. By using controlled
feedback around 2 VCQF's in series, you can get a nonlinear filter
that IMHO sounds even better than any of the aforementioned filters.
The patch may seem complicated so it's good to start with an overview.
Clearly, two two-poles in series gives us a four-pole. The problem
with this open-loop series configuration is that you'll get large
amplitude swings when sweeping over a harmonic signal at high Q.
Unless you like this sound, you'd remedy this by using a compressor.
But what can you do about self-oscillation? The trick is to use
feedback to generate Q externally, but control the amplitude of this
feedback by monitoring the overall output amplitude. How is feedback
used to generate Q? We use Moog's old trick of feeding back the
inverted output of the open-loop 4-pole combination. By themselves,
the filters provide 180 degrees of phase shift at the cutoff
frequency. Inverting this generates another 180 degrees of shift,
which regenerates the signal at the cutoff frequency, providing a
resonant peak at that frequency. We get a screeching self-oscillation
at uncontrollable high amplitude. This establishes the need for
external amplitude control. If we can monitor the signal amplitude
somewhere in the feedback path, then we can use a VCA to attenuate the
feedback gain, keeping the "screeching" well-behaved. Now when we
play with If the feedback gain gets too large (>4), this configuration
will go unstable.
Patch:
The input for this configuration will be the regular input
(the "IN") for the first filter. The output will be the LO output of
the second filter. Track the cutoffs of both filters in parallel
(*make them equal at all times*), and set the Q to minimum, since
we're generating the resonance externally. Also if there is a "gain"
knob set this to maximum. (on the VCFX the gain is always set to
maximum). Patch the LO output of the first filter to the input of the
second. Now create the feedback loop. Patch the BAND output of the
second filter to an inverter (use one of the dual processor sections,
with the scaling knob full left). Take the output of the inverter and
run it through a VCA. Patch the output of the VCA to the AGC input of
the first filter, thus closing the loop. It is highly recommended to
use a UAP as the VCA, because we want something capable of large gains
(4x is necessary for self-oscillation). Offset the output of the
second processor section to +5 volts, and patch this to the VC input
of the VCA.
At this point, the only thing left to do is to implement
amplitude control. Monitor the VCA output by patching it to the input
of an envelope detector. Invert the output of the envelope detector
using the processor controlling the VCA. You won't need to do a full
inversion: turn the knob only partially left. Now you should have a
great sounding four-pole filter!!! You can play with the Q control on
the first filter to vary further nuances in the sound. It's best that
the input to this filter be full-amplitude, like a sawtooth or square
wave from NTO.