SergeModular

"Invert the output of the envelope detector using the processor
controlling the VCA."

To directly answer your question, the envelope output is patched to
the feedback VCA (through the processor, which in Harvey's patch
instructions had already been patched to the feedback VCA)

To elaborate, the output of a processor is connected to the VC input
of the VCA in the feedback loop. The input to that processor comes
from the envelope detector and, in addition to inverting that input
using one of the bipolar attenuators, the offset control is set to
+5v so that the feedback VCA is fully open except when the envelope
detector is active, reducing feedback amplitude to keep the overall
gain from getting out of hand. You may need to play with the amount
of the processor inversion to get the behavior to suit your taste.

Hope this helps!

Chris


--- In SergeModular@yahoogroups.com, "simulacreant"
<matfhew.carpenfer@...> wrote:

>
> 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.
>