[sdiy] filtering cv

[Cynthia Webster]

on 7/15/04 6:50 AM, Scott Stites at scottnoanh at peoplepc.com wrote:

> Hi Gregory,
> 
> I read a post from Cynthia Webster on a different forum (can't remember which
> now) about the Low Pass Gate.  According to Cynthia, if you eliminate the 1 uF
> input cap, the LPG can process CV's in the low pass mode and actually act as a
> lag generator.  After all, a lag or (or slew generator, portamento, etc.)
> function is really just a low pass filter with a really low cutoff.  Varying
> the cutoff of the filter should determine the time constant of the lag.  So,
> if your LP filter is DC coupled, or can be DC coupled, it should give you
> varying amounts of slew, depending on the cutoff.

Hi guys!

Yes, the Quad Low Pass Gates do work great as four Voltage Controlled Lags.

Because this was originally a Buchla circuit, it reflected Don's philosophy
of keeping the signal path and control voltage paths separate, (and thus had
all audio going through a capacitor to strip out potential DC pops and
clicks).  

I saw no reason to keep this limitation, and removed these AC Coupling caps
from all of the low pass gates, (why shoot oneself in the foot?)
Though AC Coupling is a fine way to raise the specs on a typical VCA circuit
for example.

There is provision on the boards to add a 1.0 Mfd electrolytic capacitor
in series with the inputs in order to eliminate DC inputs, (this is known as
"AC Coupling") for those who desire the utmost faith in this vintage
circuit's reproduction.

That is all about the audio, but from the control side, the Vactrols
just by nature of the way they work smooth out any unwanted glitches
there quite nicely.

> I've actually been beta testing some designs for Thomas Henry - he's coming
> out with a killer DIY book sometime before Christmas, and I'm lucky enough to
> be beta testing some of the circuits to see if they fly.

Oh!  So you're one of the guys testing these!  I hear that they are quite
lovely and have an amazingly tiny parts count...  (Polyphonic crazies please
take note)

> This particular 
> filter has an AC/DC switch input switch, with one purpose of the DC switch
> being to allow the filter to process CV's.  And, guess what?  I haven't even
> tried that part of it out yet (some beta tester, huh?) - I've been so busy
> just enjoying the filter itself - super sweet.

Try adding this very same feature to the inputs of a ring modulator -
Very Handy!

> But, I digress.  This filter
> also provides quadrature outputs in self oscillation, so that's another bonus
> to consider/search for when deriving control signals from a filter.

This last comment suggests that it is a multi-mode or state variable design.
(for example the Serge Filters are known for this same technique.  I haven't
tried this yet with a Modcan 10A Multimode filter yet, (should work?) as it
is one of the very few Modcan module types that I don't already own).

Greg, I don't think turning-up the resonance control would be terribly
exciting in your example below, but definitely keep thinking the good
thoughts!

Cynthia 

http://www.cyndustries.com/modules_quadgate.cfm


> 
> Cheers,
> Scott
> 
> 
> -----Original Message-----
> From: gregory zifcak <zifcak at hotmail.com>
> Sent: Jul 14, 2004 5:54 PM
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] filtering cv
> 
> what happens if you patch an analog sequencer through a resonant low-pass
> filter to the pitch cv input of an oscillator? i'm assuming that depending
> on the cutoff, the change would be slewed, giving a portamento effect. then
> what happens if you turn up the resonance? what does the change sound like?
> what about self-oscillation? what would the sequencer output look like?
> 
> sorry for the lack of diy. just suppose they are homebuilt modules:)
> 
> greg
> 
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