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Subject: Re: Triple Filter: who needs it?!?!?

From: "Dave Bradley" <daveb@...
Date: 1999-03-12

I believe I posted this before, the first time I proposed this filter. The
url below on Mark Pulver's site has a good review of the MAM filter, a nice
block diagram, AND 2 really nice RealAudio samples of a dry string patch,
and the same patch through the filter. Also some background info on
Vactrols. This will help you answer the question "who needs it".

Now that we've been talking about this filter for awhile, go check it out if
you haven't already. It's neato!

Dave Bradley
Principal Software Engineer
Engineering Animation, Inc.

> -----Original Message-----
> From: Paul Schreiber [mailto:synth1@...]
> Sent: Thursday, March 11, 1999 9:09 PM
> To:
> Subject: [motm] Triple Filter: who needs it?!?!?
> From: "Paul Schreiber" <synth1@...>
> This filter is a klone of the Korg PS3100 synth's filter. It is
> also a klone
> of the MAM filter box (made in Germany) which is a stereo version
> (hence Dave stereo remark).
> This filter (like the Moog filter in it's own way) is good for 1
> sound, but
> ∗BOY∗ what a sound!
> It makes the most W O N D E R F U L string and ambient pads you've ever
> heard!!
> The best demo I can offer is buy either "Equinox" or "Oxygene" by Jean
> Michael Jarre.
> The "swooshing" string sound is a Korg PS3100 (he also uses an
> old EH Small
> Stone phaser)
> The "smoothness" of the effect is because the filter is not really a
> traditional VCF. Rather, it is
> a filter that we "wiggle" the cutoff pots with a special device called a
> Vactrol.
> A Vactrol is a LED touching a cadmium-sulfide photocell. This
> resistance of
> the photocell varies as the
> LED intensity. The range of resistance is like 1K-100K. The Vactrol is the
> size of a sugar cube.
> Now, the "badness" of the photocell is that is is S L O W
> to react
> to changes in the light from the LED.
> This means the filter sorts of drags behind the LED, trying to play "catch
> up". This effect causes the filters
> to "slur" and "smear" when the control voltages changes slope.
> Like......the
> peaks of a triangle wave.
> It's sort of like pulling a toy car with a Slinky.
> This "badness" of Vactrols is used in 95% of all audio compressors.
> Including the $2300 Joe Meek box.
> So, for $13 buy a JM Jarre CD and check it out! (plus, they are damn fine
> CDs. I play them all the time)
> Paul S.
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