[sdiy] Oberheim Xpander VCF

[Tim Ressel]

Folks,

How about using digital pots for adjusting the mixer and setting the Q/gain of the filter? Then you can have a library of settings to select from. 

Fascinating thread.

--TimR


--- On Thu, 1/15/09, Scott Nordlund <gsn10 at hotmail.com> wrote:

> From: Scott Nordlund <gsn10 at hotmail.com>
> Subject: RE: [sdiy] Oberheim Xpander VCF
> To: 
> Cc: synth-diy at dropmix.xs4all.nl
> Date: Thursday, January 15, 2009, 10:29 AM
> You can get all of these responses by simply using a mixer
> to combine the outputs of each filter stage (positively or
> negatively).  It actually helps as well to mix the input
> signal (5 input mixer), though this isn't done in the
> Matrix 12/Xpander because the oscillator mixing is done
> inside the chip.
> 
> I have made a quick and dirty software model to experiment
> with this (to see if a mixer is a good idea), and it seems
> to work fairly well.  The way the mixer settings correspond
> to filter responses isn't always intuitive unless
> you're thinking about Laplace transforms, so it may be
> difficult to simply dial up some exotic response.  Also, as
> has been mentioned, a lot of the filter types don't
> sound radically different from each other, but I prefer to
> think of it as a useful way to subtly adjust the tone of the
> filter (in ways that aren't ordinarily possible), and
> this actually works better with a continuous mixer.
> 
> One roadblock I ran into is that the filter must not reduce
> its passband gain as the resonance is increased.  Without
> some sort of gain compensation, the mixing coefficients
> needed to get a certain filter response may change
> (sometimes drastically) as resonance is adjusted.  I'm
> not sure how this is usually done.
> 
> A question about the N2P2N2T, etc..   Are you adjusting the
> cutoff frequencies of the individual filter stages
> independently to get some of these (continuously or by
> switching a capacitor)?
> 
> ----------------------------------------
> > Date: Thu, 15 Jan 2009 15:52:40 +0000
> > From: andylist at vellocet.com
> > To: ajhuovil at cc.hut.fi
> > Subject: Re: [sdiy] Oberheim Xpander VCF
> > CC: synth-diy at dropmix.xs4all.nl
> >
> > And you can get many more responses than the ones in
> the oberheim
> > xpander service manual, one of my favorites is -12 st
> Notch, 0 st
> > Resonant peak, +12 st Notch called N2P2N2T (T for twin
> or tripple in
> > this case)
> >
> > Here are some plots:
> > http://www.vellocet.com/dsp/CascadedFilterResponses/
> >
> > Andy
> >
> > Antti Huovilainen wrote:
> >> On Thu, 15 Jan 2009, George Hearn wrote:
> >>
> >>> I've been pointed in the direction of the
> Oberheim XPander VCF. I think
> >>> it's quite incredible how 16 filter modes
> can come out of one
> >>> filter. Does
> >>> this not make it about the most versatile
> analogue filter design ever?
> >>
> >> You can get the same modes (and over a dozen more)
> from any 4 pole
> >> cascaded RC style filter (Moog ladder, CEM, OTA
> etc).
> >>
> >> Antti
> >>
> >> "No boom today. Boom tomorrow. There's
> always a boom tomorrow"
> >> -- Lt. Cmdr. Ivanova
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