[sdiy] Oberheim Xpander VCF

[Scott Nordlund]

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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