[sdiy] Oberheim Xpander VCF

[Scott Nordlund]

With a microcontroller (and some clever programming), it could also be possible to save settings or interpolate between them, possibly surpassing (depending on how much work is put into it) the Doepfer A-107's capabilities.

----------------------------------------
> Date: Thu, 15 Jan 2009 11:50:23 -0800
> From: madhun2001 at yahoo.com
> Subject: RE: [sdiy] Oberheim Xpander VCF
> To: synth-diy at dropmix.xs4all.nl
>
> 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  wrote:
>
>> From: Scott Nordlund 
>> 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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