[sdiy] Oberheim Xpander VCF

Thu Jan 15 23:59:46 CET 2009

Doesn't this technique require inverted versions of the outputs as  
well? Balanced modulators instead of VCAs?

Mark

On Jan 15, 2009, at 5:30 PM, George Hearn wrote:

>
>> 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.
>
> 	How about a single 8bit microprocessor IC running four PWM channels
> filtered to give four DC values controlling the gain of four VCAs  
> one on
> each filter stage with their outputs mixed.  The buttons on the MCU  
> port
> pins could select between 'preset' filters like LP/BP/HP and the ADC  
> inputs
> (if the microprocessor has them) could be used to sweep the  
> different gains
> to give any desired response?  Yes it's a bit of programming but  
> wouldn't it
> be a clever little beast!  G
>
> ----------------------------------------
>> 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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