[sdiy] Oberheim Xpander Multimode VCF Question

[karl dalen]

Or simply take five trim pots one for each tap and
trim until your ears declares a nice swoosh sound.
Sometimes what looks neat on paper dont sound to
interesting. 

KD

--- Den tors 2010-03-11 skrev Magnus Danielson <magnus at rubidium.dyndns.org>:

> Från: Magnus Danielson <magnus at rubidium.dyndns.org>
> Ämne: Re: [sdiy] Oberheim Xpander Multimode VCF Question
> Till: "Steven Cook" <stevenpaulcook at tiscali.co.uk>
> Kopia: synth-diy at dropmix.xs4all.nl
> Datum: torsdag 11 mars 2010 15.34
> Steven Cook wrote:
> > Hi,
> > 
> > Is it possible to get a -24dB/oct allpass response out
> of an Oberheim Xpander type of 4-pole filter arrangement?
> I've got the following for an -18dB allpass, from a post by
> David G. Dixon...
> > 
> > input + pole1*3 - pole2*6 + pole3*4
> > 
> > Which seems to work fine (in software).
> 
> I'm in a hurry, so you will not get the complete answer
> now... from me at least.
> 
> First of all, you can't talk about -24 dB/Oct slope for an
> all-pass filter. While I am happy that you say -24 dB/Oct
> for low-pass, it is meaningless for all-pass as they are 0
> dB/Oct. This only once again brings out the point... the
> slope does not tell how many poles and zeros there is in a
> filter, just how many there at least needs to be.
> 
> Now, the actual question is, can you create a 4-pole/4-zero
> all-pass filter from the Xpander module? The quick answer is
> yes.
> 
> What I don't have time to help you with is how to combine
> the outputs from the different taps, but you are on the
> right path when you combine input with the taps.
> 
> For non-complex poles, you have (s+a)/(s-a) for each stage.
> For complex poles (s+a-jb)(s+a+jb)/(s-a-jb)/(s-a+jb). Notice
> how the real term a shift sign from poles to zero. Any
> all-pass filter is then just a combination of those using
> various sets of a and b. You should be able to pull it out
> from there with some paper-and-pen work, which is the thing
> I don't have time for right now.
> 
> If you still have problem, let me know. I might be able to
> do the homework for you later.
> 
> Cheers,
> Magnus
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