[sdiy] Re: moog high pass

[Karl Dalen]

 --- Magnus Danielson <cfmd at swipnet.se> skrev: > From: Karl Dalen
<karldalen at yahoo.se>
> Subject: Re: [sdiy] Re: moog high pass
> Date: Wed, 30 Jul 2003 02:56:53 +0200 (CEST)
> 
> > >Ethan Duni <eduni at ucsd.edu> skrev: > Except for the inversion, isn't
> > >that exactly what you'd expect out of a highpass filter with a cutoff
> > > of 0? 
> > 
> > Yes, but his suggestion with comparing with inverted "unprocessed"
> > input signals will leave a inverted input signal at the output when Cf
> > is closing down to zero. Imagin this topology with high Q. 
> 
> Actually, what you gain is a fiddeling of the zeros in a sufficiently
> close  fashion as one wants.
> 
> Summing with the input signal isn't a bad idea,

It still dont give me real HP filtering, it fades between a 
inverted input signal and a LP filtered signal and that is not
a real HP filter. (holding my thumbs real hard, hoping im not
entierly wrong into this :-)..!!).
 
>easpecially if you
> consider the alternative, which is to dump multiple, if not all, of the
> diffrential voltages over the caps in the ladder and then combine these
>in a matrix  form  into the state-variable signals of a 4-pole
>state-variable filter. It's  possible but messy.

Exactly, that is implementing the OBI Xpander filter
solution to a Ladder design as you say messy but should work.
Infact i had some ideas of going this way! 

> You should really invert the lowpass signal then. You get a difference
> signal
> either way, the only thing is the sign of the difference signal.

Yes, and as far as i understand that wont give me a real 
HP filter either, as you say it only changes sign. Still as i can
understand the input signal goes straigt trough when Cf is closed
no matter if the LP signal or the input signal is inverted!

Reg
KD

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