[sdiy] Re: moog high pass

[Magnus Danielson]

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, 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.

> >Lots of filter topologies do exactly
> > this to obtain a highpass (state variable for example). I've certainly
> > used
> > this trick lots of times to get my sampler to do highpass filtering. You
> > can
> > also construct bandpass/bandstop filters this way by taking the
> > difference
> > of the outputs of two LP filters with different cutoff's.
> 
> Yes, but you are dealing with a "HP filter" allready in your examples!
> Think in this way, take two signals run them in paralell one is inverted
> the other one is LP filtered, now sum them at the output. Now remove
> the LP filtered signal (equal to closing down the Cf point).
> What do you have at the output? A inverted inputsignal!!

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.

Cheers,
Magnus