[sdiy] Re: moog high pass

[Karl Dalen]

>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. 

>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!!

Reg
KD

> Ethan
> ----- Original Message -----
> From: "Karl Dalen" <karldalen at yahoo.se>
> To: <synth-diy at dropmix.xs4all.nl>
> Sent: Tuesday, July 29, 2003 5:18 PM
> Subject: Re: [sdiy] Re: moog high pass
> 
> 
> > I suggest that it wont work, because you got one LP
> > filtered signal phase comparing/canceling a inverted input signal!!
> > When the cutoff point are moved close to zero (or even before that)
> > what you have is a straigt inverted "inputsignal" at the output!!
> >
> > KD
> >
> >  --- Gavin & Delia <elmystico at earthlink.net> skrev: > sum the output
> of
> > the low pass with the inverted input signal, through
> > > an op
> > > amp is fine, the low frequencies phase cancel and you're left with
> the
> > > highs.


_____________________________________________________
Gå före i kön och få din sajt värderad på nolltid med Yahoo! Express
Se mer på: http://se.docs.yahoo.com/info/express/help/index.html