[sdiy] SVF & phase

[patchell]

    Yes, this is correct...however, I hate to be a real Naysayer...When I was in
college, one of our labs was to build a Brick Wall Low Pass Filter.  It had no
passband ripple and at 1000 Hz, it attenuated the passband by 0 db, and at
1000.01Hz it was over 100 db of attenuation, and, there was no phase
shift....sounds great, huh?  How did we implement this filter?

    We used a crude F->V converter, and when the voltage (frequency) got up to a
certain point, it would trip an analog switch preventing the signal from getting
through.  On the Network Analyzer it looked like the perfect filter, however, if
you were to use this in practice, it would not be a good filter at all...still, I
cannot say you can't make a filter with zero phase shift.  I seem to recall from
the same class that it was mention that certain digital implementations could
realize filters that you could not realize with standard components.  But, we
never got into that.  But, if you are using good old poles and zeros, you will
alsways have phase shift.  Although, like I said before, having no phase shift I
would think, would make the filter sound uninteresting anyway...

Grant Richter wrote:

> Mark's question makes me question my assumptions about filters. Maybe
> someone can clarify.
>
> I am under the impression that a filter without phase change would not be a
> filter at all?
>
> I mean, the phase change with frequency is what allows the amplitude change
> with frequency?
>
> Because the amplitude response is the "real" axis of the S plane and
> movement along the "imaginary" (phase) axis is how the shape of the real
> axis is determined?
>
> > From: patchell <patchell at silcom.com>
> > Date: Tue, 26 Nov 2002 12:08:37 -0800
> > To: mark s <n0nspaz at mindspring.com>
> > Cc: synth-diy at dropmix.xs4all.nl
> > Subject: Re: [sdiy] SVF & phase
> >
> > Well, if you could build a filter like that, it would probably not sound
> > very interesting.  The changing phase relationship between the overtones is
> > what make a filter sound interesting...IMHO...
> >
> > One thing for sure, you will have a difficult time making a filter with
> > zero change in phase.  You can make one that has a constant (or near
> > constant) time delay vs frequency, but that is about it (time delay and
> > phase shift are related)..  Your only hope may be in the digital domain,
> > using finite impulse response filters (a subject I am not at all familiar
> > with), but even with that, I am doubtful.
> >
> >
> >
> > mark s wrote:
> >
> >> greets,
> >> what are some options for building a state-variable filter with a
> >> better-than normal phase response ? what would one have to do to make an
> >> SVF that had zero change in phase  (theory or reality) ?
> >>
> >> mark
> >
> > --
> > -Jim
> > ------------------------------------------------
> > * Visit:http://www.silcom.com/~patchell/
> > *-----------------------------------------------
> > *I'm sure glad Merry Christmas comes just once a year
> > * -Yogi Yorgensen
> > ------------------------------------------------
> >
> >
> >

--
 -Jim
------------------------------------------------
* Visit:http://www.silcom.com/~patchell/
*-----------------------------------------------
*I'm sure glad Merry Christmas comes just once a year
* -Yogi Yorgensen
------------------------------------------------