[sdiy] SVF & phase

[patchell]

jhaible wrote:

>
> A filter in the EE sense (not in the musical sense, where nonlinearities
> are an important part of the sound) is a *linear* circuit. (It's certainly
> a matter of definition, but that's the definition I learned.)
> Your circuit is not only highly nonlinear, it's also unable to separate
> the frequencies of a two-tone signal, letting one pass, but not the other.
> Which is a result of being nonlinear, of course: In linear circuits
> you have the principle of superposition (sp?), i.e. you can look
> at each spectral component independently. You loose this with
> nonlinearity, and the circuit you described is the most excellent
> example to show this. (They didn't show this example in college
> just for that very reason, did they ?!)

    Actually the lab we did was more about making a F->V converter rather than a
filter..we ran the "filter" through the  network analyzer just to show it
"worked".  And like I said, not a good filter, for just the reasons you gave.
Some of my professors had a rather strange sense of humor.

>
>
> > 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...
>
> What you can do in digital is *linear* phase (constant delay).
> Zero delay or zero phase is not possible.
>
> Now let _me_ play the devil's advocate:
>
> There is a remarkable filter effect that attenuates high frequencies and
> might not result in a phase shift: The reproduction of an analogue recording
> over a mis-aligned tape head (wrong azimuth):
>
> Questions:
>
> (1) Does this really cause no phase shift?
>
> (2) Of course it does *not* go against the above rules, even
>       if it doesn't cause phase shift. Why ?
>
> JH.

--
 -Jim
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