[sdiy] SVF & phase

[jhaible]

>     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.

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 ?!)

> 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.