[sdiy] SVF & phase

[jhaible]

Yes. Phase and Amplitude are linked in any filter (with one small exception,
see below), just as real and imaginary part are linked. (Hilbert
transform !)

The exception is that you can build filters which have *more* phase shift
than the above, i.e. you could separate such a filter into the basic
amplitude
function (with its linked phase function!), plus an all pass filter part.

So you can get phase shift with constant amplitude (all pass filters), but
not amplitude filtering without phase shift.

JH.


> 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?