VC Phaser using the LM13600

[jhaible]

> A phase shifter circuit really has no delay, above or below the center
> frequency.  It just shifts the phase.  This is important: any change
> at the input of a phase shifter will always show up in the output
> immediately.

At least *something* will show up in the output immediately. But the
shape of your input will be altered, as different spectral components of
your
input signal will get different delay. That's how I think it works, at
least.
The concept of group delay is difficult to apply to real world signals, as
the d phi / df at a certain frequency asumes a sine signal that was
switched on a long time before. Nevertheless it has some relation to
finite, real life signals, too. Think of a burst of several cycles of a
certain
frequency - the group delay should be seen at least approximately.
Or think of the response to a input step - the change in shape at the
output resembles the (different) delay for each frequency component
of the input step.
Another argument that a "real" delay is involved in all pass filters:
You can make a nice vibrato just with a variable all pass filter. Just
as in a BBD, you can get positive and negative frequency deviation
because there is some delay to "store" the signal for a short time.

JH.