VC Phaser using the LM13600

[Magnus Danielson]

>>>>> "j" == jhaible  <jhaible at primus-online.de> writes:

 >> 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.

 j> At least *something* will show up in the output immediately. But the
 j> shape of your input will be altered, as different spectral components of
 j> your
 j> input signal will get different delay. That's how I think it works, at
 j> least.
 j> The concept of group delay is difficult to apply to real world signals, as
 j> the d phi / df at a certain frequency asumes a sine signal that was
 j> switched on a long time before. Nevertheless it has some relation to
 j> finite, real life signals, too. Think of a burst of several cycles of a
 j> certain
 j> frequency - the group delay should be seen at least approximately.

Well... there are two types of delay, phase delay and group delay.

If phi(w) is the phase responce of a network...

Phase delay is the delay of a single sine signal, the phase delay is

         phi(w)
tau(w) = ------
           w

Group delay is the delay of a packet of nearby frequencies, the group
delay is

       d phi(w)
D(w) = --------
         d w

Who every said things where simple?

 j> Or think of the response to a input step - the change in shape at the
 j> output resembles the (different) delay for each frequency component
 j> of the input step.
 j> Another argument that a "real" delay is involved in all pass filters:
 j> You can make a nice vibrato just with a variable all pass filter. Just
 j> as in a BBD, you can get positive and negative frequency deviation
 j> because there is some delay to "store" the signal for a short time.

Yes, but while delay is a linear property, this is infact not a linear
property since a lineary network should be time invariant which the
allpass obviously isn't since you modulate it.

On the other hand, synths becomes sooooo boring when they are truely
linear... no VCA, VCF, no envelope, only unstable sine oscillators etc.
Be happy and slightly nonlinear ;)

Cheers,
Magnus