Phase vs. Delay (was VC Phaser using the LM13600)

[Don Tillman]

   From: "jhaible" <jhaible at primus-online.de>
   Date: Fri, 8 Jan 1999 03:25:25 +0100

   Don:
   > 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.

Well, let's see...  Imagine a single phase shift stage in front of
you, the classic opamp circuit.

At low frequencies you'll have near 0 degrees phase shift, there's no
delay because the signal goes straight through.

At high frequencies there's 180 degrees phase shift.  Well not exactly
phase shift; it's a straightforward polarity inversion.  No delay
there either.

At the center frequency there's 90 degrees phase shift (-90 actually),
but this happens instantaneously -- it's computed from the derivative
of the the signal (or integral, depending the circuit).  That happens
instantly in the same way the you can find the slope of a curve
instantly.  So there's no delay there either.

At any point in between it's just a mix of the above three signals.
Thus no actual delay.

   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.

Good argument!  It's certainly true that a phase shift circuit makes a
fine vibrato.  But I'll claim that that this vibrato effect happens
because the phases of the component sines are changing and not because
of any Doppler-like pitch shift due to storing the signal in a medium
and pulling it out at a different rate.

  -- Don