AW: AW: VC Phaser using the LM13600

[Haible Juergen]

	>To my knowledge you are theoretically right, but practically a
"phaser" is  
	>not a perfect delay. 

The delay is a function of frequency. It's not constant.
(but it's "fairly constant at low frequencies.")
My point was that it's always a *positive* delay, regardless of
the implementation of the all pass filter stage.

	>It is an allpass filter, which makes a delay, but only  
	>below the centerfrequency. Around and over the centerfrequency, the
delay  
	>gets shorter. Therefor Tietze/Schenk do not call it delay but
"frequency  
	>depending phase shift".

Sure. Which doesn't mean that you can't look at it as a "frequency
dependent delay" as well. One is the d/df of the other. Phase is
ambiguous in respect of N*360 degrees, and a 180 degrees
difference looks the same as a (zero delay) signal inversion.
Looking at the Delay rather than just the Phase may help avoiding
some confusion (in the original question). When we're going
on to look for the peaks and notches of the whole phaser, the use of Phase
rather than Delay is much more comfortable, of course.

	>The (audible) difference between phaser an flanger is the
difference of the  
	>combfiltershapes which result from the difference between the
perfect digital  
	>delay that is used for the flanger and the "nonlinear" (don't know
how to  
	>explain) delay of the phaser.

A delay that's constant over frequency (as in a Flanger) will result in
an extremely high amount of phase shift for high frequencies.
(The same very basic mathematical relationship
of phase shift, delay and frequency again.) So for high freq
you're crossing N*360 degree lines all the time. Thus the 
"comb" filter effect when you add the non-delayed signal.

As for implementation, a Flanger is no way restricted to digital
implementation. Reportetly it was invented with two tape reels
running in parallel (therefore the name), and there are many 
BBD-based flangers out there.

And as for "perfect digital delay", I always have problems with 
the words "perfect" and "digital" in one sentence (;->).

JH.