Hi Bakis

> dieter doepfer: "[...]Maybe someone
> of the newsgroup knows detailes too (Florian ).[...]

The "comb filter" thing I already mentioned in the mail from 4th of
February. I wrote
"[...]by an allpass filter (which is used also for phaser) or by a delay.
Both do similar but technically not identical effects."
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I thought this was very clever, since I had not to explain the difference
in detail ;-) ok, now you've got me...

The difference between the (analogue) phaser and the flanger in reality is
the technical way, how a signal delay is created.

The delay in the flanger is realized with some kind of signal storage,
which does not change the signal itself generally. The stored signal is
kept for a while and then played back. This is realized either with digital
storage or in older flangers and guitar efx with so called bucket bridge
delays.

The phase works also with a delay, but this time the delay is realized with
an so called allpass filter. An allpass filter is a circuit which does
not(!) filter. This sounds senseless as long as you think filtering is the
only thing that a filter does; but a filter does a lot more:
- it does a very(!) small delay (this is, what the phaser wants to use)
- and it shifts the phase of partial notes above the cutoff-frequency
(yes an allpass filter has still something like a cutoff-frequency,
though it does not cutoff anything). This phaseshift is originally
not intended, but it is typical for all analogue filter designs.


As we already learned: A comb effect is generated by elimination of partial
tones, which happen when a delayed signal is mixed together with the non
delayed signal. Now you can imagine, that the phase shift of the partial
tones in the allpass will cause, that other partials will be eliminated.

So far the technical background.


__Which__ special frequencies are eliminated and how this looks (linear or
logarithmic...) I do not remember. Sorry. I know there is a paper somewhere
deep in the big paper sink in my room ;-) but I cannot promise, that I will
find it. So I fear the answer won't be better than the one, you already
received by Dieter.

Florian



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