Dear sir, you made my day!
I will go to bed a bit less stupid tonight.
I didn't think trough the filtering of other frequencies, with anything other than all pass.

Thank you very much for the explanation.

--- In Doepfer_a100@yahoogroups.com , Florian Anwander <fanwander@...> wrote:
>
> Hello
>
> > Now: isn't the phase shift of this shifted signal, due to the
> > physical effect of passing through some sort of filter (something
> > like notch or BP)
> No.
>
> What we call a "filter circuit" is a circuit which has two
> characteristics, which are both related to the center frequency, and
> which change above and below the center frequency.
> 1.) The phase of the output signal in relation to the input signal
> 2.) The level of the out signal in relation to the input signal
>
> In both cases this is not an abrupt change, but the change starts slowly
> somewhere below the center frequency and ends somwhere above.
> Both characteristics change at "the same time".
>
> We know five kinds of filters: Lowpass, Bandpass, Notch, Highpass, and
> Allpass
>
> All of them have the phase behaviour in common.
>
> But only four of them have the level behabviour in common:
> * Lowpass: the output level above the center frequency is less than below
> * Highpass: the output level above the center frequency is more than below
> * Bandpass: the output level around the center frequency is more than
> below and above
> * Notch: the output level around the center frequency is less than below
> and above
>
> The allpass does not change the level, but still changes the phase.
>
> You could use e.g. a Lowpass also for a phaser, but since there is also
> this level-thing, the phaser effect would stop soon above the center
> frequency: one ocatve above the center frequency there is nothing to mix
> with the input signal anymore - it is filtered away already. Same for
> all other types of filters. With exception of the allpass: The allpass
> provides the wonderful phase shift, wothout spoiling the game by muting
> the shifted frequencies.
>
> A phaser works only with an allpass - nothing else. But the other way
> around: a phaser never can be used as a filter.
>
> Florian
>