Doepfer

Hi Jonathan

> Sorry, i meant comb filter.

comb filters are based on the principle of phase cancelation around a 
certain frequency. They can be achieved by many different means:

1.) Each analog filter does a phase shift around the center frequency. 
For example: a multimode filter like the A121 does this: if you mix the 
high pass and the low pass, you will get a notch-filter, which one may 
regard as a single notch comb filter. If you would add several 
multimodefilters at different center frequencies, you would get a multi 
notch comb filter.
2.) This kind of a "multi notch comb filter" can be achieved more easily 
using several stages of so called allpass filters, which in fact do not 
filter at all, but they still do the phase shift around their center 
frequency. If you mix this "filtered" signal with the original signal 
you will get again a comb filter. If you modulate the center frequency 
of this chain of allpass filters you will get what we call a "Phaser".
3.) A different approach is the usage of a delay line. A delay will 
cause phase cancelations at certain frequencies, if the delay time 
corresponds to the wave length of this frequency. If you modulate the 
delaytime you will get what we call a "Flanger".

These three cases differ by the position of the notches in the frequency 
spectrum
In case #1 it depends on the different center frequencies of these 
filters, which could be selected free (in theory).
In case #2 it is spread in linear steps over the frequencies (e.g. 200, 
400, 600, 800... Hz).
In case #3 the notches appear each doubling of the frequency (e.g. 200, 
400, 800, 1600 ... Hz).

Florian