once again, thanks Florian for your useful and detailed reply!


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--- On Mon, 11/15/10, Florian Anwander < fanwander@... > wrote:

From: Florian Anwander < fanwander@... >
Subject: Re: 1 Re: Doepfer Combi Filter
To: Doepfer_a100@yahoogroups.com
Date: Monday, November 15, 2010, 10:44 AM







 









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

























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