Doepfer

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


Bakis Sirros - Parallel Worlds / Interconnected / Memory Geist

[Doepfer_a100] group owner

www. parallel - worlds - music. com

www. myspace. com/ parallelworldsmusic

www. myspace. com/ interconnectedmusic

www. myspace. com/ memorygeist

www. DiN. org. uk

www. musicamaximamagnetica. com

www. vu-us. com

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

From: Florian Anwander <fanwander@mnet-online.de>
Subject: Re: [Doepfer_a100] 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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