Begginner with questions about VCF's..

[Prof. Antonio Guerrieri]

>Hello,
>I'm new to the list and I've been kinda hanging back for awhile trying
>to absorb some of the posts... I'll admit alot of it goes over my head..
>I was wondering if there was any difference,other than the resonance
>adjustment, between a VCF and an electronic crossover like those used in
>high-end home and car audio??? Also, what exactly does the resonance
>potentiometer do?? I know what it sounds like, but what technically is
>going on when you change the resistance???
>                                           Thanks in advance, Jon
>
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	Dear John,

in the following the main differences:

- a VCF is an "active" filter whereas a crossover is usually a "passive"
filter: generally speaking, this means  that the output signal level is
lower with respect to the input signal one (gain lower than 1);

- in this respect, the gain of such devices is not the unique parameter
that make the difference: in fact, an active filter permits also other
features such as higher roll off slopes;

- both can be designed and used as low pass, band pass, notch or high pass
filters: however, crossovers are not tunable, i.e. the frequency cut-off is
usualy already setted at the desired frequencies (for example, to the low,
medium and high frequencies needed for a 3-way loudspeaker); at the
contrary, in VCFs the cut-off frequency can be continuosly changed
(manually, by changing the resistance of a potentiometer or better
indirectly using a control voltage) in all the four modes, accordingly to
the desired musical needs;

- finally, in a first approach I can say you that some active filters are
designed in a such a way that the frequency gain is much higher at the
cut-off frequency than the others one; the resonance parameter controls
such a gain: increasing the resonance increases the gain of the filter at
the cut-off frequency; note that in some (and the more interesting)
designs, the gain can be so increased that the filter become an oscillator,
i.e. its output is a sine wave of frequency equal to the cut-off frequency.

That's all.
Sorry for my bed English and for some inevitable mistakes.

Antonio