[db filters?]

[Harry Bissell]

This will get everybody going... list not quiet anymore
db / filters. A single RC (resistor capacitor) network will usually give a
6db/octave attenuation. This is often called a "single pole" filter. It
doesn't roll off very quickly. Two of these stages in series would be a
"two-pole" or 12db/octave. This is useful and gives a bright, "synthesizer" 
sound (like the ARP oddessy) The next filter that is coomon is the four pole,
or 24db/octave. Like the Minimoog, it gives a darker sound with the high
frequencies falling more rapidly. I've assumed you mean low-pass filter, there
are also high-pass (cuts lows), Band-pass (cuits lows and highs) Band-reject,
or notch (cuts out a center band of frequencies), and all-pass (which is the
basis for the "phase shifter" effect.) All of the above can have resonance
added, which makes a "peak in the response near the frequency where the
attenuation starts. This gives a "wah" effect to high and low pass filters,
makes bandpass and notch filters sharper (they boost or cut a more narrow
range of frequencies). Add wenough resonance and most filters (at least 12db
or higher) will oscillate.
    Other unusual features have to do with how the filter behaves when the
resonance is increased. The Minimoog (also called "ladder") filter will reduce
the amplitude of the input signal, so the the peak level remains nearly
constant when the resonance in increased. The "state-variable" filter (like
the oddessy and Oberheim SEM) filter keeps the passband amplitude the same,
and makes the peak louder as resonance is increased.
    Filters behave differently when they are in high resonace mode. In some
the input signal is still the the most obvious sound, and in others, once the
filter begins to oscillate, it drowns out the input signal. FFilters will also
give unique sounds when overdriven, some clip the signal, some limit, some
"fart" etc... There are varing degrees of control voltage interaction which
give other thumps and clicks wich endear some people to some filters, and make
others hated.
    All these filters are usually controlled by voltage for cutoff frequency
and resonance, and are varied over time. I'd say the filter is the single most
important determinant of the character of a synthesizer. Discussions in this
forum will bear that out... Now, count the number of flaming messages that
follow this one as everyone votes for their favorite. 
    P.S. You want as many filters as you can get. All kinds of them. The more
you have the more your peers will envy you...   :-) Harry Bissell
    P.S.S. My favorites are the Moog Ladder and the ARP Oddessy (2-pole).



I have a question..

I've noticed that some synths have different filters you can switch to;
like a 24db filter, 12 db filter 8 db filter (do i say this correctly? i
hope so..) and i was wondering:
how important are those features in an analogue or virtual analogue syth or
synthmodule (in terms of the _variety_ of sounds it can make through
them..) more specifically i mean:
how important are these? and is it like the more you have of these the
better you are off..?

sorry for the rather strange question :) hope i don't offend anyone :)

Joel


____________________________________________________________________
More than just email--Get your FREE Netscape WebMail account today at http://home.netscape.com/netcenter/mail