Hello,

Hopefully someone will correct any mistakes I make...

>	Can anyone, please, explain to a begginer what's a nTh order
>filter. i.e. like a 2nd order lowpass and what is a 12db/24db filter?

The order of the filter corresponds to the order of the differential
equation which describes the filter.  A first order filter has one pole and
has a roll of of 6 db/octave.  Each order adds another 6 db.  An Oberheim
Matrix has a 2-pole filter so the frequency response rolls off at 12
db/octave.  A MiniMoog has a 4-pole filter that rolls off at 24 db/octave.
In other words frequencies one octave above the cut-off point would be
attenuated (reduced in volume) 24 db with a 4 pole filter.  The frequency
response is the ratio of two polynomials 
(a0+a1*x+a2*x^2...)/(b0+b1*x+b2*x^2...)

The biggest exponent on the polynomials is the same as the number of poles.
 (I think the a's are called zeros and the b's are called poles actually
but lets not get too complicated here...)

Digital filters are a bit different.  Instead of the differential equation
they are based on the difference equation.  The frequency response is

(a0+a1*exp(-i*theta)+a2*exp(-i*2*theta)...)/(b0+b1*exp(-i*theta)+b2*exp(-i*2
*theta)...)

Which looks to me like the zero term is an offset, and each of the other
terms traces out a circle whose radius corresponds to the coefficient.  The
first term would trace out a semi-circle as the frequency goes from 0 to
sr/2 (the Nyquist frequency) (theta goes from 0 to pi)  The second term
would complete a full circle, the third term a circle and a half etc.

>	BTW, what's resonance and how i can implement this in csound?

One of the things that give analog synthesizers their distinctive sound is
resonance.  This is a boost or emphasis of frequencies at the cut-off
frequency.  (well just before the frequency starts dropping.)  In the case
of the Moog diode ladder filter the phase at the cut-off frequency is
shifted by 180 degrees and an inverted signal is fed back into the system
which results in the resonance.  With digital filters things are a bit
different again.  For some implementations review this lists archives and
for some simple examples:

http://www.werewolf.net/~hljmm/csound/

>	This is the emphasis in the frequencies near the cutoff point?

Yes, see above.

>	And i presume that resonance and "Q" are the same thing, is this
>correct?

Umm yes pretty much. (I think) Although I think Q is sometimes used to
describe the band-width of a band pass filter but that's sort of the same
thing.

>	
>	Thanks in advance!

Your welcome,
Hans Mikelson