[sdiy] FatMan Hacking ( and I dont mean hitting me with anAxe!)

[Magnus Danielson]

From: harry <harrybissell at prodigy.net>
Subject: Re: [sdiy] FatMan Hacking ( and I dont mean hitting me with anAxe!)
Date: Sun, 28 Oct 2001 01:19:19 -0500

> Again... "guru mode off"

On who? Jürgen?

> The Oberheim Matrix filter is more like a cascaded integrator filter... local
> feedback around each stage, one global feedback path. very much like
> the SSM2040, CEM3320, etc.  If you tap each section, and sum with correct
> amplitude and polarity you can get "any" response you want.

Yes, in a similar sense to that of a state variable filter. What
causes them to be so "nice" is that they make a simple mapping from
the mathematical theory board of pole and zero polynoms, you just take
the coefficients of the polynom and convert them one by one into the
mixing values. If this makes a usefull filter is however another
thing. What makes me wonder is why they (I've just looked at the
Xpander schematics) did not go for a 4 pole HP. What makes it more
difficult to get different response out of a non-SVF filter is simply
because you don't got the simple relations. The Xpander schematics
shows just which type of processing which is needed. The variable Q
value can make this a bit more of a hairpulling experience. Let's just
say that this is the kind of thing you can destroy a otherwise
perfectly good day by lock you to a block of paper and a pencil. As
one has got out of that intense work you fall down and let it be till
another day that you actually try it out, but it can be done.

> They "kill" one filter by switching in such a small cap that that stage is
> effectively
> gone from the audio range (but still stable...)

Yes, the first, so that the signal passes rather unmodified.

> This is NOT an SVF in any way I understand it.  Its closer to the Moog Ladder
> in that the resonant gain will be fixed, and the passband will drop.

Indeed.

> BTW a SVF can be configured to have constant peak response by putting the
> input into the Q feedback, rather than the normal point.
> 
> So why can't you do the same with the Moog Ladder ???

Because in the Moog Ladder you need the partial canceling of the
lowpass output and input signal. It is really in this balance where
the Q value is set. As the high term is suppressed the others stick
out more and the filter becomes resonant.

If you where using a different method of making the Moog ladder
resonant, then you could get a different effect alltogether. One such
would be to feedback from the middle tap.

> Can any "mathy" types put this in "non-guru" form for us ?????

Nope.

Cheers,
Magnus