[sdiy] Mood Ladder Filter: Many Questions!

[Magnus Danielson]

From: HOSHUYAMA <houshu at muj.biglobe.ne.jp>
Subject: Re: [sdiy] Mood Ladder Filter: Many Questions!
Date: Sun, 06 Mar 2005 04:01:43 +0900
Message-ID: <422A0217.2070603 at muj.biglobe.ne.jp>

> Hello List,

Hi Osamu-san,

> harrybissell wrote:
>  > iirc Osamu Hoshuyama has a scheme to do highpass with the ladder
> 
> This schematic?
> 
>    http://www5b.biglobe.ne.jp/~houshu/synth/Vcfv0212.GIF

It can be said again, brilliant analysis! I felt really stupid when I looked at
it the first time, it is obvious when you see it, but I had failed to think
according to those lines. 

> Transistor/diode ladder can be used as a Sallen-Key filter as in Alisa.
> 
>    http://www.ruskeys.net/eng/pasp/alisa1377/pasp.php#sc
>    http://www.ruskeys.net/eng/pasp/alisa1387/pasp.php#sc
>    http://www.geocities.com/rusynth/alisa.htm

Yes, but that's a two-pole.

> High pass input and band pass input can be added to obtain a multimode 
> filter like the Steiner VCF, which is the easiest way IMO.
> 
> http://www-timc.imag.fr/Yves.Usson/personnel/SDIY/Modular/EN/STEINERVCF/

I agree. I have been looking into the the multi-tap method on the Moog ladder
a few times, but it becomes a bit uggly to do it "perfect" in the
state-variable kind of outputs. The trouble is not really the filter itself,
but the action of the feedback. This does not happend when doing multiple
insertions. The inputs will not be "pure" in the state-variable sense, but that
may not be important. However, with some simple preprocessing they can be
"purified" if needed. I don't think such "purification" is needed for normal
musical use.

Best of Regards,
Magnus