[sdiy] stringfilter

[Magnus Danielson]

From: Dino Leone <dino.leone at stanford.edu>
Subject: [sdiy] stringfilter
Date: Sat, 12 Feb 2005 10:58:26 -0800
Message-ID: <p06200700be33ff3affd2@[171.64.60.224]>

Dino,

> I've been looking again at JH's stringfilter webpage as I do once 
> every other week... ever since I became obsessed with that idea of 
> adding some "organic" quality to the sounds (as he puts it):
> http://home.debitel.net/user/jhaible/jh_string_filter.html

... which I totally missed!

> Now I finally had this idea (not a very clever idea in fact...) to 
> run my MOTM through the parametric equalizers that are built into 
> Logic since these are the only such EQs available to me.
> After setting peaks to emulate the main wood and air resonances of 
> violins or violas I was rather disappointed in the results; although 
> the addition of these formants does add some vague hints of "stringy" 
> character to the sounds, it's miles and miles from what JH's demos 
> sound like.

Indeed.

> So, I wanted to ask you guys about any tricks and equipment that 
> would make such an endeavor more successful?
> 
> Are there any schematics out there that would allow building of 
> parametric EQs to add to a MOTM system?

Well, to start off, the General Impedance Converter (GIC) is very simple.
You just put the resistors and one of the condensators in a string like this:

                   |\
  +----------------|+\___
  |              +-|-/   |
  |              | |/    |
  |  ___     ||  |  ___  |  ___     ___  |
o-o-|___|-o--||--o-|___|-o-|___|-o-|___|-||.
          |  ||  |               |       |
     R1   |  C1  |  R2      R3   |  R4
          |   /| |               |
          |__/-|-+               |
             \+|-----------------+
              \|

The inductance becomes

    R1*R2*R4*C1
L = -----------
        R3

Now, just toss a second capacitor on the left terminal either in series or in
parallel with the above circuit depending on your needs and you got yourself
the high-Q resonator using the GIC Jürgen is talking about.

To interprent what he is saying, that the resistors and capacitors are equal it
becomes

R1 = R2 = R3 = R4 = R and C1 = C2 = C

we end up with

     2
L = R C

          1                 1              1
f = -------------- = --------------- = --------
    2*pi*sqrt(L*C)   2*pi*sqrt(R²C²)   2*pi*R*C

So, calculating R and C for a given frequency is really trivial under these
conditions. Similar benefits could be done by choosing R3=R4 to be some other
value common for all resonators (say 100k). The only R1=R2=R.

It you need lower Q, just add a resistor in parallel with the capacitor.

Now, with this at hand, you realize that it is fairly trivial to make a bunch
of these on a board and you only need a handfull of op-amps on the input and
output of this to make it alive and kicking. Actually, now you get more out of
looking at Jürgen photos. Each IC is a dual op-amp and they are neatly put in
two 4x5 banks. Then you have 3 extra ICs per bank.

Cheers,
Magnus