[sdiy] stringfilter

[JH.]

Well Magnus is on the right track, but my idea was not to
use textbook GICs for this, but a circuit derived from that
which would need even less components. This then turned into
a problem later.
The thing is, the circuit works fabulous as I built it, but only
with a handful of dual opamps I had in a drawer. *No*
way to get it stable with any other dual opamp, nor with
opamps of the same manuafacturer, but different time code!!
I have even discussed this problem with a former Burr Brown
opamp guru who also said it *should* work. (;->)
Well actually it does, but only for - I told you.
I _should_ try it with some improovements some time,
I have some ideas, and the BB man had some too -
but as so often, I just moved on to the next project.
The way it is now, I cannot publish it, as building it would
run you into the same problems.
If I were to do it again (but there's no pressure in that
direction, really!), I'd go straight for textbook GICs.
Or digital (;->)
(Seriously: Ken Elhardt's string sounds are more convincing than my
demos, and he used a handful of readily available FX boxes.
Maybe he likes to talk about that.)

JH.





----- Original Message ----- 
From: "Magnus Danielson" <cfmd at bredband.net>
To: <dino.leone at stanford.edu>
Cc: <synth-diy at dropmix.xs4all.nl>; <mans at common.se>
Sent: Saturday, February 12, 2005 9:01 PM
Subject: Re: [sdiy] stringfilter


> 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
>
>