Lin detune,MS20 Core,Polysix/Trident

[Bjorn Julin]

Hi JH and ALL.

>>I dont agree, when i look at a Korg CCO i see a core
>>wery similare to a Moog Rouge etc, core for example.
<> The principle of "charge full" instead of the usuall
<> short the capp is wery similare. But you are saying VCO
<> and that is confusing!
>[...]
>>Still i dont agree in this reasoning about what is expo and lin
>>CCO's to me they are all lin CCO cores but with different principles
>>in the front end of the Core.I find it quite confusing when
>>people refer to it as a expo or lin VCO, i think this issue has
>< to be sorted out once for all!?

>... and I had thought I was the one who helped sorting it out on
>this mailing list (;->).

Yes you have, the problem is that i have missed that
discussion, and im to lazy to search trough the archives!

>But seriously, it all depends on where you make your imaginary
>"cuts" in the schemos to separate function blocks:

>All VCOs have a CCO core, of course.

Yes!

>Most VCOs drive that core from an exponential current source,
>so you have an inner block - linear CCO - and an outer block
<- expo VCO.

Yes and no, yes to inner block, no to outer block, its
confusing saying "expo VCO", say instead, "expo converter"!
Now say, im right and you are wrong!:-)

>You could also define a different block, with the cut
>at the expo converter's summing node, then you'd have an intermediate 
>exponential CCO - and the resistor to
<that summing node makes it a exop VCO. It's all just
<a matter of definition. But one thing you cannot
<do: find a cut where a partial block acts as a
>linear VCO (unless you're using the reference current
>input - which is exactly what the MS-20 does.)

But that is what i find confusing, why would the MS20 be
a more exponetial VCO then linear! To me its a linear CCO
with a exponetial driver who hapends to be driven exponetial.
The point is, for me they all are linear CCO's they just get
driven differently, that then eventually makes them "behave"
in a linear or exponetial manner. This might sound silly to
be reasoning like this but im slightely backwards!!
Thats why i do such wirdo stuff like 4046 VCO! :-)

But "what" actually are the benefits by doing what they do
in the MS20? Getting rid of a expo voltage scaling term?
Or the first order temp term?

>The Polysix and Trident oscillators are performing an exponential
>conversion that includes an expo *voltage* node. Then this expo
<*voltage* is converted to an expo current with a *linear* voltage
<controlled current source. Which then feeds the CCO core. The big
<difference to an ordinary expo VCO is that you *can* make a cut
<here such that an inner block is a linear VCO. Sure, the innermost
<block will always be a CCO, but next larger block (CCO + VC current
<source) *is* a linear VCO. So it does exist (;->) - and if you have
<offset voltage at that point you must compensate it (what Korg
<called "Low trim") - and that's also a point where you can add an
<offset voltage on purpose, which results in that infamous "linear
<detuning".

Yes that striked me to, by removing all the low trim pots and insert
a buffer who then drives all "first" 8 CCO,s one could do lin detune
by adjusting this - offset against the second row of CCOs.

But what kind of problems would a current regulator might have?
In the textbook it dosnt talk much about many, it seams to work
nice and dandy?

Korg did a neat solution to the requirement of negative voltage
input to the current regulators,by shifting the entire expo
cirquit by -10 volt.
Another neat thing that came along with that is they didnt
need level translators from the CPU to the 4051 muxes.
Yet another clever design!

I did a redesign of the current regulators by using a -5volt
reference voltage, the CCO sweps from 0 to 20kHz with 3Hz from
0 to-5V. If you folks would use this for LFO purpose wich i
recomend use a 10k Log poti for the freq controll.

A interesting thing with multiplexed expo converters is
that one can use sinking or sourching current regulators,
or why not driving a VCO directly, like multi vibrators!

Not to mention the combined S/H buffer and current regulator!!

>All right now - I almost went on writing "but Korg has chosen to
>do it at a different point, changing the trigger threshold of the
>CCO core." But you have tricked me - it's true they do it that way,
<and it's also beyond doubt that this results in "ordinary" detuning
<(just as in every expo VCO), and *not* in linear detuning. (At least
<not in what I called linear detuning when I coined that word.
<Ok, I see the word was misleading from the start.)

>Why is this ? You're changing the thresholds to a certain amount,
>and depending on the VCO frequency it will take a different time
>for the ramp to pass that additional voltage. Detuning is related
>to the current frequency, and not constant.

>So - nice idea, and I really believed it for a moment ! But in >reality the 
>Trident's detuning is quite ordinary, and when there is >an additional
<linear detuning effect (and from the sound, I have no doubt about it), then 
it originates in less than accurate "low trim" - i.e. some >remainingoffset 
voltage in the linear VCO block. And in the fact that >there is something 
like that at all - it can not happen in VCOs that >consist of an exponential 
current source + linear CCO without >anything in between.

Trick and treat! Yep, your right Juergen (looking into the shemos again), 
;-> How sad, i thought i had found somthing interesting !!! Booooooo to me!!

Hmmmm, it seams that i was "to fast" on the Trident!!! :-)
But, when i first started to reflect over the Trident way
of detune i then went to your "linear email" and what you
said was "by introducing a constant current", i think i didn't
read your email close enough. Now as it seams the "only" way
to get true lin detune in a linear CCO core is to add or
subtract current into the core itself or offset the sourced or
sinked current"  to a point in the CCO cirquit where the point
are not frequency dependent.

But then we look at the Bit 99 who offsets the second oscillators
frequency by a voltage who then are converted to a current by a
linear current regulator. So this is by offseting the voltage rater
then the current directly, but the principle are the same. Its just
converted to linear current later. So its constant current offseting
by a voltage, or voltage controlled linear detune in a linear VCO.
(if i may talk in JH language! ;->). Now this is just another example
how confusing it all is when people talks about lin or expo VCO's!!
Hehe!

>Which does not mean that a synth that would actually use lin detune
>*only* would not make sense. I made some experiments with the JH-4
<meanwhile, turning the ordinary detune as close to zero as possible,
<and switching in Lin Detune does then result in a fixed beat rate over
<the whole keyboard range. Is this desirable ? As an option, yes.
>But it's quite a restriction, too. A bit like PWM which is often used >to 
>sort of emulate a second VCO on single oscillator synths. IMO, for
<most applications a mix of lin and expo detune is desirable. Just
<like adding a bit (a quarter octave per volt) LFO tracking to PWM
<maybe. Modulation rate increasing with higher notes, but not doubling
<every octave. That's what I built this lin detune circuit for in the >JH-4, 
and that's what (involuntarily ? or rather on purpose - who >knows ?)
>happens in early Moog Modular VCOs, the Taurus, the Trident, the
>CS-80: expo detune dominating, but a tiny bit of lin detuning (due to
<inaccuracies) added to add charm in the lower range.

Absolutely.

Perhapps, the Crumar bit 99 is the only machine with
"on purpose linear detune" ??
A aproach to get a interesting string sound is when
one LFO modulates the reference frequency of one of
the oscillators. The Bit had two LFO's so interesting
chorus string sound can be achived by using separate
LFOs for each OSC.

And i just "love" the sound of that machine!!

PS:The MS50 seams dated to 78-11-09.

Reg
BJ

_________________________________________________________________________
Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.

Share information about yourself, create your own public profile at 
http://profiles.msn.com.