From: <
groovyshaman@...>
> > >Craig Critchley wrote:
> > >I'm interested in filtering control voltages,
>
> > <media.nai@...> responded:
> > Afaik, only the 830, 820, 700, and half of the 110 can process DC. I
> don't
> > know about the 130 or 190, even though I've asked a few times. I guess
a
> > 320 can be considered a DC processor of some sort -- it has DC inputs
and
> > can produce a DC output for considerable length of time.
>
> I think only the 820 would be usable for LP filtering control voltages
(slow
> waveforms). Of course, LP filtering a flat DC signal would not produce
> much. :)
Well, that's true. :-) The 101 vibrato has a low frequency bandpass and
putting two together I got curious about low frequency filters in general.
>
> > >Craig Critchley wrote:
> > >inspired partly by the '101
> > >vibrato circuit and partly by trying to make a PLL with two '300's and
> the
> > >ring modulator from the '110.
>
> Ok, correct me if I'm wrong, but isn't soft sync basically a form of PLL?
It certainly produces a different sound; you don't get the overtones from
the reset, but you do get some FM-like effects, particularly around the
attack while the loop vco is still hunting for the right frequency, if you
set the filter time long enough.
>
> > <media.nai@...> responded:
> > My, that sounds ambitious :) Let me see if I understand what you mean.
> You
> > are trying to have one 300 track another 300, taking the square outputs
of
> > both 300's, using a 110 ring mod as a "phase comparator", smooth the
> output
> > with an 820, then send the output to the second 300 using the FM input??
> > Or am I completely lost here?? I think I understand how a "four
quadrant
> > multiplier" would give the sum and difference frequencies using sines,
but
> > I don't see how it would give you just the difference like an XOR using
> two
> > squares. Wouldn't it give you something much more complex??
>
> Hmmm... The 110 produces both sum and diference sidebands, so when one
tunes
> both 300s perfectly at 1000Hz, there will be a resultant 2000Hz upper
> sideband signal and a DC output corresponding to the difference in the
phase
> of the two input waveforms. Now if they're slightly detuned, say by 1Hz,
> then the 1Hz low band signal could be retrieved from the output by LP
> filtering with the 820. In any case, I would think the PWM settings of
the
> two 300s would be critical in this application. If set to anything other
> than precisely 50%, it would add either positive or negative offset to the
> LP filtered control voltage. It could be interesting to try using the PWM
> input to control the offset.
>
> Another issue, when using square waves through a ring mod, there are
∗tons∗
> of side bands due to all of the odd harmonics present. Why not use sine
> waves?
Actually, I am using sine waves from the '300 as the inputs to the ring
modulator, patching other outputs through a filter for the audio output.
I'm not using the PWM inputs, I'm using FM. I had the note voltage patched
to both 1v/oct inputs, with the loop vco detuned so it would have some work
to do, then I switched the loop vco to another fm input so I could change
how well it tracks the 1v/oct note. Different amounts of initial detuning
produce different timbres as well, I suppose because of phase and some
leftover FM, but the VCO's can't start too far apart or the loop won't find
the frequency.
I would imagine that a ring modulator and an XOR would produce similar
results for square waves, since an XOR is multiplying if you call the states
+1 and -1. Since the next module after that is a low-pass filter, the extra
harmonics might not make much difference.
>
> <snip>
>
> > Regardless whether it works as a PLL or not, that sounds like quite an
> > interesting patch!!
>
> Agreed! I'm curious Craig, what led you to this experiment?
Mostly curiosity after reading about PLLs. I'd seen various 4046-type PLLs
in the past but then saw a block diagram for a sine vco with a balanced
modulator and thought "hey, i have all that stuff in my motm..."
>
> George
...Craig