[sdiy] MIDI VCO Control - but not CV?
cheater cheater
cheater00 at gmail.com
Tue Jul 15 15:22:24 CEST 2008
OK, so:
in a DCO, the trigger pulses basically replace the comparator.
In a CV FM VCO, the CV controls the current going to the accumulator.
This indeed could be problematic to put together.
What if you use the pulses to create a DC CV? Take the pulses and put
them through a reeaaalllyyy strong LPF. Sort of like a DSD DAC. Then
you get DC voltage, dependent on how many pulses you have per second.
The problem here is to have this voltage respond to note-ons without
lag. However, if you want your keyboard to have a range from 10 Hz up
to, say, 24000 Hz (I would say that could satisfy even the best
'golden ear'! :^) ) you could still output the pulses quickly enough
to be able to complete the cycle several thousand times a second.
Unless you are Jordan Rudess, that will be OK.
Then you can simply use that as CV.
What do you guys think about this?
Cheers
On 7/15/08, Christian Bergmiller <cbergmiller at gmail.com> wrote:
> look at the asm-1 schematics for a typical saw core.
> the input cv goes through an exponential converter with current output.
> this current charges a timing cap that is reset via a fet. The fet is controlled
> by a comparator (lm 311) comparing the charge of the timing cap with a preset
> value..
> more input cv -> more current to the timing cap -> higher pitch
>
> hope this makes some sense
>
> christian
>
> 2008/7/15, cheater cheater <cheater00 at gmail.com>:
> > > A digitally hard synced VCO can't be frequency modulated via an analog LFO/VCO.
> >
> > Very good point, Christian!
> > Wonder how to tackle this problem....
> >
> > For one thing, you could perhaps have CV inputs into the DCO. This
> > would easily allow sub-audio-range modulation.
> >
> > For higher-rate modulation, maybe something could still be invented. I
> > wouldn't be giving up so fast.
> >
> > The important question to solve this problem is: In a normal saw-core
> > VCO, what does the pitch CV change actually? How does that work?
> >
> > > If you want to build it nonetheless, you'll need a Midi to CV Module that puts out a hard sync trigger
> >
> > Exactly what I'm thinking of doing!
> >
> > Cheers
> > Damian
> >
> > On 7/15/08, Christian Bergmiller <cbergmiller at gmail.com> wrote:
> > > A digitally hard synced VCO can't be frequency modulated via an analog LFO/VCO.
> > >
> > > If you want to build it nonetheless, you'll need a Midi to CV Module
> > > that puts out a
> > > hard sync trigger. A phase accumulator oscillator (for example) can
> > > run in an interrupt routine nicely. search PAO on the net..
> > >
> > > Christian
> > >
> > > 2008/7/15, cheater cheater <cheater00 at gmail.com>:
> > >
> > > > Tom,
> > > > I guess what I'm asking about is: how easy is it to modify my
> > > > favourite VCOs (e.g. the MOTM VCOs or the moog ones) to work with
> > > > this? What would I need to look for?
> > > > Not all VCOs use an accumulator design - do they?
> > > > I am thinking it could be very difficult to do with a sine oscillator.
> > > >
> > > > Cheers
> > > > Damian
> > > >
> > > > On 7/14/08, Tom Wiltshire <tom at electricdruid.net> wrote:
> > > > >
> > > > > What you're talking about sounds like a MIDI-controlled DCO, so all the
> > > > > recent comments about DCOs apply here too.
> > > > > Sampa's quite right to bring up the amplitude compensation problem. This is
> > > > > the main snag with the typical DCO design, but the problem is by no means
> > > > > insoluble.
> > > > >
> > > > > You're also right to point out that you'd still need a CV to control the
> > > > > filter. But you're going to need a CV to control the amplitude too, so
> > > > > you're going to have a DAC of some type in there.
> > > > >
> > > > > It's do-able, but it isn't going to be much simpler or easier. In some
> > > > > ways, MIDI2CV with a high accuracy DAC is more straightforward. But by all
> > > > > means go ahead and prove me wrong!
> > > > >
> > > > > Just my 2 pence,
> > > > > T.
> > > > >
> > > > >
> > > > > On 14 Jul 2008, at 19:13, cheater cheater wrote:
> > > > >
> > > > >
> > > > > >
> > > > > > Samppa,
> > > > > >
> > > > > > I'm only talking about taking existing VCO designs, and modifying them
> > > > > > to work in such a setup.
> > > > > >
> > > > > > Cheers
> > > > > >
> > > > > > On 7/14/08, Samppa Tolvanen <samppa.tolvanen at gmail.com> wrote:
> > > > > >
> > > > > > > On 7/14/08, cheater cheater <cheater00 at gmail.com> wrote:
> > > > > > >
> > > > > > > > Wouldn't it be *much* easier to make a midi2trigger module, that
> > > > > would
> > > > > > > > then hardsync the VCOs?
> > > > > > > >
> > > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > If I got You right, it would be MCU based squareware oscillator, eh?
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > >
> > > > > > > > Are there any popular/interesting VCO designs that wouldn't work well
> > > > > > > > with being synced this way?
> > > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > Sawtooth oscs would be just fine as long as You don't mind amplitude
> > > > > > > multiplying/dividing by powers of 2 per octave :D
> > > > > > >
> > > > > > >
> > > > > > > Samppa
> > > > > > >
> > > > > > >
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