Fw: [sdiy] Digitally controlled VCO core

[Paul Maddox]

----- Original Message -----
From: Paul Maddox <Paul.Maddox at wavesynth.com>
To: <mikko.a.helin at nokia.com>
Sent: Saturday, June 09, 2001 10:46 PM
Subject: Re: [sdiy] Digitally controlled VCO core


> Mikko,
>
> Get hold of a copy of the ROLAND JX8p/3p manual,
> it in there, Its also in the JUNO range aswell...
>
> The amplitude thing is easily done by one of two methods..
> one icreasing the VCA level as you move up the scale
> and/or,
> Haveing ranges and different capacitor chargeing currents for each range.
>
> An easier solution would be to use a micro to generate the sawtooth
> useing phase accumulator techniques... you can have a resolution as low as
> 0.00001hz if you want,
> and if you worry about output resoltion you could have 16Bit without much
> hassle..
>
> My monowave uses two of these, loaded by the main CPU , which receives the
> Midi data and decodes it.
> software conversion to exponential isnt too much hassle, you can either do
> it by calculating the value or have a Look up table (quicker).
> My monowave uses a lookuptable for the top octave and simply shifts the
> value right for each octave below that.
> The value is a 24bit number, giving me a resolution of 0.003hz...
>
> TTFN
> Paul
> _______________________________________
> Wavesynth home page;-
>     Http://www.wavesynth.com
> Modulus synthesizers home page;-
>     Http://www.wavesynth.com/modulus
>
> Mikko Wrote;-
>
> > Has anybody tried to use a microcontroller to control the current source
> and
> > capacitor decharging circuit for ASM-1 style VCO core? I thought it
would
> > simplify the design as the CV to exp. current converter doesn't have to
be
> > that precise - so no expensive transistor pairs or T.C. resistors were
> > needed and you could also use a 8 bit DAC - if the uC sets the time when
> the
> > cap is decharged and the exp converter only takes care of the amplitude
of
> > the waveform.
> >
> > I see (only) some problems with this design. One is the difficulty to
keep
> > the output amplitude constant and the other is that (software)
exponential
> > frequency modulation and portamento are little bit tricky to implement,
> and
> > you can't easily (without A/D) use your analog modifiers. Good thing is
> that
> > you could build the thing into your MIDI-2-CV converter.
> >
> > -Mikko
>