[sdiy] MIDI VCO Control - but not CV?

[harrybissell at wowway.com]

The thing to consider is what does a shift in the core
oscillator (assume sawtooth for arguement's sake) do to the
triangle wave.  

An amplitude change causes a discontinuity in the triangle... the two
points of the triangle do not meet, there is a sudden (maybe dramatic)
step.

This produces even harmonics that should not exist at any level in a triangle
wave.

Through the normal sine shaper, it will make a sudden step near the peak of
one sine wave, again a harmonic generator where ~no~ harmonics should be present.

Worse, the step may be different at different frequencies, so the harmonic
content will change.

I'd say that by the time you reach maybe 5% harmonic distrotion the sine wave
is VERY un-sinusoidal...

It depends on 'how much distortion can you stand' ???

I found that the typical 'OTA overdrive' sine-shaper does not work to my
satisfaction... even with trims for offset and amplitude.

Check out the Aries VCo (is it AR-314 iirc) ... it uses a triangle based core
with a sawtooth output which is a mirror of the triangle's current source with
a reset pulse from the triangle core. Sounds pretty good on its own but if you
feed some critical waveshaper (maybe a Wiard Mini-Wave ?) the amplitude
differences will be really obvious.

H^) harry


On Tue, 15 Jul 2008 18:39:41 +0100, cheater cheater wrote
> John,
> it wasn't review, and was very informative. I know little about
> oscillator design :)
> 
> Do you know within what boundaries the amplitude should be for a sine
> shaper oscillator?
> 
> Alternative idea:
> what about a circuit analyzing the exact pitch of the signal coming
> from the saw/triangle core, and makes up for that in a feedback path?
> This could be done digitally while keeping an 'all analog' path.
> This could also allow CV modulation to be added: if you have two 
> cores with the parts placed tightly together (temp. coupled and experiencing
> the same magnetics) and very well matched, you could have one core
> controlled by 'static' CVs (from keybard etc) while the other core
> could accept those static CVs as well as LFO CVs etc. Then analyze 
> the 'static' core and make up to both cores.
> 
> Best regards
> Damian
> 
> On 7/15/08, John Mahoney <jmahoney at gate.net> wrote:
> > At 12:20 PM 7/15/2008, cheater cheater wrote:
> >
> > > What could be done to circumvent the problem of the shapers that
> > > create the sines?
> > > Cheers
> > >
> >
> >  AFAIK, a sine shaper requires a clean triangle wave of precise amplitude.
> > Distortion results if the amplitude is not just right. Part of calibrating a
> > VCO involves setting a trimmer or 2 to get the cleanest sine possible.
> >
> >  Therefore, you need to ensure that the triangle wave has constant amplitude
> > and no glitches, which usually means ensuring that the sawtooth wave has
> > constant amplitude.
> >
> >  That's the goal. I've no suggestions on how to achieve it.
> >
> >  Apologies if that was all review material.
> >
> >  John
> >
> >
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Harry Bissell & Nora Abdullah 4eva