[sdiy] VCO tuning philosophy re-visited

[karl dalen]

The superior linearity of the core, the disadvantage are
the 30usec flat period of the ramp unless Ian have made
something magic?

Fun to notice are the top and bottom flatness problem
in the Chroma! Software created non oscillator linearity! :)  

KD

>Tom Wiltshire <tom at electricdruid.net>:

> I can see the attraction of a simple counter run from the
> LM311 pulses as the base of a wavetable oscillator. But the
> LM311 has a linear response, so you're going to need a
> exponential convertor in front of it, in which case, you're
> back to the usual problems of VCO tuning.
> 
> So what's the advantage of the LM311? Why not run the
> counter from the comparator reset pulses of a standard
> ramp-core VCO, but tuned higher than typically? Is *that*
> the advantage of the LM311 - it can cope with higher
> frequencies?
> 
> Thanks,
> Tom
> 
> 
> On 7 Feb 2011, at 19:40, David G. Dixon wrote:
> > Still, it begs the question as to why the LM331
> clocking an 8-bit counter to
> > a DAC isn't seen more often as a basic variation on
> the sawcore VCO.  Of
> > course, it would give a staircase, but at 8 bits, the
> steps would be pretty
> > darned small, and could be filtered to smoothness
> pretty easily, or
> > interpolated using Henry Walmsley's scheme.
> > 
> > Ian, you and I discussed this LM331 business a year or
> so ago (after I
> > started a thread about Henry Walmsley's 16-step
> waveform generator) but then
> > I forgot all about it, but now I'm all excited again,
> and I think I'm going
> > to design a wavetable oscillator around it.  I
> prefer this method to the
> > ramp-controlled method that Olivier mentioned, because
> it should be
> > glitch-proof -- no worries about the ramp being too
> small or too large,
> > particularly with changing frequency, and no need for
> an ADC.
> > 
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