AW: Fast VCOs/V->F converters

[Haible Juergen]

Hi Douglas,

most of your mail looks very reasonable, but I'd like to mention a few
things anyway:

	>At A440 this is 1.837Hz, at A up four octaves (x 16) this is 29Hz.
	>This rule of thumb is based on tests that were done with pro
musicians who
	>could gauge pitch errors *of isolated tones*.
	>Relative errors between two or more simultaneous pitches are much
more
	>stringent simply because the ear is extremely good at detecting
beat
	>frequencies.  Chorus effects will be detected at sub 1Hz errors,
	>Vibrato/Tremolo like effects at around 6Hz.  When I used to do this
stuff
	>almost 25 years ago we worked darn hard to get near perfect (much
less then
	>1 Hz) tracking over five octaves that would hold for long periods
of time
	>(after thermo warm-up).  It wasn't easy but it was doable.  The
reason for
	>all this attention to accuracy between VCOs was simple.  If I
wanted
	>chorusing/flanging I'd patch it, I didn't want it on its own
initiative!

If I want *no* chorusing, I use a DCO, or I sync the VCOs.
IMO, 90% of building a good pair of VCOs is getting the "right"
beat rate over the whole range. I agree that 29Hz is too much;
and I'd go further and say there are applications where the fun
just starts *below* "1.837"  (but not zero !) Hz. 

	>PLL are particularly bad hear because they are by
	>there nature (If the have any reasonable loop response time) always
hunting
	>for the perfect frequency match but never quite getting there.  

I share your doubts about PLLs, for the reason Martin and others
mentioned earlier: No clean transition between notes. But I don't
understand "never quite getting there" - once it's locked, it's locked
in *phase*, which implies being locked in frequency, too. (In theory,
you might have an infinite response on pitch changes in the PLL, but
in reality it will be really locked after some time.)

	> I think you can see why DCOs became the norm!

Yes. And I suspect that for complex waveforms (and that's what this
thread was about, wasn't it ?), locked frequencies might be
musically more pleasant than beating / chorusing ones. The only
problem I see with DCOs is that they were also widely used for
basic traditional waveforms like saw and pulse, where you often
want the phasing. Two completely different worlds, IMO.

	JH.