[sdiy] Hard Sync on DCOs

[Dan Snazelle]

Sent from my iPhone

On Nov 9, 2011, at 12:37 PM, Scott Nordlund <gsn10 at hotmail.com> wrote:

> 
>> Normal hard sync is exactly as you describe, Florian. You need a master osc, and a slave osc. The master osc's reset pulse from it's comparator is OR'd with the slave's reset pulse from its comparator and acts to reset the slave too. Easy enough.
>> 
>> However, with a DCO, you don't need a separate master osc. Instead you can *include* a comparator in the DCO (as you say, most DCOs don't have them) and then OR the uP's reset pulses with the ones from the DCOs own comparator. Essentially the DCO becomes a slave oscillator to the uPs master reset pulses. If the DCO compensation CV is set correctly, this produces a straight sawtooth. If the DCO compensation CV is increased, the DCO becomes a free-running VCO synced to the uP master reset pulses.
>> 
>> My question was basically "For the cost of one comparator, why didn't more DCO synths do this?" It just struck me as a bit odd, that's all.
> 
> I think in that case you'd need a more accurate and higher resolution CV (though still not what a true VCO would require). If you're using it for either sync or PWM effects, the CV needs to be both smooth and consistent between all the voices. Inaccuracy in a normal DCO synth just results in slightly incorrect sawtooth amplitudes. I think incorrect pulse width or sync would be a lot more audible. I guess you could split it into two cases: global PWM or "slave oscillator ratio" could use a global analog CV that's summed to each voice (as is sometimes done with pitch modulation in DCO synths), while per voice modulation (more of an expected feature for sync than PWM) would need a smooth demultiplexed CV per voice (plus software modulation routings, etc.). Maybe that adds a little bit too much to be economical. Or maybe it just never occurred to anyone.
> Also, didn't the ill-fated Moog SL-8 do something like that?                         
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Sad...the SL-8 looks interesting!