[sdiy] Hard Sync on DCOs

[Tom Wiltshire]

On 9 Nov 2011, at 14:59, Florian Anwander wrote:

> Hello Tom
> 
> 
>> Looking at the schematics, the JX-8P (not sure about the 3P, but I doubt it's different)
> http://fa.utfs.org/diy/jx3p/schemo/JX3P_schematics_08_mainboard.jpg
> 
>> uses a 'standard' sync where the reset pulse from one osc is fed to the other. In this case, it's fed to the 8253 counter's reset input, but it's the same thing.
>> Neither osc has a reset comparator which would enable it to produce a sync sound without the influence of the other osc.
> 
> Hm, I do not really understand your question. Hardsync on a saw-core VCO is: send the reset trigger (=output of the comparator) from the master oscillator to the switch (transitor) which resets the capacitor of the slave oscillator. There it will be OR'ed with the reset from the original Comparator
> 
> At a DCO there is no comparator, but there is the trigger from the counter.
> Now at the JX3P/JX8P the signal from  the master oscillators counter is OR'ed with the the signal from the slave oscillators counter and then resets the capacitor.
> 
> In both cases the reset signal of the master is OR'ed with the reset of the slave. That is, what hardsync is. Hard sync does not work with the comparator input. That would be soft sync (see the Roland SH-7's oscillator B).
> 
> Florian

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.

Thanks,
Tom