[sdiy] Hard Sync on DCOs

[Tom Wiltshire]

On 8 Nov 2011, at 22:17, Richie Burnett wrote:

>> Given that it is easy enough to make a hard sync sound on a DCO by including a reset comparator as well
>> as the uPs reset pulses and simply ORing the two sets of reset pulses together (allowing you to boost the
>> DCO amplitude compensation CV to produce sync bend), would anyone care to speculate why more DCO
>> synths don't/didn't do this?
> 
> The JX-3P and JX-8P both offered oscillator hard sync on their DCOs from what I remember.
> 
> I seemed to remember that the sync on the JX-3P was quite strange though.  I think a sync'd sawtooth waveform starts again from half-way up the ramp where it's value crosses zero, rather than starting right from the very bottom of the ramp when reset like most synths.  Maybe this was done to minimise DC offset in the output of a sync'd oscillator, I don't know.  It definitely sounded different to a usual hard sync'd sawtooth.  You could think of it as the difference between hard syncing a cosine wave vs a sine wave.  They reset back to different places in the cycle.

Looking at the schematics, the JX-8P (not sure about the 3P, but I doubt it's different) 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.

I hear you about the cosine/sine sync being different. There were some weirdnesses in that synth, like the 'Metal' cross mod.

Tom