[sdiy] Pro One CEM3340 Sync magic

[David G. Dixon]

When I've added sync to tri-core VCOs, I've simply summed a syncing pulse
with the triangle going into the schmitt trigger.  This gives the classic
triangle-style sync shown on the website Grant linked to.  This style of
sync will work with both polarities of pulse.  Hence, square waves give
double the sync frequency of saw waves.

My saw-core VCOs have the classic saw-style sync which resets the
comparator.

Both styles can be attenuated for variable, or "soft" sync (a stupid and
misleading name, IMHO).  In either case, the height of the pulse is varied
from 0 to just above the height of the wave which is working against the
threshold voltage.  This gives sync which 'may' or 'may not' cause reset.

I prefer triangle-style sync, personally, because the triangle and sine
waves maintain their "soft" timbre, unlike in saw-style sync, where the
obtain falling edges and thus sound much like saw waves.


> I had an interesting discovery this weekend (for me anyway). As I'm sure
> many of you know, the Pro One has a single (free-running I call it) LFO
> and
> this lone LFO also drives the sequencer/arpeggiator.  While working on my
> "replacement CPU" I noticed, that while the LFO was free running, it would
> always sync to the user when they first played notes with the Arp on. That
> is, the first note would sound immediately and then the LFO would clock
> the
> next note one LFO period later. My initial attempts at duplicating the
> arpeggiator resulted in no note sounding until the LFO "came around" to
> the
> proper edge used to clock the notes. So I'm thinking, how the hell do they
> reset the LFO to the user?  Low and behold they used a rather clever
> method
> of squashing the triangle wave of the 3340 which resets the LFO. This
> seems
> to be somewhat undocumented since the part has other pins for sync. Anyway
> I
> guess it's more well  known to others, since today I found a mention of it
> in this thread:
> 
> http://electro-music.com/forum/post-176134.html