ASM-1 sync amount

Martin Czech martin.czech at
Wed Jan 28 08:32:06 CET 1998

> Hi there,
> I´m finishing my ASM-1 synth (2 ASM boards) and I´m trying to add some
> features and aditional modules to complete the system.
> I´d like to control,the sync amount in the oscillators. I use a lot the
> "weak" sync output in my System 100, and I think that a soft or medium
> sync is more "usefull" than hard sync...
> Do you know if thre are a easy way to have Sync amount in a knob in the
> front panel, or may be 3 different inputs?
> I´m building the ASM1(with many difficults) but I don´t know pretty much
> about electronics. All info is greatly apreciated.

a) Saw-Oscillator
The "soft sync" input usually is added to the discharge comparator
reference, such that the comparator threshold is lowered.  The higher
the incomming amplitude of the sync spike, the more likely the
discharge is. Ie. if the saw is almost at the top end, a little spike
will start discharge, but not if the saw has just begun.  A larger
spike will need, say 50% of the saw cycle to discharge.  A very large
spike will allways discharge, this is "hard" sync.  Given that the sync
spike is large enough a simple pot for the sync spike input (maybe some
buffering) should be sufficient to blend from softest to hardest sync ;->.

b) Triangle
Basically the same, but CEM3340-like osc will reverse direction if
synced, there is no build in discharge. This will sound even
softer the saw-"soft"-sync, since there is no edge. In this case
it is possible the apply negative as well as positive spikes to the comparator reference, to make rising or falling direction switching more
or less likely. 

If the tri-osc has an extra discharge trany for "real hard"-sync the same applies as in a).

It could be interesting to use a MOSFET to discharge and to set the
level to which the capacitor is discharged (a bipolar can do this too,
depends on circuit). This way interesting applications are possible:
If you want to play a low frequent (100Hz) sine wave, with quite fast
attack, you will hear random clicks. This is because the osc is free
running and if you start the envelope say at the sine maximum, this
will give an egde and this is heard as a click.  Now , if the osc is
synced to some adjustable level (another pot) it is possible to varry
the loudness of the click from maximun to none at all. Another example:
If the master and slave osc are at the same frequency, hard sync will
not be very interesting. But if the "reset" value is slowly change, an
adjustable phase offset betwen master and slave should be possible.
This is voltage controlled phasing. Should sound nice with two saw


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