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[Martin Czech]

> >>The internal hard sync is a little strange indeed, and doesn't sound
> >>like "ordinary" hard sync at all. But very few commercial synths used
> >>it anyway (the Synton Syrinx being one exception). They added one
> >>external transistor instead (I think this is in the 3340 data sheet as
> >>well),

Yes.

Please, could someone shead some light into this ?

It is in the data sheet, but I don't quite understand this schematic.
What I can see out of it is that they clamp the triangle output buffer
with an external trany to minimum voltage and in the same moment
the  internal vdd/3 reference is pulled down via the soft sync pin.
This will cause the tri osc. to change to a certain direction
(I think rising). But after the sync pulse has gone, the tri output
buffer will recover and deliver almost the same level as before
the sync event (since the coupling capacitor will give only a
very short sync spike, compared to the triangle integrator time).

So this proposed "hard sync" will only lead to a sudden change in
charging direction, not to a jump in output level.

On the other hand, the data sheet says, that this schematic will
give hard sync like in conventional sawtooth osc. (that is sudden
and resetting discharge to minimum level).

Therefore I think that this schematic is wrong, instead of pulling down
the triangle output buffer I would expect to pull down the
osc. timing capacitor. So the discharge trany should be connected 
to pin 11, and not to pin 10 (from my head, hope this is correct).

The influence into the soft sync pin is still necessary, because just
setting the timing capacitor of an tri osc is not enough to 
determine the following waveform, it is also necessary to  set up
the charge/discharge flip-flop/comparator thing.

Is this observation correct ?

Now, there are the following opportunities to experiment with:

a) "hard sync" with hard sync input on rising edge
b) "hard sync" with hard sync input on falling edge
c) "hard sync" with hard sync input on falling & rising edge
d) soft sync with soft sync pin
e) conventional hard sync discharging pin 11 to some adjustable level

I think, it is worth to implement them all. It is not expensive.
As far as I have experienced now, all these possibilities sound
different, but sometimes you need patience, certain circumstances
(frequencies, levels), to get the difference out of the circuit.


m.c.