World's best VCLFO- $4ea

[Colin Fraser]

>----------
>From: 	Haible Juergen[SMTP:Juergen.Haible at nbgm.siemens.de]
>Sent: 	29 July 1997 11:43am
>To: 	Colin Fraser; 'DIY'
>Subject: 	AW: World's best VCLFO- $4ea
>
>
>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),
>and the result is very similar to "ordinary" hard sync.
>I have done some quick measurements once, but not in depth, I have to
>admit. 
>So, is the external transistor method 100% equivalent, or only similar ?
>
>JH.
>

The single transistor for true hard sync on a 3340 sounds right,
Juergen.
According to the E&MM Spectrum article, with the add-on circuit, hard
sync pulses reset the triangle wave output to its lowest point, and
therefore intiate a positive going ramp. 
Soft sync resets the tri-wave to its mid point so the wave will continue
in the direction it was going before the sync pulse arrived.
The extra circuit for this is only a few transistors which are directly
charging the integrator capacitor to the desired voltage. For hard sync
only this should be possible with a single transistor.
I'll have a look at this when I get home.

>From what Martin descibed of the 3340 data sheet it does sound like they
have the resetting  transistor in the wrong place.
Also feeding a pulse into the 'soft sync' input as well as reseting the
capacitor doesn't seem necessary - assuming its's intended to force a
positive ramp. Maybe it there to speed up the comparator.

As regards retro-fitting this type of sync circuit to Digisound VCOs -
this is exactly what I intend to do - my modular oscillators are a
slightly modified version of the Digisound circuit.


Cf