the WASP DCOs

Haible Juergen Juergen.Haible at nbgm.siemens.de
Wed Jan 14 13:25:59 CET 1998


DCOs. Yes. Normally I hate synths with DCOs. Because usually
that means that several voices get their VCO frequencies by
dividing a common clock frequency by different division factors.
So they can't frealy beat against each other. If memory serves,
the Matrix 6 had 3 Master oscillators for 12 DCOs, the Matrix
1000 had 2 Masters for 12 DCOs. That's the "DCO compromise".
The wasp has 2 Master oscillators for 2 DCOs. There's nothing
wrong with that. The Roland SH7  does a similar thing, btw.
Ot the Korg Polysix. Similar in that a VCO is combined with
dividers for the octave switching. The Wasp goes one step
further. The master VCOs (555s) only do the tune, detune
and pitchbend stuff (so they stay within one octave!). Then
there are dividers for the keyboard position (flipflops, and a
programmable down-counter for each oscillator). So we are
still talking pulse waves, less than 3-octave range and no
portamento at this point.

Now comes a PLL for each oscillator. Simple (CD4046), but
effective. The PLL loop filter is variable, and that's how Portamento
is produced. And a symmetrical square wave restored.

Now the saw wave converter and the octave switching:
The PLL runs at the highest footage (2' I think). With two
monoflops and two integrators in each oscillator (around
CD4013 and LM3900) there is a tachometer circuit and a
saw converter built. The first monoflop is always triggered 
by the 2' clock, and feeds the first integrator to produce a
voltage that increases with frequency. This voltage controls
the input of a second integrator, i.e. the rise time of the saw
ramp voltage. This second integrator is then reset by the second
monoflop, which is triggered by a pulse of the selected footage.
(you get these by sucessive flipflops from a 4024.)
Now you have the saw wave over the full 7 octave range, and you
can also feed a PWM circuit (built from 4069's of course).

Now, think of it, you have in this little insect two complete DCO
chains consisting of
1) HF-VCO
2) Keyboard octave divider flipflops
3) Note-of-one-octave-counter
4) PLL with portamento
5) Frequency to Voltage converter
6) Divider for Footage
7) Saw wave integrator
8) (only for one oscillator) PWM converter

Quite impressive. If I would have designed it, I would have 
ended up with 3 or 4 PCBs in size of the Wasp. That's
why I am so impressed. 

BTW, as the whole waveshaping and footage division
takes place *after* the PLL, it might be interesting to build
just this "back end" and use it as a guitar synth etc.
All with a single 5V supply.

JH.




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