Doepfer

Now with a day or two under it's (read: my) belt, it seems I was 
full of it on my initial commentary on the A149 on a couple of 
fronts:

It seems that the top and bottom half are running off the same 
random source.  When you trigger them with the same signal 
they do track each other.  Not that they give the same voltage 
result mind you due to the basic conventions of the module, but 
they definately are NOT going in their own directions.

Secondly, this module triggers on both rising and falling edge of 
a trigger signal, not  just the falling edge only as I reported 
before.  

I have come to the conclusion that this is a bit of a pain.   Here's 
why: 

If you want to sync envolopes to the 149 you need to trigger them 
from the same source. If you trigger them both off a square, you 
will get two voltages out of the 149 per envelope.

The only workaround I've found for this is to use a trigger source 
that has both square AND inverted (ramping downward) 
sawtooth outputs.  You use the square for the EG and the 
inverted ramp for the 149.  I guess it needs a hard elbow to 
trigger from the falling edge.

The BIG problem with this is depending on your VCO, the square 
and saw outs aren't always in phase aligned*  The workaround 
for this is to offset the riding edge of the square using the PWM 
pot.  Once it's set up, it all work perfectly, but it takes a bit of 
juggling to get it there.

* - By phase aligned, I mean the saws and squares of some 
VCOs are by design 180 degrees out of phase to one another.  

It depends on two things: the manner of waveshape conversion 
incorporated, and the type of waveshape generated at the 
devices' core.  Most being sawtooth core,  some manufacturers 
use the reset pulse in which to generate the square, and this is 
OK as the reset causes the saw to charg up again so they are 
cool.  However, in triangle core VCOs (Buchla and Aries being 
two of them) , instead of a retrigger PULSE to the flyback switch, 
it generates a square wave who's trailing edge is exactly at the 
peak of the triangle.  Add to the mix, the sawtooth waveform from 
triangle core VCOs usually use both the triangle and square 
wave signals through an anlog switch for sawtooth conversion 
and without going into boring details, by the nature these saws 
are almost always rising only and 180 out of phase with the 
square wave.  

As an unrelated yet possibly interesting sidebar, this is why Don 
Buchla used his somewhat unorthodox method of square 
generation via an adaptation of the sine shaper found in 
Electronotes volume 1 (even though his use preceeded that 
publishing).  He doesn't use the retrigger signal as the square, 
he generates another one.  In doing so, his VC waveshaping 
(square to sine) is in fact put back into phase so that that 
function doesn't go all wack in the middle (passing a half sine/ 
haf square waveshape).

Man, this letter went totally OFF topic, but I thought some of you 
might find it interesting.

- P