Why do older Obies sound fatter?

Haible_Juergen#Tel2743 HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de
Thu Feb 15 22:41:00 CET 1996


Got the OB-X service manual recently. (Thanks again, Michael!)
The circuits were nice to look at, but even more interesting was
the description of the autotune routine.
If I understood it right, the OB-X just tests for one single pitch
from each VCO. So you have to do exact scaling and even HFT
with trimpots.
The OB-8, on the contrary, has no HFT trimmer at all, and even the
scale trimmer is just for a roughtsetting. Then the autotune routine
checks for each octave (?), and puts correction values into a RAM, fromwhich 
it interpolates the exact CV for each and every note.
While the latter method is surely more correct, the old and simple
method has a great advantage:

"Fat" (in the sense of JUMP) means that you have a certain amount
of detune in the lower and mid range. The amount of detune is
limited by the beat rate in higher octaves, however, where excessive
beating starts to sound dissonant from some point onward. (With
PWM it's just the opposite, btw. Here the low range is the critical one).
The trick to have mucho fatness and still a smooth sound is detuning
the lower range more than the upper range.
Asuming the OB-X uses a quite high pitch for autotune (but here I'm only
guessing; please correct me!), and also assuming a certain amount
of omnipresent scale error, You'd have exactly what you want for
a fat sound after hitting autotune: Highs iin track, Lows beating.
On a OB-8, you have *everything* in tune, and it comes to real life
only a few minutes later. (I rarely hit autotune, therefore)
But this (tuning it and then let it drift) is not exactly the same as the
tuning at certain frequencies and letting other ones free, thus
causing a *systematic* "error".

I know, the OB-X has different VCO's, too. But am I completely wrong
with my theory (never had a OB-X in front of me, so it really is just
theory!) ?
And, does someone know how the autotune works in the intermediate
model, the OB-Xa? (The real Jump synth, after all ...)

Any comments welcome,

JH.



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