[sdiy] Oberheim OB-XA autotune procedure questions

[Kevin Lightner]

>  This may be one reason why JP-8's sound "fatter" than
>Jupiter 6's.  Yes, there are several other reasons, too -- but the JP-8's
>tuning is rarely ever as accurate as the that of the JP-6, so it will always
>sound a little warmer and wetter, at least in terms of the relative tuning of
>VCO's.  A freshly autotuned JP-6 almost sounds digital and clinical 
>in comparison!

Very true.
Put your finger on a CEM3340 in a JP6, it doesn't drift.
Put your finger on the vcos of a JP8, it's outta here.
How 'bout them exposed thermistors? ;-)

The comparisons between the 6 and 8 are funny too.
The JP8 came out beforehand and has an almost failsafe cassette 
load/save circuit.
You could probably save your patches on a wax cylinder and they'd reload.
But it and the autotune were too slow in Roland's opinion, so whent 
the 6 was designed, they addressed these issues. So the 6 tunes and 
saves much faster, however the 6 can be picky on it's FSK, while the 
8 is almost bulletproof.

>Mike B.
>
>P.S. -- the lack of a "perfect" VCO hardware calibration function in the Rev3
>Prophets is why the pitch gets so wacky when you do large pitch bends.  The
>pitch bend CV's are fed directly to the VCO CV summers, and if the hardware
>scaling is off, they will not bend together -- even if the autotune has been
>recently hit, and the VCO's are playing perfectly in tune from the 
>keyboard.  This
>is especially apparent in unison mode, or when you pitch bend large chords. 
>The same is true of wide LFO pitch modulations -- unison-tuned VCO pairs and
>notes in chords go further and further out of tune with each other, 
>the farther
>you modulate them from center pitch (try it with a squarewave LFO...).  The
>Prophet designers just didn't bother (and likely couldn't bother, due to
>hardware and software limitations) to run the pitch bend and LFO mod 
>CV's through
>some sort of autotune compensation, like they did with the keyboard and
>transpose CV.  If you think about it for a minute, you'll probably 
>realize why this
>would be really, really hard to do, or at least highly impractical to do in
>1982.  I've never had the occasion to check this on the OB8, but I'm 
>betting that
>it has a similar problem with wide pitch bends, too.

This is also common with Minimoogs.
While they used 1% resistors in the summers, they aren't matched and 
putting up the pitchwheel can detune them. The only resistors in the 
mini summer that aren't 1% are for the main tuning knob, where Moog 
likely felt it unnecessary.

-- 
Regards,
Kevin Lightner

http://www.synthfool.com