[sdiy] Polyphonic temp compensation in VCF's!

[karl dalen]

Tom Wiltshire <tom at electricdruid.net>:
> To be honest, I think the autotune solution is perfectly
> acceptable.Rarely does the temperature you're operating a
> synth in vary enormously within a few minutes, so if you
> make sure the synth is at the ambient temp and switch on and
> allow it to warm up before you autotune, you'll get a result
> which is pretty consistent between sessions.

It's still +3300 ppm in there, and some manufacturer considered
it necessary to use tempcos, best argument are the ARP Chroma
due to the way it uses VCF's to generate sine waves and FFM.
Imagine a Chroma FM patch with VCF sines and no tempcos?
 
But your argument could stand well considering the OB M6 design.

One interesting test would have been if i had a Chroma to remove
the tempcos from one voice and comnpare with a voice with tempcos
in various voice modes.

> I once took my Polysix around to friends house on a cold
> winter's day. Although it was only outside for five or ten
> minutes or so when I arrived the tuning was unplayably bad.
> And there's no autotune on a polysix...

>It was a long time ago. I only remember the very-cold-then-warmed-up
>Polysix going "Blong! Bling! Blang!" when you pressed successive notes
> i.e. the voices were no longer close to identical.

>Florian's point about the single exponential convertor is very pertinent, >since that suggests that the problem was mostly from the filters >drifting. Which is what we were talking about - crumbs!

When the "polysic" goes boing-bling-bloing are not the filters
but the VCO's, try this, hold a chord, now turn the power on and
you will get a neat poly glide. :)

The glide are due to the heat up of the expo, different out
of order voltages are muxed out during this heat up procedure
until the expo settled. 

Harry mentions the PV, the OB xpander uses similarly method.

KD