If you try to simply play the filters in self oscillating mode, without applying any kind of Audio Mod, then you should be able to hear clearly if you have perfectly calibrated filters or not... you can hear that simply by playing thru the voices... they should be perfectly in tune... all 16 voices... mine are. If you do this following the guidelines I've been giving several times, you should get perfectly in tune filter oscillation.
Now, Audio Mod is a completely different matter... if you want rock solid FM, REV2 is NOT the way to do it... then you need digital oscillators, like on the Prophet 12. I also have inconsistencies between the voices the more i crank the Audio Mod up... it is simply unavoidable with the analog oscillators and the unstable filters (for FM at least).
Yes... i use Audio Mod a lot, but I use it in a way that will not make you notice these inconsistencies... i usually use Audio Mod to induce a clangorous transient to a sound... like a bell etc... you do this by routing the AUX envelope to Audio Mod, so that it may start out briefly at max, but then quickly go down to very low Audio Mod values... that is the key... the lower the Audio Mod amount, the less inconsistency in the sound.
There really is no other way about it... if you crank up Audio Mod, and let it stay there, you WILL get serious inconsistencies! ... if you really need to have static high values, then use it on monophonic sounds where the same voice is fired again and again... then you will not hear any inconsistencies except at extreme settings where sometimes the pitch start to drift a bit.
Also remember, that oscillator 1 is the modulator, and that for the clearest and most useable FM timbres, you should use either Triangle or Pulse... Triangle usually works best, as it represent a sine wave the most.
The problem is that FM is extremely sensitive to very slight pitch or amplitude deviations that analog simply cannot deliver.... if you want precise FM... get a digital synth... at least a digital front-end synth.
Besides... you can actually do bell'ish sounds even without the use of Audio Mod... just use the two oscillators in tandem with the self oscillating filter... with the right ratios you can also get some convincing inharmonic timbres that way.
I always consider the Audio Mod as a sort of "effects parameter"... it will introduce some weird and cool sounds, but not in a very harmonically tuned fashion... actually the inconsistency is good when you use Audio Mod to create metallic transient sounds because that makes every keystroke sound a bit different... you simply have to take the disadvantages and make them into advantages
...and yes... when you switch oscillators on or off, you sometimes hear that the tuning completely stabilizes.. i cannot tell you why that is, i bet it's a hardware thing... also turning up the SUB Oscillator may stabilize the pitch... you simply have to experiment.... i often find that having the second oscillator "OFF" makes the pitch unstable... so i simply make sure the OscMix parameter is turned as to only hear oscillator 1, and then set Oscillator 2 to anything but "OFF" to keep the pitch stable... for many sounds you really do not need the second oscillator when you are doing FM sounds... there will be situations where it just simply does not work too... trial and error is the keyword here.
This is just what you have to accept, with a purely analog synth...
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