Why has audio range FM (via X-Mod) an instable tuning ?

[nertsch]

Hi,

When using audio Range FM via Poly-Mod (target is "Freq 1" and both oscillators operating in audio range) it is possible to create great "metallic" sounds.
But when doing that every voice has a noticeable different beat frequency and the "octave purity" gets worse with increasing modulation level.
Therefore it is not possible to play melodic lines over a few octaves or even chords, without sounding "wrong" and out of tune.
Imho this form of FM can just be used for non-melodic effects or monophonic operation.

To be said: Slop and Detune are set to 0. The device is tuned (in the range of a few cents, checked with a tuner)  and also it sounds in tune when played without FM

Is there a reason why the P6 (and also the OB6) behaves in this way ?
Or should that not be the case and I have a non properly working device ?

br
Gerhard

[the8bitdeity]

I believe this the nature of exponential FM, whereas linear FM can be more stable but generally requires the digital domain to pull off successfully.