Bill is correct. When building your own Performances on the CP40 STAGE, if you select an acoustic piano sound to be layered with an instrument sound like strings or a synth pad, you will want to avoid the Acoustic Piano Voices that are “stretch tuned” and use an Acoustic Piano that is “flat” tuned.

“Stretched tuning” (is a fairly widespread method of tuning acoustic pianos) works well when playing the piano alone; “Flat tuning” means it is tuned properly at the extremes… as in ‘not stretched’ — the so-called “flat” tuned acoustic piano Voices work well when layering with synth sounds. If you select a stretch-tuned Voice you will hear beating on notes at the extremes.

The regular “flat” tuned acoustic pianos will have “Fl” in their name “CF StFl” is a Yamaha CF acoustic piano Stereo Flat-tuned.

Other Possibilities:
Of all the possible things that could be causing the unwanted modulation, the MW would be the very last on the list. An Effect inadvertently left on — effects can include Detuning. But the most likely cause is the tuning issue between synth sounds which are tuned across all keys, and the main acoustic piano Voices that are stretched.

Let us know.

I think this is an interesting topic! Although I'm not under the illusion that my hearing is very sensitive to these kinds of phenomena, I'm curious how the tuning of the CP88 pianos is (flat or stretched). I have not yet observed the effect described above with a layered piano-string combination. Hopefully someone can shed some light on this, thanks!

Typically, you’ll find that when stretch tuning is used, Yamaha will indicate this in the product and/or in the documentation. The design concept of the CP88 (Stage Keyboard) is weighted toward quick, on-the-fly programming when necessary, and the layout of the sections lends itself quick and easy layering. And thus the piano sounds are tuned so they encourage layering the acoustic pianos with other instrument sounds.

The CP4/40 STAGE delve a bit deeper into the programming details that invite sound designing… they feature SCM (Spectral Component Modeling) and are designed for those who want to get a bit deeper into the programming area.

I see, thanks for the enlightening answer! My first stagepiano was a P250 and I believe that some of these things were also possible with regard to tuning. It was a great board, but I love my CP88 more.