Tuning

Because Pianoteq is a piano, it is not tuned by using the rules used for synths, but by using the technique that is used by piano tuners with acoustic pianos. That means that the notes do not follow a rule based on octaves with exact ratio 2, but are tuned in order to have the most consonant chords and the best possible compromise between pure fifths, pure octaves, pure 12ths, etc... (talking here about the equal temperament case), which is done by tuners by simply listening to how these chords sound (and not by measuring the frequency of the fundamental: it wouldn’t be sufficient because overtones wouldn’t be taken into account).

The consequence is that the tuning depends strongly on the overtones driven by the inharmonicity, which depends itself on the length, the tension and the diameter of the string. For that reason, the Erard preset for example has a different tuning than the C2 (or C1) series. The C2 series is however very standard in its tuning, and if you listen to it together with for example a digital Yamaha, you won't hear any noticeable beats, which means that the tunings are very close.

If you are looking for a very flat tuning to be compatible with synths (themselves flat tuned), you may want to augment the piano length in a C1 or C2 preset and of course reduce the octave stretching. Note that the octave stretching stretches the octaves mostly in the treble notes, and has a smaller effect in the medium or bass ranges (you can make it evident how it works when you push the slider completely to the right: then the high notes become exaggeratedly too high).

So the stretch parameter in this case should be set to zero? Or does the inharmonicity aspect still make a difference - in other words, should stretch be set to zero and the piano size to something very large?

I really would like to avoid having to attach a meter, make lots of measurements, and then wonder just how to make Pianoteq intone nicely with a flat-tuned synth or a definitely-not-flat-tuned guitar - none of them are!.

It's just that I find it difficult in Pianoteq to translate mentally its index of the stretch parameter into an idea of intonation in relation to other instruments. (I appreciate the stretch curve is only an abstraction, and that adjacent notes may be above or below the curve, as determined by other considerations). A similar situation applied to the unison index. I suppose what I'm really wanting is for the numeric index to be replaced by a set of verbal attributes I can understand, such as "flat tuned", "optimally tuned for solo", "tuned for ensemble", "a bit out of tune but passable", "nicely out of tune - use at your peril", etc. I don't want a headache to stop me from playing Pianoteq

in the meanwhile you can obtain a flat tuning by saving the text below in a file named ptqflat.scl and loading it in Pianoteq in microtuning mode (click on the "mu" button in the tuning section, then click in the scale menu for browsing the file).  The last value 1200.01 prevents Pianoteq to take inharmonicity into account, thus tunes Pianoteq like a synth. In that case no stretching is possible.

! ptqflat.scl
!
flat temperament
12
!
100
200
300
400
500
600
700
800
900
1000
1100
1200.01

Voxas, your questions are interesting, and you are right, it is not possible to have uniform octave stretching in the current version because it involves the sophisticated acoustic piano tuning technique which is based on inharmonicity, and inharmonicity is not uniform across the keyboard (it is much higher in the treble). But with the flat tuning that I mentioned before, we will implement a uniform octave stretching, independent of inharmonicity, like in synths.