I've been wanting to hear for myself if the "Railsback" adjustments make a difference. I finally took some time to make two sound files, one which uses pure equal temperament and the other which incorporates "railsback" adjustments, using the chart linked earlier that came from a Yamaha digital piano manual.
Folloing are the two links. The timbre used is a pretty common variant of a DX7 string synth patch. It's being played by a Java-based FM synth that I wrote.
6 seconds of a low drone: A1 (midi 33, 55 Hz)
6 second drone of a high drone added, A6 (midi 93, 1760 Hz)
then an arpeggio that spans A2 to A5 comes in.
With the "railsback" version, the pitch adjustments range from -8 cents for the low A1 to +9 cents for the high A6.
The difference between the two tracks is pretty subtle! I can hear it most readily when running both files at the same time, but flipping (solo/mute) back and forth between them quickly. I find it's easiest to distinguish the pitch difference between the high drones. It does seem to me that the "railsback" version sits a little easier, somehow. But I wonder if this high string synth would sound still better if it were yet a few cents sharper.
There's some detuning built into the patch, but the Op stack which predominates is a pure 1 : 1 : 4 (where 1 is the carrier and 1 and 4 are modulators stacked in series). The other stack is a "feedback" loop with the pitch set via a 0.998 factor (i.e., for 440 Hz, the detuned stack is 439.12 Hz, and in that region, 0.247 Hz is 1 cent). I think its overtones phase with the main stack overtones more than they directly affect the perceived pitch. Maybe this stack does have an impact on the sense of the octaves. But I'm not at all clear how, especially since the exact same patch is being used for all the notes being played.
I think the internal phasing of the timbre that results would tend to make clashes between the overtones and fundamentals of concurrent notes less prominent, as there is a baseline of considerable timbral activity regardless of the exact pitches being played.
So, is it worth the trouble to correct for the octave stretch? IDK. I think it depends on the context. The producer (and the musicians involved) need to use their best judgment. Sometimes if a note is slightly "off" or "pitchy" it actually works better. A subliminally sharp note might give it a bit of edge or assertiveness, a subliminally flat note might add a bit of despondency or passivity that fits the moment.
