I thought it was common knowledge, folks When you tune a piano, you`ll use forks which give a certain frequency. You`ll get a dozen in the box. Then you tune on the octaves until you get to the top end of the keyboard. It then needs to be stretched beyond the harmonic frequency of the octave in order to sound in tune and possess that brilliance! Really does . . .every grand or upright acoustic will be tuned thus originally in the old fashioned way (digital tuning devices will be used now, I should think). Certainly, 20 years or so ago, Technics had digitals on the market which had this. . . for further reading,see this link. http://en.wikipedia.org/wiki/Stretched_tuning
There are 2 main reasons for stretch tuning.
1, For the piano. (inharmonicity) Piano strings have lots of harmonics in them. Some of them, especially at the lower end of the range, have harmonics which are out of tune with the higher notes of the piano. Stretch tuning alters these lower notes to try to bring these out of tune harmonics into tune with the higher notes on the piano. In doing this, low strings that are quite thick compared to their length end up being tuned flat. The thicker the strings get, the flatter they tend to become - hence the term "stretch tuning".
2. For the human ear. The human ear doesn't have a linear response to frequency. The perception of pitch at the low and high limits of the piano are not as accurate as the middle part of the keyboard. This is due to a certain amount of inefficiency in the anatomy of the ear itself. It is simply designed for a middle range - where most sounds occur, especially human voices. Low notes sound more true to their middle octave counterparts when they are tuned slightly flat. High notes sound more true when they are tuned slightly sharp.
However - not all humans are the same in this respect. While most humans are equal in their desire not to hear inharmonicity in their piano, humans vary quite a bit in terms of the efficiency of their ears. To some, the stretch needs to be stronger than others depending on how sharp or flat they hear those upper and lower notes. The ideal of course is if you are happy with the amount of stretch the piano's inharmonicity compensation gives. That means you aren't acting against what is ideal for the piano in terms of harmonics being in tune with the middle notes.
People need less additional stretch over what was added for harmonicity, probably have more efficient ears. The high notes don't need much stretching - they really aren't that high compared to the range of human hearing and shouldn't need to be stretch much, if at all. Bass notes are more problematic for the ear because they are getting close to the lower limit of human hearing. The inefficiency is greater there.
In terms of DPs, the inharmonicity part has already been done - pretty much only on the bass side. But they usually don't adjust for the individual desires for stretch at the outer octaves. That's where somebody might want an adjustable stretch tuning. Be warned though, when you stretch that tuning yourself, you are going to be working against the piano that was sampled - you might hear the fundamental frequencies as more accurate (especially as single notes), but when you play chords, you might get some unpleasant clashes because too many harmonics are fighting with the middle notes.
Personally, I have never liked the stretch tuning adjustments on DPs, they do more harm than good in my opinion. Most DP makers do a good job of adjusting for inharmonicity - which they should because if the original acoustic piano that they sampled was well tuned, they really shouldn't have to compensate anything beyond that - the inharmonicity adjustment has already been done!
Most DP samples are done from very large acoustic pianos so the inharmonicity is fairly low anyway. I actually doubt you would add much stretch manually because it will just make other things worse.
Peter, do you hear the extreme upper and low notes as being badly out of tune?