Peterson AutoStrobe 490ST - Manual (Page 17)

Page 19
Once a temperament is activated, its name will appear at the upper right-hand corner of the RUN screen:
KEY= C TMPR= PYTH
A=440 C 00 ¢
AUTO
SETUP MODE
1 2 3 4
d) VIEWING AND ALTERING STRETCH FILES
“Stretch”

refers to specific adjustments in the frequencies (pitches) of notes—both within octaves and over multiple
octaves—to enhance the overall quality of tuning within an instrument. While such adjustments have traditionally been
applied only to the acoustic piano, it has become increasingly popular to exploit this tuning consideration in allied
instruments such as the harp, bass, and guitar as well as in percussive instruments such as the xylophone, vibraphone, and
steel pan. The need for stretch adjustments arises from the “non-ideal” vibrational mechanisms that are inherent to such
instruments. This is not to say that they are inferior in some way; on the contrary, it is the non-ideal vibrational modes of
these instruments that lend unique interest to their tones. Rather, “non-ideal” refers to the fact that these sound vibrations
do not precisely follow simple mathematical models that yield frequencies of sound at integer ratios to one another. In short,
with no stretch tuning in these types of instruments, the overtone frequencies produced when striking lower-pitched notes
do not match the fundamental frequencies of notes in higher octaves. This results in a subjective impression of discord even
when the fundamental pitches of these notes are completely harmonious with one another!
In order to minimize this disparity, the pitches of notes in lower octaves must generally be made increasingly flat and the
pitches of notes in upper octaves made increasingly sharp—hence, the term “stretch”. However, the proper degree of
stretching depends on many factors. In a piano, for example, the thickness, tension, and type of construction of the strings
as well as the “scale” of the piano all play a role in determining how much stretch should be employed. In reality, no two
pianos would ever be properly tuned to the exact same note frequencies throughout.
Moreover, stretch tuning is a highly subjective adjustment which is necessarily a compromise between minimizing overtone
beats and maintaining the “cohesiveness” of fundamental pitch. In fact, no two critical listeners (or even one listener at two
distinct times or locations) would ever truly agree on the “ultimate” tuning of any given piano! Thus, customizing a stretch
for a piano based on the vibrations of three or four strings—as at least one tuner manufacturer does—is, in some ways,
providing a false sense of security. Even if your piano matches another perfectly at such points (yielding identical stretch
curves under such a method), does your piano “break” into wound strings at the same note as the another? Have each of your
strings really aged uniformly and in the exact same way as those on another piano? Seems unlikely, doesn’t it?!
Is there no hope! Are we destined to mire in a sea of cacophony!! Not really. True, there is no possibility of mathematical
perfection in the tuning of a piano, but, after all, this is not why pianos exist. A piano is a wonderful tool for creating sounds
that please the human ear and soul. The piano technician’s goal then is to maximize the playing and listening pleasure of
his clients. A reasonable compromise (or at least a good starting point) for tuning pianos and related instruments is to employ
stretch tuning to match those factors which have the greatest impact on the perception of pleasing pitch and harmony. For
pianos, these factors are, overwhelmingly, the type (upright or grand) and scale (string lengths) of a given instrument. The
Model 490-ST tuner addresses this need by supplying a number of preset stretch profiles which correspond to the range of
instruments most piano technicians will likely ever encounter.
The Model 490-ST comes with 9 non-erasable, unmodifiable stretch profiles (including a proprietary “G5TH” stretch for
guitar which cannot be copied). The first 8 “piano” stretches vary steadily in order from very minimal (CNGD—Concert
Grand) to extreme (SPNT—Spinet). A table describing all included stretches is presented here: