Apple Logic Pro 9 - Manual (Page 1228)

Detuned octaves are more noticeable to the ears, so your choice should be obvious.
The Comma
The difference between a perfectly tuned octave and the octave resulting from a tuned
circle of fifths is known as the comma.
Over the centuries, numerous approaches have been tried to solve this mystery, resulting
in a range of scales (before arriving at equal temperament—the 12 tone scale).
Other historical temperaments that have been devised emphasize different aspects of
harmonic quality. Each compromises in some way or another. Some maximize pure thirds
(Mean Tone) while others emphasize pure fifths at the expense of the thirds (Kirnberger
III, for example).
Every temperament has its own character, and a given piece of music may sound fine in
one key but awful in another. Transposing a piece to a new key can completely change
its character.
Careful attention must be paid to the selection of temperaments for authentic
performances of historic keyboard music. The wrong choice could result in an
unsatisfactory and historically inaccurate musical experience.
About Equal Temperament
Equal temperament takes the tuning error (the comma), and spreads it equally between
each step of a chromatic scale. The result is actually a scale of equally mistuned intervals,
with no interval grossly out of tune, but none in perfect tune. Equal temperament has
become the de facto standard for two main reasons:
• Convenience: Retuning an instrument to a temperament that is better-suited for a
particular piece of music is a hassle. Many instruments are not capable of being
alternately tuned (fretted string instruments, for example).
• Portability: All Western musical pieces can be performed (adequately) on an instrument
tuned to equal temperament. Obviously, some of the nuances may be missing in pieces
that were originally performed in another temperament.
What Is Hermode Tuning?
Hermode Tuning automatically controls the tuning of electronic keyboard instruments
(or the Logic Pro software instruments) during a musical performance.
In order to create clear frequencies for every fifth and third interval in all possible chord
and interval progressions, a keyboard instrument would require far more than 12 keys
per octave.
1228 Chapter 43 Project Settings in Logic Pro