Apple Logic Express 9 - Manual (Page 576)

• Harmonic Correction: When audio is transposed, not only is the fundamental pitch
transposed, but all of the formants are shifted as well—as though the whole instrument
or singer is being reduced or enlarged. This, of course, is not natural. Voices transposed
up will sound like Mickey Mouse, or like Darth Vader if transposed down. These artifacts
can be circumvented by using Harmonic Correction.
Select the Harmonic Correction checkbox to leave the formants in the transposed
material unchanged. This means that the original timbre (or the physical size of the
resonance body) is maintained, resulting in a more natural-sounding transposition. The
only trade-off is that calculation takes more time.
Note: The quality of Harmonic Correction is heavily dependent on the source material,
because the algorithm has to make intelligent decisions between tonal and atonal
components of the recording, and handle them separately. This is not an exact science
and is more accurate on monophonic material than on complex stereo material, but
you can certainly use it on a complete mix. The phase correlation of stereo recordings
is maintained.
You can also use Harmonic Correction to shift the formants without transposition. In
this way, you alter the physical size of the sound source’s resonance body—to give
female voices a male character and vice versa, for example—while keeping the pitch
in tune.
This effect (sometimes known as gender-bending) allows you to change sounds so
that they appear to have been made by unusually small or large instruments. It is useful
for beefing up thin or brittle sounding parts, such as guitars, or instruments and vocals
that were recorded through a microphone with a limited frequency response.
As a usage example on a vocal part: If you set Harmonic Shift to −300, and the
Transposition value to 0, the sonic character of the singer will be changed as though
transposed three semitones down—but without an actual transposition in pitch. This
means that a musical C remains a C, but the timbre of the vocal becomes darker.
• Harmonic: If Harmonic Correction is turned on, you can also use the Harmonic (Shift)
parameter to independently alter timbre. The units are shown in 1/100 semitone units
(cents).
• If you select the same value in both the Harmonic and Transposition fields, no
correction occurs, and the result is as if Harmonic Correction is turned off.
• If you set Harmonic to 0, the formants don’t change. This prevents the unwanted
side effects of traditional pitch shift algorithms.
Tip: If you need to find the exact transpose value by trial and error, turn off Harmonic
Correction. As soon as you’ve found the right transposition value, do an independent
Harmonic Correction, with the same value, in a second step.
576 Chapter 17 Editing Audio in the Sample Editor