Apple Logic Express 8 - Manual (Page 534)

534 Chapter 21 Editing Audio in the Sample Editor
Using the Time and Pitch Machine
You can use the Time and Pitch Machine to radically alter the time structure of audio
files, including time compression or expansion, and pitch transposition. When changing
the pitch, you can also correct any alteration of the formants. Pitch shifting without
formant correction causes a phenomenon commonly known as the Mickey Mouse
effect.
The Technology of the Time and Pitch Machine
The Time and Pitch Machine analyzes the spectral components and dynamics of the
digital audio material, and processes the result. The algorithm endeavors to retain as
much spectral and dynamic information as possible, and minimizes phase variations.
In stereo files, the phase relationship between the left and right channels is fixed, and
not altered. Doubled sound events are kept to a minimum.
You should, however, bear in mind that apart from resampling (transposition), the
Time and Pitch Machine has to achieve the “physically impossible”: When a sample is
lengthened, information needs to be invented. This should be as realistic as possible.
Conversely, when a sample is shortened, information has to be cut out. This cut
information should be as unimportant to the overall character of the sound as
possible. Lengthening is more difficult than shortening, and if you have a choice, it’s
better to speed up a drum loop than to slow it down.
There’s always a small deviation between the set stretch or compression factor, and
the actual result. This is because the algorithm needs some freedom to optimize the
spectral and dynamic integrity (the sound quality). The deviation from the set value is
only a few milliseconds (or fractions of a bpm). This shouldn’t present a problem, as
the absolute deviation is independent of the length of the processed section. Put
another way, this means the deviation is no greater in longer files.