Apple Logic Pro 7 - Manual (Page 407)

Chapter 11 Sample Editor 407
Time Machine—Technical Background
The Technology of the Time Machine
The Time Machine analyzes the spectral components and dynamics of the digital audio
material, and processes the result. The high-grade 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. This approach produces a high
quality result, which is smoother than what you’re probably used to hearing from other
products, despite the speed of the process.
You should, however, bear in mind that apart from resampling (transposition), the Time
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 it is 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.
About Harmonic Correction
The entire spectral structure is usually shifted when audio is transposed—whether the
method used is digital (editing samples), or analog (changing the tape speed). During
this process, not only is the fundamental pitch transposed, but all of the sound source’s
resonances (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 by
“normal” methods will sound like Mickey Mouse, or conversely, if transposed down, like
Darth Vader.
The Harmonic Correction feature of the Time Machine allows you to correct this
unnatural spectral shifting of formants.
You can also shift the formants without transposition. This means you can 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 is a
remarkable effect, allowing you to change sounds so that they appear to have been
made by unusually small or large instruments.
Harmonic Correction is completely different from the effect of an equalizer. You could
never use an equalizer to change the apparent size of a resonant cavity.