Owner`s manual

5
Pitches are often described relative to one another as intervals, or ratios of
frequency. For example, two pitches are said to be one octave apart if
their frequencies differ by a factor of two. Pitch ratios are measured in
units called cents. There are 1200 cents per octave. For example, two tones
that are 2400 cents apart are two octaves apart. The traditional twelve-
tone Equal Tempered Scale that is used (or rather approximated) in 99.9%
of all Western tonal music consists of tones that are, by definition, 100
cents apart. This interval of 100 cents is called a semitone.
How Auto-Tune detects pitch
In order for Auto-Tune to automatically correct pitch, it must first detect
the pitch of the input sound. Calculating the pitch of a periodic waveform
is a straighforward process. Simply measure the time between repetitions
of the waveform. Divide this time into one, and you have the frequency in
Hertz. The AVP does exactly this: It looks for a periodically repeating
waveform and calculates the time interval between repetitions.
The pitch detection algorithm in the AVP is virtually instantaneous. It can
recognize the repetition in a periodic sound within a few cycles. This
usually occurs before the sound has sufficient amplitude to be heard. Used
in combination with a slight processing delay (no greater than 4 millisec-
onds), the output pitch can be detected and corrected without artifacts in
a seamless and continuous fashion.
The AVP was designed to detect and correct pitches up to the pitch C6. If
the input pitch is higher than C6, the AVP will often interpret the pitch an
octave lower. This is because it interprets a two cycle repetition as a one
cycle repetition. On the low end, the AVP will detect pitches as low as 42
Hz. This range of pitches allows intonation correction to be performed on
all vocals and almost all instruments.
Of course, the AVP will not detect pitch when the input waveform is not
periodic. As demonstrated above, the AVP will fail to tune up even a
unison violin section. But this can also occasionally be a problem with solo
voice and solo instruments as well. Consider, for example, an exceptionally
breathy voice, or a voice recorded in an unavoidably noisy environment.
The added signal is non-periodic, and the AVP will have difficulty deter-
mining the pitch of the composite (voice + noise) sound. Luckily, there is a
control (the Sensitivity control, discussed in Chapter 4) that will let the
AVP be a bit more casual about what it considers “periodic.” Experiment-
ing with this setting will often allow the AVP to track even noisy signals.