11
Pitch
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 the ATR-1 detects pitch
In order for the ATR-1 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 ATR-1 does exactly this: It looks for a periodically repeating waveform
and calculates the time interval between repetitions.
The pitch detection algorithm in the ATR-1 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 (about 1 to 10 milliseconds), the
output pitch can be detected and corrected without artifacts in a seamless
and continuous fashion.
The ATR-1 was designed to detect and correct pitches up to the pitch C6. If
the input pitch is higher than C6, the ATR-1 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 ATR-1 will detect pitches as low as A0
(55Hz). This range of pitches allows intonation correction to be performed
on all vocals and almost all instruments.
Of course, the ATR-1 will not detect pitch when the input waveform is not
periodic. As demonstrated above, the ATR-1 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 ATR-1 will have difficulty determining
the pitch of the composite (voice + noise) sound. Luckily, there is a control
(the SENSITIVITY control, discussed in Chapter 4) that will let the ATR-1 be a
bit more casual about what it considers “periodic.” Experimenting with this
setting will often allow the ATR-1 to track even noisy signals.