Pitch class

A pitch class is the set of pitches separated by whole octaves and treated as equivalent. It captures a note’s identity while ignoring its register.

Pitch (music): Pitch is the perceived highness or lowness of a sound, associated with its frequency. Pitch class retains pitch identity while discarding the pitch’s register.
Modulo 12: Modulo 12 arithmetic identifies integers that differ by a multiple of 12. It models pitch classes when semitones are numbered in twelve-tone equal temperament.
Pitch-class profile: A pitch-class profile records the relative prominence of each pitch class in a musical passage or recording. It summarizes pitch-class distribution for key estimation and music analysis.
Musical register: Musical register is the range or relative height at which a sound or part is produced. Pitch class deliberately ignores the register that distinguishes these sounds.
Octave: An octave is the interval between two pitches whose frequencies have a 2:1 ratio. Pitches an octave apart belong to the same pitch class.
Twelve-tone equal temperament: Twelve-tone equal temperament divides the octave into twelve equal logarithmic intervals. Its twelve semitone steps produce twelve distinct pitch classes per octave.
Chroma feature: A chroma feature represents audio energy across pitch classes while collapsing differences of octave. It applies pitch-class equivalence to audio signal processing.
Frequency: Frequency is the number of repetitions of a periodic signal per unit of time. Unlike pitch class, frequency distinguishes octave-separated tones by numerical value.
Semitone: A semitone is the smallest standard interval in twelve-tone equal temperament. Adjacent pitch classes in twelve-tone equal temperament are separated by a semitone.
Pitch-class space: Pitch-class space is a mathematical representation of pitch classes and their relationships. It treats octave-equivalent pitches as points in a compact musical space.

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