A histogram of pitch classes
Chroma is an audio feature that attempts to represent the composition of pitch classes in a sound. Each pitch class, or “chroma”, is an equal division of the octave. By default, the Chroma algorithm will return 12 pitch classes which conveniently maps on to a western model of tonal harmony (C, C#, D, D# etc…).
We should be aware that this is not the only way to use this audio feature – the number of pitch classes (chromas) is modifiable. We should also be mindful that the Chroma algorithm is not a fundamental pitch tracker and it won’t tell you about the structure of pitch classes, only their relative strength to each other. Furthermore, the entire spectrum is wrapped into a single octave, meaning any notion of register is lost. As such it is useful for perhaps identifying rough tonal components a sound might be comprised of, but nothing about their hierarchy as a chord.