chroma

Pitch-class is the 12-tone theoretical concept from Western music theory (centuries old). Chroma is a 12-dimensional feature vector that encodes pitch-class energy distribution — it was designed to represent pitch-class information in audio analysis. Chroma derives from and implements the pitch-class concept.

Chroma is a specific pitch-class representation: it folds all octaves of the pitch spectrum into 12 pitch classes, representing the equivalence of pitches separated by octaves in music. Chroma is a particular form of pitch-class encoding.

Chroma IS a specific kind of spectral representation — it represents the spectral energy distributed among octaves/pitch classes. A competent speaker would call chroma 'a spectral representation.' Files against the nearest kind.

Chroma extraction operates on signal processing to function — FFT, filtering, and spectral analysis are the computational primitives that make chroma computation possible. Remove signal processing and chroma algorithms collapse entirely.

Chroma extraction operates on the magnitude spectrum — removing spectrum and chroma has no spectral data to quantize into pitch-class bins. Removal test passes.

chroma IS a specific kind of spectral feature — it represents the distribution of acoustic energy across pitch classes (C, C#, D, ... B), computed from a frequency spectrum. A competent speaker would call chroma 'a spectral feature'. Nearest kind: spectral-feature (not stft, not magnitude-spectrum).

chroma computation requires magnitude spectrum as input — chroma bins accumulate energy from magnitude spectrum frequency bins into pitch class bins. Remove magnitude-spectrum and chroma has no data to bin. Removal test passes.

Chroma extraction is designed and maintained for the sake of music information retrieval — it extracts pitch-class profiles specifically to support MIR tasks like key detection, chord recognition, and genre classification. The designed purpose is to further MIR's operation. Law 8d: servant→master, purpose-by-design.

Chroma features are computed by folding mel-frequency-spectrum data into pitch-class bins. Remove mel-frequency-spectrum and chroma has no spectral input to fold — it stops operating. Removal test (Law 8) passes.

Chroma features are designed for tonal analysis — they fold spectral energy into pitch-class bins specifically to support tonal classification and key detection. By design, chroma serves tonality's operation.

Chroma features compute spectral energy distributed across 12 pitch-class bins. Remove pitch-class and chroma has no bins to partition energy into — its entire operation depends on the pitch-class concept. This is constitutive, not just historical: pitch-class is the organizational structure chroma computes upon.

PCP is computed from chroma vectors — the chroma representation is the direct input to PCP calculation. Remove chroma and PCP cannot be computed. The removal test passes: PCP does not operate without chroma as its data source.