Chroma Features and Chromagrams

A chroma representation maps spectral energy to pitch classes, typically the twelve semitone classes of Western equal temperament. Frequencies separated by an octave map to the same class, so the representation emphasizes harmonic content while discarding register. A chromagram displays chroma values over time, with one axis for pitch classes and another for frames. Chroma can be derived from a short-time Fourier transform, a constant-Q transform, or related frequency representations. The constant-Q transform uses frequency bins spaced geometrically, which can align naturally with musical pitches. Spectral energy from relevant bins is accumulated into pitch classes. The exact mapping depends on tuning reference, frequency range, normalization, and transform parameters. Librosa offers chroma functions such as chroma_stft and chroma_cqt with different assumptions. This folding makes chroma useful for comparing harmonic patterns when octave differences should matter less. It can support chord recognition, key estimation, music segmentation, or cover-song alignment. But chroma alone does not identify a chord with certainty. Different chords can share pitch classes; bass position, note duration, rhythm, voicing, and context provide information that simple chroma discards. Percussion and noisy spectra can also produce misleading energy. A chromagram can be normalized frame by frame or across a recording, which changes what amplitude variation means. Tuning differences shift energy among neighboring bins; a globally detuned recording may appear to have weaker or smeared chroma unless tuning is estimated or corrected. Time resolution depends on hop length, while frequency resolution and leakage depend on transform settings. Use chroma as a feature rather than a musical truth label. Pair it with temporal context or other features for tasks that require chord sequences. Evaluate on recordings with varied instruments, tuning, and production styles. Report transform choice, tuning assumptions, normalization, and frame rate so analysis can be reproduced.