Harmonic pitch class profiles

Harmonic pitch class profiles (HPCP) is a group of features that a computer program extracts from an audio signal, based on a pitch class profile—a descriptor proposed in the context of a chord recognition system.

[1] HPCP are an enhanced pitch distribution feature that are sequences of feature vectors that, to a certain extent, describe tonality, measuring the relative intensity of each of the 12 pitch classes of the equal-tempered scale within an analysis frame.

By processing musical signals, software can identify HPCP features and use them to estimate the key of a piece,[2] to measure similarity between two musical pieces (cover version identification),[3] to perform content-based audio retrieval (audio matching),[4] to extract the musical structure (audio structure analysis),[5] and to classify music in terms of composer, genre or mood.

The result of HPCP computation is a 12, 24, or 36-bin octave-independent histogram depending on the desired resolution, representing the relative intensity of each 1, 1/2, or 1/3 of the 12 semitones of the equal tempered scale.

The HPCP feature has been used to compute similarity between two songs in many research papers.

Fig.1 General HPCP feature extraction block diagram
Fig.2 Example of a high-resolution HPCP sequence
Fig.3 System of measuring similarity between two songs