Towards Timbre-Invariant Audio Features for Harmony-Based Music

Chroma-based audio features are a well-established tool for analyzing and comparing harmony-based Western music that is based on the equal-tempered scale. By identifying spectral components that differ by a musical octave, chroma features possess a considerable amount of robustness to changes in timbre and instrumentation. In this paper, we describe a novel procedure that further enhances chroma features by significantly boosting the degree of timbre invariance without degrading the features' discriminative power. Our idea is based on the generally accepted observation that the lower mel-frequency cepstral coefficients (MFCCs) are closely related to timbre. Now, instead of keeping the lower coefficients, we discard them and only keep the upper coefficients. Furthermore, using a pitch scale instead of a mel scale allows us to project the remaining coefficients onto the 12 chroma bins. We present a series of experiments to demonstrate that the resulting chroma features outperform various state-of-the art features in the context of music matching and retrieval applications. As a final contribution, we give a detailed analysis of our enhancement procedure revealing the musical meaning of certain pitch-frequency cepstral coefficients.