Inverse Harmonic Clustering for Multi-Pitch Estimation: An Optimal Transport Approach

In this work, we consider the problem of multi-pitch estimation, i.e., estimating the fundamental frequencies of multiple superimposed truncated harmonic series from noisy measurements. We phrase this as recovering a harmonically-structured measure on the unit circle, where the structure is enforced using regularizers based on optimal transport theory. In the resulting framework, a signal’s spectral content is simultaneously inferred and assigned, or transported, to a small set of harmonic series defined by their corresponding fundamental frequencies. In contrast to existing methods from the compressed sensing paradigm, the proposed framework decouples regularization and dictionary design and mitigates coherency problems. As a direct consequence, this also introduces robustness to the phenomenon of inharmonicity. From this framework, we derive two domain-agnostic, frame-level estimation methods, one for stochastic and one for deterministic signals, and propose efficient numerical algorithms implementing them. In numerical studies on both synthetic and real data, the proposed methods demonstrate superior performance relative to frame-based reference methods, whereas methods utilizing multi-frame information can attain higher performance when such context is available.