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A signal processing method is described, which separates harmonic sounds by applying linear models for the overtone series of sounds. Time-varying sinusoidal parameters are estimated in an iterative algorithm which is initialized using a multipitch estimator that finds the number of concurrent sounds and their frequency components. The iterative process then improves the estimates using the least-squares criterion. The harmonic stucture is retained by keeping the frequency ratio of overtones constant over time. Overlapping frequency components are resolved by using linear models for the overtone amplitudes. In practice, the models retain the spectral continuity of natural sounds. Simulation experiments were done using some basic structures for the linear models. These include polynomial, mel-cepstal and frequency-band model. Demonstration signals are available at http://www.cs.tut.fi/∼momasv/demopage.html.