Automatic identification of chords is being investigated and several methods has been proved. A method using spectral pattern recognition has been presented previously by the authors. In that method, the chord is detected iteratively, note by note, by means of a set of patterns generated by an acoustical model of the piano. In each iteration step, the spectral components of the detected note are eliminated from the signal spectrum using a mask related with the pattern of that note. When a "staccato" chord is played on a piano, the string is struck with a higher velocity and the excitation becomes more nonlinear. As a result, intermodulation components appears in the spectrum. Taken into consideration the inharmonicity of the string vibration, the effect of those intermodulation products is a widening of each spectral component of the chord. This widening must be considered when the patterns and masks are applied during the identification process. We have found out that this widening varies with the partial and the note. It also depends on the octave (differences in hammer felt dimensions and stiffness) and on the force applied to the key ("piano", "mezzoforte" or "forte") A model for the calculation of the widening due to non-linearity is presented in this contribution. The results show that the application of this model improves the recognition process.
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