A functional approach to automatic melody harmonisation

Melody harmonisation is a centuries-old problem of long tradition, and a core aspect of composition in Western tonal music. In this work we describe FHarm, an automated system for melody harmonisation based on a functional model of harmony. Our system first generates multiple harmonically well-formed chord sequences for a given melody. From the generated sequences, the best one is chosen, by picking the one with the smallest deviation from the harmony model. Unlike all existing systems, FHarm guarantees that the generated chord sequences follow the basic rules of tonal harmony. We carry out two experiments to evaluate the quality of our harmonisations. In one experiment, a panel of harmony experts is asked to give its professional opinion and rate the generated chord sequences for selected melodies. In another experiment, we generate a chord sequence for a selected melody, and compare the result to the original harmonisation given by a harmony scholar. Our experiments confirm that FHarm generates realistic chords for each melody note. However, we also conclude that harmonising a melody with individually well-formed chord sequences from a harmony model does not guarantee a well-sounding coherence between the chords and the melody. We reflect on the experience gained with our experiment, and propose future improvements to refine the quality of the harmonisation.