Interactive Arrangement of Chords and Melodies Based on a Tree-Structured Generative Model

We describe an interactive music composition system that assists a user in refining chords and melodies by generating chords for melodies (harmonization) and vice versa (melodization). Since these two tasks have been dealt with independently, it is difficult to jointly estimate chords and melodies that are optimal in both tasks. Another problem is developing an interactive GUI that enables a user to partially update chords and melodies by considering the latent tree structure of music. To solve these problems, we propose a hierarchical generative model consisting of (1) a probabilistic context-free grammar (PCFG) for chord symbols, (2) a metrical Markov model for chord boundaries, (3) a Markov model for melody pitches, and (4) a metrical Markov model for melody onsets. The harmonic functions (syntactic roles) and repetitive structure of chords are learned by the PCFG. Any variables specified by a user can be optimized or sampled in a principled manner according to a unified posterior distribution. For improved melodization, a long short-term memory (LSTM) network can also be used. The subjective experimental result showed the effectiveness of the proposed system