Understanding and Compressing Music with Maximal Transformable Patterns

We present a polynomial-time algorithm that discovers all maximal patterns in a point set, 𝐷⊂ℝ𝑘, that are related by transformations in a user-specified class, 𝐹, of bijections over ℝ𝑘. We also present a second algorithm that discovers the set of occurrences for each of these maximal patterns and then uses compact encodings of these occurrence sets to compute a losslessly compressed encoding of the input point set. This encoding takes the form of a set of pairs, 𝐸={⟨𝑃1,𝑇1⟩,⟨𝑃2,𝑇2⟩,…⟨𝑃ℓ,𝑇ℓ⟩}, where each ⟨𝑃𝑖,𝑇𝑖⟩ consists of a maximal pattern, 𝑃𝑖⊆𝐷, and a set, 𝑇𝑖⊂𝐹, of transformations that map 𝑃𝑖 onto other subsets of 𝐷. Each transformation is encoded by a vector of real values that uniquely identifies it within 𝐹 and the length of this vector is used as a measure of the complexity of 𝐹. We evaluate the new compression algorithm with three transformation classes of differing complexity, on the task of classifying folk-song melodies into tune families. The most complex of the classes tested includes all combinations of the musical transformations of transposition, inversion, retrograde, augmentation and diminution. We found that broadening the transformation class improved performance on this task. However, it did not, on average, improve compression factor, which may be due to the datasets (in this case, folk-song melodies) being too short and simple to benefit from the potentially greater number of pattern relationships that are discoverable with larger transformation classes.