An expert system for harmonizing Chorales in the style of J. S. Bach

Abstract

This thesis describes an expert system project called CHORAL, for harmonizing four voice chorales in the style of Johann Sebastian Bach. An attempt has been made to describe the Bach Chorale style via approximately 270 rules written in a form of first order predicate calculus. The rules represent musical knowledge from multiple viewpoints of the chorale, such as the chord skeleton, individual melodic lines, and Schenkerian voice leading within the descant and bass. The program generates chorales from left to right, and performs intelligent backtracking until a solution satisfying all the constraints is found. A substantial number of heuristics are used for biasing the search toward musical solutions. BSL, a new and efficient logic programming language, which forms an unexpected bridge between non-deterministic language and logic programming, was designed to implement the CHORAL system, and the theoretical foundations of BSL were formally investigated.

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