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

Abstract

This paper describes an expert system called CHORAL, harmonization of four-part chorales in the style of Johann Sebastian Bach. The system contains about 350 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, the melodic lines of the individual parts, and Schenkerian voice leading within the descant and bass. The program harmonizes chorale melodies using a generate-and-test method with intelligent backtracking. A substantial number of heuristics are used for biasing the search toward musical solutions. The CHORAL knowledge base provides for style-specific modulations, cadence patterns, and complex encounters of simultaneous inessential notes; it imposes difficult constraints for maintaining melodic interest in the inner voices. Encouraging results have been obtained, and output examples are given. BSL, a new and efficient logic-programming language fundamentally different from PROLOG, was designed to implement the CHORAL system.