Essl designed Lexikon-Sonate to never finish

The system

Lexikon-Sonate is composer Karlheinz Essl's generative software instrument for piano, begun in 1992 and, by his own description, still not finished. Essl's own work page calls it 'an interactive realtime composition environment for musical composition and live performances,' generating what it describes as 'an excerpt of a virtually endless piano piece,' sent directly to a player piano rather than performed from a fixed score. The title refers to Andreas Okopenko's 1968-70 hypertext novel Lexikon-Roman, whose alphabetically cross-referenced chapters inspired Essl's own non-linear structure.

What the documents establish

Essl's page describes the software as a set of modular 'structure generators,' each derived from analysis of classical and contemporary piano writing: 'Each module generates a specific characteristic musical output as a result of the compositional strategy that has been applied,' using random generators that 'control each other' to move along 'a scale between completely deterministic and completely chaotic behaviour.' Gesture models drawn from Bach, Mozart, Beethoven, Webern and Stockhausen appear as what Essl calls allusions, not quotations. A companion page on the Lexikon-Projekt documents a 1996-1998 multimedia extension built with Okopenko and Regina Freimueller, in which clicking keywords in a hypertext reading station sent control signals that altered the piano software's output live, producing what the page calls 'a unique and non-reproducible version' with every visitor.

Craft and rights

Because Lexikon-Sonate generates continuously rather than producing a fixed work, the usual question of what counts as 'the piece' does not have a single answer; Essl's own documentation treats each realtime output as one instance of an ongoing system, not a final composition to be reproduced identically. The stylistic borrowing here is limited to abstracted gestures rather than sampled audio or a corpus of copyrighted scores, which keeps the rights picture simple compared with systems trained directly on recorded performances, but it also means any claim that the software 'sounds like' a named composer is describing an allusion Essl designed on purpose, not an artifact of undisclosed training data.

Outcomes and open questions

Essl's site documents the software's platform migrations in detail, from 1990s MIDI output to Yamaha Disklavier and Bösendorfer computer pianos, through a 2018 version adding VST plugin support that removed the MIDI dependency, to a 2020 64-bit release, which means a description of 'what Lexikon-Sonate sounds like' depends heavily on which decade's version is meant.

• Is a generative system being treated as a single fixed work when its own creator describes it as an ongoing, still-changing environment?
• Does a system's stylistic 'allusion' to a named composer come from a deliberately designed gesture model or from unexamined training data?
• Which technical version of a decades-old generative system does a given demonstration or recording actually represent?

Essl's own account leaves little ambiguity: Lexikon-Sonate was designed from the start as a process rather than a product, and thirty years of version history have kept it that way.