7 Assessing Disputed Attributions for Organ Fugues in the JS Bach
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7 Assessing Disputed Attributions for Organ Fugues in the J. S. Bach (BWV) Catalogue Peter van Kranenburg Dept. of Information and Computing Sciences Utrecht University P.O. Box 80.089 3508 TB Utrecht, Netherlands [email protected] Abstract We describe a computational approach to musical authorship problems in which ma- chine-learning algorithms are used to recognize personal musical styles. The algo- rithms learn characteristics from representative examples and are able to use the ob- tained knowledge to classify previously unseen compositions. The pilot project fo- cuses on several organ fugues in the catalog of J. S. Bach (BWV 534/2, 536/2, 537/2, 555/2, 557/2, 560/2 and 565/2) the attributions of which have been challenged in re- cent years. With a nearest-neighbor classifier, these disputed fugues have been com- pared to a number of fugues indisputably by J. S. Bach as well as the contenders J. L. Krebs, J. P. Kellner and W. F. Bach. This comparison has been done in a subspace spanned by a selected optimal set of features. It appears that this comparison pro- vides valuable contributions to discussions about the authorship of these pieces. Some hypotheses from musicological literature can be either confirmed or rejected. Tonal Theory for the Digital Age (Computing in Musicology 15, 2007), 120–137. 120 TONAL THEORY FOR THE DIGITAL AGE 7.1 Bases for Composer Attributions One task of musicology is to study the musical past. A common approach has been to provide an overview of the most important composers of each era together with their compositions, then to study the relationships of these compositions to each other and to musical and non-musical contexts. A complementary activity has been the prepa- ration of critical editions of scores. In these activities the question of authorship re- mains paramount. If we want to present the most important composers and their works, we need to know who composed what. If we want to make a critical edition of the works of a certain composer, we cannot escape making decisions about dis- puted compositions. Problems may be caused by conflicting attributions among plu- ral sources, the lack of an authoritative source contemporary with the composer, an incomplete source, or an anonymous source which tradition holds to be by the com- poser. Attributions of the same work to multiple composers is a common phenome- non of European works of the fifteenth through nineteenth centuries. Both external and internal evidence may be used to solve authorship problems. A let- ter from a composer in which he mentions a recent composition provides strong support to claim his authorship in the presence of a rival claim. In many cases, how- ever, external evidence of a decisive nature is lacking. Here, internal evidence be- comes more important. Internal evidence may focus on handwriting, text underlay (in vocal music), physical features of the paper (e.g., watermarks, raster-line spacing), and other philological considerations. Countless disputes about the authorship of musical works stem from diverse norms of judgment about discrepancies between internal and external evidence. Stylistic evidence, which is explored here, is unlikely to sway the opinion of someone who rejects an otherwise documented work which lacks a copy signed by the composer. However, it can bring additional perspective to discussions of works whose authorship is truly unresolved. In order to assess stylistic evidence one must have a model that is able to represent musical styles in such a way that specific instances of it can be associated with a composer’s personal style to a unique degree. In manual practice, proof by example is often used to support an attribution on stylistic grounds. The most pertinent fea- ture may be a distinctive motif or chord progression that is present both in the dis- puted work and in an undisputed composition. Such similarities might, however, be occasional. If we want to support an attribution in a statistically sound way, we have to use events which occur frequently (such as notes and intervals). Computer-based assessment of musical authorship was first extensively explored by Trowbridge (1982; 1985-6), who revealed differences in style among four Renaissance composers (Gilles Binchois, Antoine Busnois, Guillaume Dufay, and Johannes Ockeghem) by comparing the average values of 16 quantifiable features (Trowbridge 1985-6). The repertory evaluated consisted of 92 Renaissance chansons, of which two- thirds exist in a single manuscript copy with scribal attribution. Many of the rest are anonymous in at least one source, a few in more than one source. Many of the fea- VAN KRANENBURG: ASSESSING DISPUTED ATTRIBUTIONS 121 tures are coincidentally similar to those used here. They included melodic intervals, harmonic intervals, chord types, bass progressions, root progressions, root distribu- tions, prepared dissonances, chord durations, chord motion, texture reduction, me- lodic direction, rhythmic activity, average melodic range, relative melodic motion, voice crossing, and harmonic range. A good account of still earlier systems for quan- titative analysis is given in Trowbridge (1982). For polyphonic music Trowbridge’s thesis is by far the most thorough and comprehensive of its time. Far more prevalent today are studies that isolate and analyze musical features for differentiation of pieces by genre (e.g., McKay and Fujinaga 2004), mood (e.g., Dan- nenberg 1997), or idiosyncratic traits of individual composers (e.g., Cope 1991, Cope 1998). Cope’s intent was to replicate style and genre of individual composers in his very extensive Experiments in Musical Intelligence (1980-2005). 7.2 A Machine-Learning Approach to Stylistic Assessment Our approach to authorship problems employs machine-learning algorithms. These algorithms learn characteristics of musical styles from representative examples, and are then able to use the obtained knowledge to classify previously unseen composi- tions. The repertory on which we focus here consists of seven organ works attributed in the Schmieder (BWV) catalogue (1950/2nd rev. edn. 1990) to Johann Sebastian Bach but disputed on stylistic (or other) grounds in recent musicological literature. In an earlier publication the authorship of the fugue for organ in F minor (BWV 534/2) was evaluated (Backer and Van Kranenburg 2005). In the current article, another classifi- cation algorithm is used and the dataset extended with six additional fugues listed in Schmieder and with control compositions by Johann Peter Kellner. The works inves- tigated experimentally here and the proponents for and against Bach’s authorship are summarized in Table 7.1. BWV number, Supporting J. S. Questioning J. S. Proposed alternative title* Bach’s authorship Bach’s authorship composer or medium BWV 534/2 Humphreys (1985) Kellner or J. L. Krebs Breig (1993), with reservations about dating Dirksen (2000) W. Fr. Bach BWV 536/2 Humphreys (1989, Attributed to Kellner 2000) BWV 537/2 O’Donnell (1989) Fugue ending by Joh. Ludwig Krebs Breig (2000) 122 TONAL THEORY FOR THE DIGITAL AGE BWV number, Supporting J. S. Questioning J. S. Proposed alternative title* Bach’s authorship Bach’s authorship composer or medium BWV 555/2 Keller (1937) Possibly by Krebs Tittel (1966) Not by Krebs BWV 557/2 Keller (1937) Possibly by Krebs Tittel (1966) Not by Krebs BWV 560/2 Durr (1987) BWV 565/2 Williams (1981) Transposed from solo violin work in A Minor? Humphreys (1982) Attributed to Kellner Claus (1998), but suggests late rather than early work. Rejects possibility of Kellner’s authorship. Billeter (2004) For cembalo? Table 7.1. Summary of recent claims about the authorship of particular Bach organ fugues. [The indication (*/2 signifies the second movement (i.e. the fugue) of a prelude-fugue pair.] (Questions of performing medium—violin vs. keyboard, harpsichord vs. organ— have not been investigated here, since they would require evaluation of different fea- tures from those used to evaluate authorship and a different set of control works.) 7.3 Modeling Musical Style In Style and Music Leonard Meyer developed a theory of musical style that can be used as a starting point for studies that compare musical styles algorithmically. He defines style as a replication of patterning, whether in human behavior or in the artifacts produced by human behavior, that results from a series of choices made within some set of constraints (Meyer 1996: 3). In the process of composing, a composer is subjected to certain constraints while making his choices. Meyer distinguishes three levels of con- straints. Laws (1) are universal. One cannot, for example, ask a piccolo to play a con- tra G. Rules (2) are intracultural. It is in the rules that music from the Renaissance dif- fers from music from the Baroque. Strategies (3) are constraints to which the com- poser subjects himself within the rules of a certain culturally established style. Thus it is in the strategies that the music of G. F. Handel differs from the music of G. Ph. Telemann. Not all strategies reside on a conscious level. Certain patterns are ingrained during the training and development of a composer and are not replicated consciously every time during the process of composing. (Meyer’s “unconscious strategies” correspond VAN KRANENBURG: ASSESSING DISPUTED ATTRIBUTIONS 123 to the concepts of unconscious “fingerprints,” “signatures,” and “earmarks” in the extensive work on style simulation by David Cope (1991, 1998 et al.). Meyer indicates the necessity of statistics: since all classification and all generaliza- tion about stylistic traits are based on some estimate of relative frequency, statistics are inescapable (Meyer 1996: 64). It can be expected that each composer has idio- matic, countable patterns that are more often replicated in his works than in compo- sitions by other composers. The task is to find features in which such patterns are re- flected. 7.4 The Dataset 7.4.1 Selected Features There is no well-tested theory available that predicts which features have to be used to solve a particular authorship problem.