Variance in Repetitive Games Music

_412 _413

VARIANCE IN REPETITIVE GAMES MUSIC

Axel Berndt

Institute of Software and Multimedia-TechnologyFaculty of Computer Science, Technische Universitat Dresden

ABSTRACT

The musical scoring of video and computer games is facedwith the unpredictability of interaction. Music has to fol-low the process of the interactive scene but this leads totwo basic problematic situations: (i) the player is too fastand the music has to react before the current piece is fin-ished, and (ii) the player is too slow and music has tobridge a longer period than the current piece does. Whileearlier papers were mainly treating the first problem thispaper focusses the second.

As the length of an interactive scene is usually im-possible to predict it is likewise impossible to say howmuch musical material is needed. The common way tobypass this problem is to loop the music for as long as thescene lasts. This approach involves an existential danger:Sooner or later the player becomes aware of the repetition;the game scenario emerges as a mere mechanical arrange-ment and loses much of its integrity. Variance is neededthat renews the music each time it repeats. This paperpresents several views to this problem and introduces avariety of possible approaches.

1. INTRODUCTION

An important basic principle of audio-visual media scor-ing is that musical change indicates (and therefore neces-sitates) a corresponding change in the scene, the narration,or the dramaturgy, even if not visible [12]. In interactivemedia such changes are highly dependent on user interac-tion. It is generally impossible to predict the amount oftime a player will spend in a scene or how long a certainsituation lasts until the player makes that triggering inter-action. Thus, it is equally impossible to plan the lengthof the corresponding music in advance. The endless loopis the most common means today to musically stay at asituation for an uncertain period.

But the exact repetition of a complete musical piece isa very specific means too that can become very conspic-uous to the player and, therefore, desires a corresponding(narrative) reason. Most commonly, it indicates the re-currence of scenic content or action. Its symbolic qualityand associative power can evoke a deja` vu like effect. Butthis is not the case in the common gaming situation wherethe music is just waiting for the player. The player didnot reenter a scene, nothing recurs, time passed on. Exact

repetitions emit the unnatural aura of a time warp whichusually contradicts the scenes actuality. Music becomesa foreign object to the scene, disturbing the dialectic unityof the multimedia. This effect is perceived even more in-tense since the player, when recognizing repetition, be-comes consciously aware of the music and its contradic-tion to the scene. At this point even the best musical com-positions is in danger to disturb, bore, and annoy. Collinsreports that in this situation the players even interrupt thegameplay and manually switch off the music in the gamesettings menu [7].

To prevent such undesirable effects the repetition hasto concealed from the listener. There is a variety of waysto do this which ultimately lead to the concepts of musicalvariation. In the following we will introduce and discussdifferent views and approaches to variance in repetitivegames music.

2. THE COMPOSERS APPROACH

Memorable features of the compositional structure are theprimary hints that listeners recognize and remember. De-rived from this insight, games music composers try to con-ceal repetition by a more diffuse structural layout that im-pedes the recognition of specific features [14].

A leading melodic part is one of the strongest catch-ers of attention. It is very often built of motivic figuresthat recur and vary over time. Due to its inherent formalprinciples, the melody mediates a strong feeling of struc-ture and form [8]. A common way to conceal this is toabdicate motivic work and to split the melody into multi-ple preferably overlapping figures, diversified into a poly-phonic formation. This technique is also well known fromcomposers of the romantic era.

Furthermore, clear structural borders are easily memo-rizable features. Fluent structural borders can be achievedby polyphonically overlapping structural layers and seam-less connections of consecutive form elements.

Most effective is a clever timely disposition. Prelim-inary user studies can determine the average playing be-havior. This gives clues to dispose the appropriate lengthof the accompanying pieces of music. Structural diffusionis a cheap and easy way to conceal musical repetition. Butit works only for the average player. Repetition itself isnot eliminated and recognition is just delayed, in the bestcase for long enough.

3. THE ARRANGERS APPROACH

Variety can be achieved by varying the sequence of mu-sical pieces or segments. A very coarse musicbox likeapproach in implemented in the The Elder Scrolls roll-playing games Morrowind and Oblivion1. For each stateof gameplay there is a set of musical compositions withalike characters of expression fromwhich one is randomlychosen. When the piece is over, another one is selected.

Amore fine-grained approach is to arrange on the levelof inner-musical phrase and section structure as proposedwithin several research prototypes (e.g., [6, 22]). They in-troduce two types of segments: One Shots and Loop seg-ments. So called One Shot segments are played back onlyduring the first iteration of the musical loop and skippedlater on. The Loop segments remain. Thus, the first repeti-tion appears to be a rearrangement instead of a repetition.Furthermore, by providing multiple alternatives for eachsegment (just as in musical dice games [17]) a huge num-ber of recombinations with big diversity become available[1].

But still, after a while, when all the precomposed mu-sical material in enough permutations and combinationswas introduced to the listener, its recurrence can becomeconspicuous again. More variance and a less binding mem-orable structure can be approached by very short musicalsnippets that play single figures or even just single soundevents (tones and chords) that fade in and out and mayoverlap with others. This cue collagemay sound very dif-fuse but it features the possibility to be combined with (ortriggered by) interactive events, creating a very reactivemusical score.

4. THE ORCHESTRATORS APPROACH

Orchestration deals with the different timbres of instru-ments, their playing techniques and articulations in or-der to implement a certain aesthetical concept, emphasizestructural properties of the composition, and promote adesired mood [2, 20]. Varying the instrumentation canshed different light on the otherwise unchanged musicalmaterial.

The musical polyphony offers further potentials forvariation. Music set in multiple counterpoint allows thetransposition and interchange of parts; a tenor part can,for instance, serve as a soprano and vice versa. Examplescan be found in J. S. Bachs The Art of the Fugue.

Moreover, the building set principles of the baroquemanner of the so-called rural compositions offer prospec-tive potential. Building set music can be performed inmore than one combination of parts, see [4] for examples.We implemented a music engine concept that uses loca-tion based sound sources and fading techniques to add/re-move musical material to/from the playback according tothe position and movement of the player in a virtual envi-ronment [6].

1developed by Bethesda Softworks and released in 2002 and 2006.

5. THE GENERATIVE APPROACH

Variation and improvisation are probably the oldest con-cepts of music. We can find printed evidence for sophis-ticated variation techniques already in medieval music.Later, variation became a kind of a compositional aspect.Performers had to learn how to vary a musical materialcorrectly. The baroque was the era were variation and im-provisation became a high art and necessary ability for theperformers [19]. Up to the extensive variation works ofthe classic era the development of a multitude of variationtechniques can be posted. Todays musical morphologydistinguishes variations by two aspects [3]:

Subject of Variation: The direct variation is applied tothe theme/motif, whereas the indirect variation re-tains the theme/motif unchanged and varies its ac-companiment.

Type of Variation: The strict variation saves the harmo-nic and architectural characteristics of the theme/motif. Its shape and gestalt quality remain unchan-ged. On the other hand, the free variation changesnot just melodic and rhythmic aspects, but also har-monic and formal. Each one of such variation canafford new gestalt and quality.

The variation, adaptation, and improvisation over a givenmusical material is also a classical subject in computermusic research. We have chosen representative prominentapproaches from the last decade for discussion.

5.1. Embellishment

A strict and direct type of variation is the melody em-bellishment. A given plain melody is enriched by vari-ous ornaments. In this respect the systems MeloNet andJazzNet are very interesting. They utilize a neural net-work to learn melody ornamentations, i.e., ornamentationfigures/patterns, including the melodic and harmonic con-text where they were applied [11]. This is demonstratedwith melody variations in the style of J. Pachelbel and Jazzimprovisations imitating Charlie Parker. The learning setdirectly influences the stylistic imprint of the network.

A generative music approach that utilizes genetic al-gorithms is described by Gartland-Jones,MusicBlox [10].It combines several (predefined) input patterns to createvariants. The fitness function can measure the relationaldistance to the input patterns. Thus, it is possible to applymutation and recombination operations and vary the resultwithin the domain spanned by its input patterns. This isment to be used as a combinatorial tool or toy for musiccomposition. But it can also be used to combine, for in-stance, a plain melody and several embellished versionsto create new embellished versions.

5.2. Improvisation

The improvisation can be seen as a free variation. It canchange all aspects of the original, its structure, melodic,rhythmic, and harmonic properties.

_412 _413

VARIANCE IN REPETITIVE GAMES MUSIC

Axel Berndt

Institute of Software and Multimedia-TechnologyFaculty of Computer Science, Technische Universitat Dresden

Dresden, [email protected]

ABSTRACT

The musical scoring of video and computer games is facedwith the unpredictability of interaction. Music has to fol-low the process of the interactive scene but this leads totwo basic problematic situations: (i) the player is too fastand the music has to react before the current piece is fin-ished, and (ii) the player is too slow and music has tobridge a longer period than the current piece does. Whileearlier papers were mainly treating the first problem thispaper focusses the second.

As the length of an interactive scene is usually im-possible to predict it is likewise impossible to say howmuch musical material is needed. The common way tobypass this problem is to loop the music for as long as thescene lasts. This approach involves an existential danger:Sooner or later the player becomes aware of the repetition;the game scenario emerges as a mere mechanical arrange-ment and loses much of its integrity. Variance is neededthat renews the music each time it repeats. This paperpresents several views to this problem and introduces avariety of possible approaches.

1. INTRODUCTION

An important basic principle of audio-visual media scor-ing is that musical change indicates (and therefore neces-sitates) a corresponding change in the scene, the narration,or the dramaturgy, even if not visible [12]. In interactivemedia such changes are highly dependent on user interac-tion. It is generally impossible to predict the amount oftime a player will spend in a scene or how long a certainsituation lasts until the player makes that triggering inter-action. Thus, it is equally impossible to plan the lengthof the corresponding music in advance. The endless loopis the most common means today to musically stay at asituation for an uncertain period.

But the exact repetition of a complete musical piece isa very specific means too that can become very conspic-uous to the player and, therefore, desires a corresponding(narrative) reason. Most commonly, it indicates the re-currence of scenic content or action. Its symbolic qualityand associative power can evoke a deja` vu like effect. Butthis is not the case in the common gaming situation wherethe music is just waiting for the player. The player didnot reenter a scene, nothing recurs, time passed on. Exact

repetitions emit the unnatural aura of a time warp whichusually contradicts the scenes actuality. Music becomesa foreign object to the scene, disturbing the dialectic unityof the multimedia. This effect is perceived even more in-tense since the player, when recognizing repetition, be-comes consciously aware of the music and its contradic-tion to the scene. At this point even the best musical com-positions is in danger to disturb, bore, and annoy. Collinsreports that in this situation the players even interrupt thegameplay and manually switch off the music in the gamesettings menu [7].

To prevent such undesirable effects the repetition hasto concealed from the listener. There is a variety of waysto do this which ultimately lead to the concepts of musicalvariation. In the following we will introduce and discussdifferent views and approaches to variance in repetitivegames music.

2. THE COMPOSERS APPROACH

Memorable features of the compositional structure are theprimary hints that listeners recognize and remember. De-rived from this insight, games music composers try to con-ceal repetition by a more diffuse structural layout that im-pedes the recognition of specific features [14].

A leading melodic part is one of the strongest catch-ers of attention. It is very often built of motivic figuresthat recur and vary over time. Due to its inherent formalprinciples, the melody mediates a strong feeling of struc-ture and form [8]. A common way to conceal this is toabdicate motivic work and to split the melody into multi-ple preferably overlapping figures, diversified into a poly-phonic formation. This technique is also well known fromcomposers of the romantic era.

Furthermore, clear structural borders are easily memo-rizable features. Fluent structural borders can be achievedby polyphonically overlapping structural layers and seam-less connections of consecutive form elements.

Most effective is a clever timely disposition. Prelim-inary user studies can determine the average playing be-havior. This gives clues to dispose the appropriate lengthof the accompanying pieces of music. Structural diffusionis a cheap and easy way to conceal musical repetition. Butit works only for the average player. Repetition itself isnot eliminated and recognition is just delayed, in the bestcase for long enough.

3. THE ARRANGERS APPROACH

Variety can be achieved by varying the sequence of mu-sical pieces or segments. A very coarse musicbox likeapproach in implemented in the The Elder Scrolls roll-playing games Morrowind and Oblivion1. For each stateof gameplay there is a set of musical compositions withalike characters of expression fromwhich one is randomlychosen. When the piece is over, another one is selected.

Amore fine-grained approach is to arrange on the levelof inner-musical phrase and section structure as proposedwithin several research prototypes (e.g., [6, 22]). They in-troduce two types of segments: One Shots and Loop seg-ments. So called One Shot segments are played back onlyduring the first iteration of the musical loop and skippedlater on. The Loop segments remain. Thus, the first repeti-tion appears to be a rearrangement instead of a repetition.Furthermore, by providing multiple alternatives for eachsegment (just as in musical dice games [17]) a huge num-ber of recombinations with big diversity become available[1].

But still, after a while, when all the precomposed mu-sical material in enough permutations and combinationswas introduced to the listener, its recurrence can becomeconspicuous again. More variance and a less binding mem-orable structure can be approached by very short musicalsnippets that play single figures or even just single soundevents (tones and chords) that fade in and out and mayoverlap with others. This cue collagemay sound very dif-fuse but it features the possibility to be combined with (ortriggered by) interactive events, creating a very reactivemusical score.

4. THE ORCHESTRATORS APPROACH

Orchestration deals with the different timbres of instru-ments, their playing techniques and articulations in or-der to implement a certain aesthetical concept, emphasizestructural properties of the composition, and promote adesired mood [2, 20]. Varying the instrumentation canshed different light on the otherwise unchanged musicalmaterial.

The musical polyphony offers further potentials forvariation. Music set in multiple counterpoint allows thetransposition and interchange of parts; a tenor part can,for instance, serve as a soprano and vice versa. Examplescan be found in J. S. Bachs The Art of the Fugue.

Moreover, the building set principles of the baroquemanner of the so-called rural compositions offer prospec-tive potential. Building set music can be performed inmore than one combination of parts, see [4] for examples.We implemented a music engine concept that uses loca-tion based sound sources and fading techniques to add/re-move musical material to/from the playback according tothe position and movement of the player in a virtual envi-ronment [6].

1developed by Bethesda Softworks and released in 2002 and 2006.

5. THE GENERATIVE APPROACH

Variation and improvisation are probably the oldest con-cepts of music. We can find printed evidence for sophis-ticated variation techniques already in medieval music.Later, variation became a kind of a compositional aspect.Performers had to learn how to vary a musical materialcorrectly. The baroque was the era were variation and im-provisation became a high art and necessary ability for theperformers [19]. Up to the extensive variation works ofthe classic era the development of a multitude of variationtechniques can be posted. Todays musical morphologydistinguishes variations by two aspects [3]:

Subject of Variation: The direct variation is applied tothe theme/motif, whereas the indirect variation re-tains the theme/motif unchanged and varies its ac-companiment.

Type of Variation: The strict variation saves the harmo-nic and architectural characteristics of the theme/motif. Its shape and gestalt quality remain unchan-ged. On the other hand, the free variation changesnot just melodic and rhythmic aspects, but also har-monic and formal. Each one of such variation canafford new gestalt and quality.

The variation, adaptation, and improvisation over a givenmusical material is also a classical subject in computermusic research. We have chosen representative prominentapproaches from the last decade for discussion.

5.1. Embellishment

A strict and direct type of variation is the melody em-bellishment. A given plain melody is enriched by vari-ous ornaments. In this respect the systems MeloNet andJazzNet are very interesting. They utilize a neural net-work to learn melody ornamentations, i.e., ornamentationfigures/patterns, including the melodic and harmonic con-text where they were applied [11]. This is demonstratedwith melody variations in the style of J. Pachelbel and Jazzimprovisations imitating Charlie Parker. The learning setdirectly influences the stylistic imprint of the network.

A generative music approach that utilizes genetic al-gorithms is described by Gartland-Jones,MusicBlox [10].It combines several (predefined) input patterns to createvariants. The fitness function can measure the relationaldistance to the input patterns. Thus, it is possible to applymutation and recombination operations and vary the resultwithin the domain spanned by its input patterns. This isment to be used as a combinatorial tool or toy for musiccomposition. But it can also be used to combine, for in-stance, a plain melody and several embellished versionsto create new embellished versions.

5.2. Improvisation

The improvisation can be seen as a free variation. It canchange all aspects of the original, its structure, melodic,rhythmic, and harmonic properties.

_414 _415

A genetic algorithm based approach is John Al BilesGenJam system [16]. The musical input of a co-perform-ing human musician is varied by mutation and crossoveroperations to generate an improvisational response. It ismelody based; the chord progression scheme, the tempo,rhythmic pace, and overall arrangement are predefined.GenJams stylistic repertoire reaches from Jazz over Latinto New Age. The musical quality and variety of its im-provisations strongly depends on the quality and varietyof the human performers input. In the games scenario,where we have static precomposed music to be varied, thismay lead to over-fitness problems over time.

A very popular means in computer music, Markovmodels, was used by Francois Pachet in his system Con-tinuator [18]. It builds a Markov model based structurefrom realtimemusical input of a human co-performer. Newpatterns are generated not just for the melody part but alsofor its accompaniment. Since it directly analyzes the re-altime input the system is stylistically independent to acertain amount. The system can run in standalone modelike a music generator, as a collaborative improviser andcomposer that creates continuations to the musicians in-put.

5.3. Reharmonization

The harmonization of a melody determines a sequence ofchords and creates a polyphonic counterpoint. Reharmo-nization changes one or more of these chords and adaptsthe voice leadings, accordingly. Well known is, for in-stance, the change frommajor to minor to achieve a darkermore pessimistic mood. To minorize a major triad onlyone tone, the third, has to be transposed down by a semi-tone. Changes to completely different chords imply moreand greater adaptations. This may also affect the melody.Hence, reharmonization is not necessarily only an indirectvariation.

Yoon and Lee describe a planning approach for affec-tive reharmonizations [24]. A system that implements re-harmonization is described by Livingstone [13]. It imple-ments the relatively unproblematic major-minor changes.Changes to completely different chords are implementedin the system by Stenzel [21]. However, it also proves thatgreat changes do harm to the musical coherence and con-clusiveness quite quickly. Naive changes in voice lead-ing tend to be unmelodious. Furthermore, the gesture ofthe new harmonization can be opposed to the gesture ofthe melody; a situation that is very hard to detect and toavoid. Thus, reharmonization can be seen as a non-trivialoptimization problem that is not solved, yet.

6. THE PERFORMERS APPROACH

The expressive performance introduces a series of trans-formations to the musical material. They affect its tim-ing, dynamics (loudness), and articulation properties. Bythese means a piece of music can be performed very dif-ferently and feature a variety of characters of expression

reaching from slow, soft, cantabile over harsh, marcato tofast-paced, and energetic.

For timing and dynamics of a performance we can dis-tinguish macro and micro features. The macro featuresdefine the overall tempo and loudness curve. These canbe varied completely which creates a new performance, orthey are kept and only the curve shape of transitional fea-tures (ritardando, accelerando, crescendo, decrescendo) isvaried which is more like another rendering of the basi-cally same performance.

The micro features are rubato (timing distortions thatare self-compensating within a certain timeframe), metri-cal accentuations, and articulation. These features can beconsidered as additional details that enrich the macro planbut do not change it fundamentally. We can apply anyshade of a swing timing. We can perform more neutral orvery pronounced accentuations. One accentuation schememay establish a fast-paced quarter meter whereas anothermay realize a more relaxed half meter. The overall artic-ulation may be legato in one performance and portato inanother, this creates either a tight or a brittle sound.

Most subtle differences can be achieved by randomvariations of note onset times and velocities, aspects thatwe know as random variation in human performance. Fur-thermore, the synchrony of the parts can be changed. Aleading part may be ahead to mediate an active progres-sive mood or it can be behind creating a laid-back kind offeeling. However, these means may already be too subtleand not suffice to give enough variety to the music. Hence,they should be used in combination with the other featuresdiscussed so far.

Approaches to create such expressive performancesare knowledge-based [9], machine learning-based [23],and derived from a mathematical music theory [15]. Theperformance engine described in [5] implements a tech-nique to seamlessly transition and combine different per-formances and orchestrations.

7. CONCLUSION

We have introduced a variety of approaches to tackle theproblem of repetitive music in games. The compositionaland arrangement approaches can already be found in to-days video and computer games. But they are restricted tothe variety of its limited precomposed material. The sameapplies to the polyphonic and orchestrational approacheswhich are based on static precomposed material as well.

This restriction can be overcome by introducing gen-erative aspects to the music processing. Embellishmentand reharmonization edit the precomposed music on thecompositional level. Improvisation contributes new mate-rial. The expressive performance renders the music in dif-ferent shades. All these approaches are not mutually ex-clusive but can be combined and thereby open up a muchwider range of possibilities.

8. REFERENCES

[1] S. Aav, Adaptive Music System for DirectSound,Masters thesis, University of Linkoping, Depart-ment of Science and Technology, Norrkoping, Swe-den, Dec. 2005.

[2] S. Adler, The Study of Orchestration, 3rd ed. NewYork, USA: Norton & Company, June 2002.

[3] G. Altmann,Musikalische FormenlehreEin Hand-buch mit Beispielen und Analysen. Fur Musik-lehrer, Musikstudierende und musikinteressierteLaien, 8th ed. Mainz, Germany: Schott, Jan. 2001.

[4] A. Berndt, Liturgie fur Blaser, 2nd ed. Hal-berstadt, Germany: Musikverlag Bruno Uetz, Jan.2008, transl.: Liturgy for Brass.

[5] , Decentralizing Music, Its Performance, andProcessing, in Proc. of the Int. Computer MusicConf. (ICMC), M. Schedel and D. Weymouth, Eds.New York, USA: International Computer Music As-sociation, Stony Brook University, June 2010, pp.381388.

[6] A. Berndt, K. Hartmann, N. Rober, and M. Ma-such, Composition and Arrangement Techniquesfor Music in Interactive Immersive Environments,in Audio Mostly 2006: A Conf. on Sound in Games.Pitea, Sweden: Interactive Institute/Sonic Studio,Oct. 2006, pp. 5359.

[7] K. Collins, An Introduction to Procedural Mu-sic in Video Games, Contemporary Music Review,vol. 28, no. 1, pp. 515, Feb. 2009, issue on Genera-tive Music.

[8] D. de la Motte, Melodie. Deutscher TaschenbuchVerlag, Nov. 1993.

[9] A. Friberg, R. Bresin, and J. Sundberg, Overviewof the KTH Rule System for Musical Performance,Advances in Cognitive Psychology, Special Issue onMusic Performance, vol. 2, no. 23, pp. 145161,July 2006.

[10] A. Gartland-Jones, MusicBlox: A Real-Time Al-gorithm Composition System Incorporating a Dis-tributed Interactive Genetic Algorithm, in Proc. ofEvoWorkshops/EuroGP2003, 6th European Conf. inGenetic Programming, G. Raidl, Ed. Berlin, Ger-many: Springer, 2003, pp. 490501.

[11] D. Hornel and W. Menzel, Learning Musical Struc-ture and Style with Neural Networks, ComputerMusic Journal, vol. 22, no. 4, pp. 4462, 1999.

[12] Z. Lissa, Asthetik der Filmmusik. Leipzig, Ger-many: Henschel, 1965.

[13] S. R. Livingstone, Changing Musical Emotionthrough Score and Performance with a Composi-tional Rule System, Ph.D. dissertation, The Univer-sity of Queensland, Brisbane, Australia, 2008.

[14] A. Marks, The Complete Guide to Game Audio: ForComposers, Musicians, Sound Designers, Game De-velopers, 2nd ed. USA: Focal Press, Dec. 2008.

[15] G. Mazzola, S. Goller, and S. Muller, The Topos ofMusic: Geometric Logic of Concepts, Theory, andPerformance. Zurich, Switzerland: Birkhauser Ver-lag, 2002.

[16] E. R. Miranda and J. A. Biles, Eds., EvolutionaryComputer Music, 1st ed. USA: Springer, April2007.

[17] W. A. Mozart, Musikalisches Wurfelspiel: An-leitung so viel Walzer oder Schleifer mit zweiWurfeln zu componieren ohne musikalisch zu seynnoch von der Composition etwas zu verstehen,Kochel Catalog of Mozarts Work KV1 Appendix294d or KV6 516f, 1787.

[18] F. Pachet, The Continuator: Musical Interactionwith Style, Journal of NewMusic Research, vol. 32,no. 3, pp. 333341, Sept. 2003.

[19] J. J. Quantz, Versuch einer Anweisung, die Flotetraversie`re zu spielen. Barenreiter, 1752, Reprint(1997), H. Augsbach.

[20] E. Sevsay, Handbuch der Instrumentationspraxis,1st ed. Kassel, Germany: Barenreiter, April 2005.

[21] M. Stenzel, Automatische Arrangiertechniken furaffektive Sound-Engines von Computerspielen,Diploma thesis, Otto-von-Guericke University, De-partment of Simulation and Graphics, Magdeburg,Germany, May 2005.

[22] H. Tobler, CRMLImplementierung einesadaptiven Audiosystems, Masters thesis, Fach-hochschule Hagenberg, Medientechnik und -design,Hagenberg, Austria, July 2004.

[23] G. Widmer and W. Goebl, Computational Modelsof Expressive Music Performance: The State of theArt, Journal of New Music Research, vol. 33, no. 3,pp. 203216, Sept. 2004.

[24] M.-J. Yoon and I.-K. Lee, Musical Tension Curvesand its Application, in Proc. of the Int. ComputerMusic Conf. (ICMC). New Orleans, USA: Interna-tional Computer Music Association, Nov. 2006, pp.482486.