Physical Modeling of Plucked String Instruments with Application to ...

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V_,LIMAKIET AL.PAPERSsign the lowest string that can play the given pitch to While pitch is a well-defined property--in practice itthe note. A more elaborate method would be allocating corresponds roughly to the fundamental frequency ofthe strings so that the movement of the left hand would a note--expression cannot be uniquely defined. Still,be minimized. This can be accomplished easily if it is everyone "feels" what it means. Although these feelingspossible to preprocess the score. However, this latter are not necessarily the same, it can be agreed that amethod cannot be used with real-time control. As a eom- low value of expression means something like colorless,promise, nearly real-time control can be used [49]. passing, or soft, whereas a high value can be interpretedas emotional, bright, emphasized, or loud. Moreover6.2 MIDI and Intelligent Synthesis Control the interpretation of the expression seems to be closelyMIDI has several caveats when used with complex related to the performance style. The realization of thissound synthesis such as physical modeling. In principle, led to the idea of combining the timbre and the mappingone could use many different controller messages for of pitch and expression to the actual synthesis paramdetailedcontrol over the synthesis. This approach, how- eters into performance styles, like classical guitar, bluesever, has severe drawbacks. It is rather difficult to con- guitar, or flamenco guitar.trol many parameters manually and there are many pa- Reducing the control parameters to only two drastirametercombinations that simply do not make a sound, cally reduces the complexity of the control interface,Another problem is that MIDI glues together the pitch and it still allows human interaction. For more detailedset and note triggering into a single "note on" message, control, the attack part of the notes, which carries theThese events are clearly separated in time on the variable most important indication of the playing technique,source instruments. Therefore when such sound is syn- could be controlled separately. On the keyboard thesethesized, these control events should be separated to expression controls could be assigned to the aftertouchcreate natural note transitions,and key velocity, respectively.In spite of these problems it would be convenient touse existing MIDI equipment with new synthesizers. 6.4 Overview of OuickMuslcWe suggest a new approach for controlling complex QuickMusic is the musical sound synthesis controlsynthesis with MIDI: let the control interface--which package of the QuickSig system (see Fig. 33) and ismaps the performance events into control events--be written entirely in Common Lisp/CLOS. It contains amore intelligent. By intelligent we mean that it should platform-independent sequencer, which uses a Lisp-synworkreasonably, even if not all details are given explic- tax musical notation and a real-time MIDI interface.itly. The missing information can be deducted from a Originally the package had been developed under Macinknowledgebase created by studying the performance tosh Common Lisp (MCL). Recently we have ported ittechniques of the given instrument and analyzing the (together with other parts of the QuickSig environment)impact of different playing techniques on the sound, to the PC platform under Allegro Common Lisp forThis approach has an additional advantage--it is much Windows.easier to play an intelligent instrument than a complexbut dumb synthesizer.6.5 The Lisp SequencerThe Lisp sequencer can be used to quickly create test6.3 Simplifying the Control Problem note sequences or even demo pieces. For describing aThe usual timbre space has three independent param- score a Lisp-like musical notation is used, allowing easyeters--pitch, level, and timbre. These parameters are parameterizing of the' individual notes by keyword patoogeneral for controlling specialized synthesizers such rameters. The sequencer is object oriented and thus casasthose based on physical models. However, during the ily expandable.performance of a musical piece the timbre will probably The score is processed in four phases--parsing, pernotexit a bounded subspace stretched by the possible forming, exploding, and playing. During the parse phasesounds of, for example, a given acoustic guitar within a high-level object description of the note sequence isa given performance style. Naturally there will be differ- generated. The perform phase was included to allowent subspaces for different instruments and performance automatic performance of the piece using performancestyles, but the main point is that the performer can rules. This part has not been exploited so far. The exchoosethe adequate subspace a priori. It may also be plode phase generates (usually several)low-level controlnecessary to allow shifting from one subspace into an- events from the notes. Finally the built-in schedulerother during the performance if the conditions change plays the piece on the DSP hardware in real time.substantially.For moving around inside one of these subspaces we 6.6 MIDI Control Interfacecan use fewer parameters--pitch and something we call The most important elements of QuickMusic are the"expression." Choosing pitch as the primary parameter different control interfaces that map the input MIDIis motivated by our interest in traditional Western music, events (such as key on/off, wheel movement) to thewhich is entirely based on the twelve-tone scale and actual synthesis parameters. They were added to allowwhere the most important property of a note is its pitch, interactive performance and easy experimenting with re-In our approach all internal parameter calculations are al-time synthesis algorithms. As an additional advantagebased on the desired pitch, of using MIDI, an external MIDI sequencer can be used348 J. Audio Eng. Soc., Vol. 44, No. 5, 1996 May
PAPERSPHYSICAL MODELING OF PLUCKED STRING INSTRUMENTSto record and replay performances. Unfortunately this technique cannot be used in real-A platform-independent MIDI message dispatch sys- time performance.tem has been built on top of the low-level interface.Message dispatching is accomplished automatically by 6.7.3 Special Control Modesthe CLOS method dispatching mechanism, since the ge- We have experimented with advanced control modesneric method process-midi-message is specialized on for the guitar synthesizer. They help in creating a naturalboth the instrument class and the type of MIDI message, sounding guitar performance using a standard MIDI key-The MIDI control interfaces are implemented as differ- board. Two basic styles have been investigated--soloent instrumentclasses,playingand strumming.Solo Guitar: Playing a guitar solo is slightly different6.7 Operating Modes from using the MIDI mono mode. On the guitar a soloThe interactivity of Lisp makes it easy to experiment is usually played on several strings. Therefore automaticwith different control strategies. We have developed sev- string allocation is used. In the solo mode all specialeral different modes, partly to overcome the limitations playing techniques that will be discussed later areposed by MIDI itself or to facilitate playing "guitarlike" available.on a normal keyboard. The keyboard, of course, by no Rhythm Guitar: One difficulty of playing voiced guitarmeans can replace a guitar controller (just as much as chords correctly on the keyboard is the relatively largethe synthesizer does not replace the acoustic guitar), but internote separation of the chord members. To reducethese specialized modes help getting the most out of the this problem we have implemented an intelligent chordsynthesizer in terms of fidelity and ease of playing, recognizer, allowing simplified chord fingering. The algorithmtries to fit different chords to the depressed keys6.7.1 Standard MIDI Modes and the most probable chord is selected (such as A, Am,In order to be able to use existing MIDI files se- A7). It is then revoiced for the guitar using look-upquenced by others we have implemented the standard tables. When alternative voicings are possible (as inMIDI control modes as an option, most cases), the position of the previously played chordMono Mode: The MIDI mono mode is often used with can be used to select the chord in the closest position.MIDI guitar controllers to send the performance data The player can influence the selection of the chord posionseparate MIDI channels for each string. This mode tion by using different octaves of the keyboard.simplifies the MIDI implementation in the receiver. Eachperformancestylecanhaveitsownsetofchords,There is one sound generator per channel, and there is which is based on the most often used chords of theno need for voice allocation. Ambiguous pitches, which given style. Furthermore, styles can contain chords thatcould be played on different strings, do not cause a use only the upper strings, leaving the lower strings freeproblem. Pitch bend can be used individually on differ- for playing bass notes, as in folk or Latin-Americanent strings without affecting other notes, styles. In this case the bass notes can be played onWhile the mono mode is perfectly suited for the MIDI another part of the keyboard using the keyboard splitguitar, it is difficult to use it with a keyboard. Although mode, or they can be played by using a special notekeyboardsplit could be used to send data on different triggering mode. The notes are not triggered immedichannels,playing chords is quite difficult if not impossi- ately by fingering a chord, but separate keys--one keyblethis way.per string--are used topluckthestringsafterward.ThisPoly Mode: In the poly mode the data for all strings technique can be used for playing nice arpeggios as well.are received on a single MIDI channel, and the control With these techniques many common guitar styles caninterface has to assign the notes to available sound gener- be played from the keyboard with little effort.ators. There is a potential problem with instruments ofthe variable source type. Assigning a different generator 6.8 Mixing Control Modesto a new note will not result in a correct transition. String It is possible to switch between control modes duringallocation is necessary for most string instruments, performance. This feature helps to play chordal solosinterleaved with short runs. It is also possible to split6.7.2 Smart MIDI Modes the keyboard and define different control modes overSmart MIDI modes are enhanced MIDI modes. While the two ranges.they accept MIDI events intended for normal use, theytry to add more details to the sound, such as by splitting 6.9 Special Techniquesthe "note on" to separate pitch set and note start events. In this section we discuss some special playing tech-Since setting the pitch happens before starting the note, niques not available on keyboard instruments. Thesethis can only be achieved by delaying the start of a techniques can be simulated with our intelligent connoteby a small amount. Other enhancements, such as trol interfaces.automatic fret noise generation, need even more delay.Allowing a relatively large (0.5-1 s) constant pro- 6.9.1 Hammer-On and Puli-Offcessing delay helps in both cases. When the synthesizer Hammer-on and pull-off are left-hand techniques thatis driven by a sequence, the delay can be compensated a guitar player sometimes uses to pluck a note. By strikbyadvancing the given track by the amount of delay, lng a finger against a fret, one can set the string intoJ. Audio Eno. Soc., Vol. 44, No. 5, 1996 May 349i I II
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