Patent Publication Number: US-9418645-B2

Title: Method of playing chord inversions on a virtual instrument

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. application Ser. No. 13/787,734, filed Mar. 6, 2013, which claims benefit under 35 U.S.C. §119 of U.S. Provisional Patent Application No. 61/607,585, filed on Mar. 6, 2012, and entitled “DETERMINING THE CHARACTERISTIC OF A PLAYED NOTE ON A VIRTUAL INSTRUMENT,” which are herein incorporated by reference in their entirety for all purposes. 
    
    
     BACKGROUND 
     Virtual musical instruments, such as MIDI-based or software-based keyboards, guitars, basses, and the like, typically have user interfaces that closely resemble the actual instrument. For example, a virtual piano will have an interface configured as a touch-sensitive representation of a keyboard, or a virtual guitar will have an interface configured as a touch-sensitive fret board. While these types of interfaces may be intuitive, they require that the user understands how to play notes, chords, chord progressions, etc., on a real musical instrument in order to implement them on the virtual musical instrument, such that the user is able to produce pleasing melodic or harmonic sounds from the virtual instrument. Such requirements may be problematic for musicians with little to no experience with the particular corresponding instrument or in music composition in general. Furthermore, some instruments may not lend themselves to virtual replicas requiring similar input articulations on the user interface. For example, brass or orchestral instruments may require air pressure or handheld instruments in order to play a note, much less a chord. MIDI keyboards are typically used as samplers to recreate these orchestral sounds and, as discussed above, require some level of keyboard proficiency and musical knowledge to produce pleasing melodic or harmonic phrases. 
     Not all users who would enjoy playing a virtual instrument are musicians who know how to form chords, construct chord progressions, or compose diatonic harmony within a musical key. Furthermore, users who do know how to form chords and play chord progressions on a real instrument may find it difficult to recreate music with the same musical proficiency on the user interface due to the lack of tactile stimulus (e.g., weighted keys, strings, bow, etc.), which the user may be accustomed to. 
     These problems lead to frustration and make a system less useful, less enjoyable, and less popular. Therefore, a need exists for a system that strikes a balance between simulating traditional musical instruments and providing an optimized user interface that allows effective musical input and performance, and that allows even non-musicians to experience a musical performance on a virtual instrument. 
     SUMMARY 
     Various embodiments provide systems, methods, and devices for musical performance and/or musical input that solve or mitigate many of the problems of existing electronic systems. A user interface presents a number of chord touch regions, each corresponding to a chord of a particular key, such as a major or minor key. The chord touch regions are arranged in a predetermined sequence, such as by fifths within the particular key. Each chord region includes a number of touch zones configured to detect and distinguish between a number of touch gestural articulations including at least one of a legato articulation, a pizzicato articulation, or a staccato articulation on a touch zone of the user interface. These gestural articulation may simulate various techniques used in playing real orchestral stringed instruments, including violins, violas, cellos, basses and the like. Depending on the touch gesture articulation that is played, a preselected audio file is associated with the touch zone. 
     In further embodiments, the touch zones are configured to provide different harmonic configurations of a base chord associated with the chord touch region. Some harmonic configurations provide progressively wider harmonic ranges across each adjacent touch zone. Other harmonic configurations can provide chords with a progressively higher relative pitch across each adjacent touch zone. 
     In yet further embodiments, a legato swipe gesture can trigger playback to two preselected audio signals for a virtual instrument, where a velocity of the swipe gesture can control characteristics of a cross-fade and amplitude of the composite audio signal. The two preselected audio signals can be loud and soft audio signals resembling the sound of a bowed stringed instrument having, where the loud audio signal includes a strong attack and heavy timbre and the soft audio signal includes a soft attack and light timbre. In some cases, as the velocity of the swipe gesture increases, the playback of the two preselected audio signals cross-fade from the soft audio signal to the loud audio signal. In contrast, as the velocity of the swipe gesture decreases, the playback of the two preselected audio signals cross-fade from the loud audio signal to the soft audio signal. 
     In certain embodiments, a method includes displaying a plurality of virtual instruments on a scrollable area of a touch-sensitive user interface. In response to a detected swipe gesture on the scrollable area, the virtual instruments cycle through or move across the display at a speed proportional to the swipe. A virtual instrument can be selected for the user interface based on its position on the scrollable area. 
     In some embodiments, a system and method is provided to modulate a life cycle of a note and, through signal processing, create smooth transitions between notes and chords played by a user by modeling a virtual momentum created by a user swiping gesture on the interface and maintaining that momentum throughout subsequent harmonic or melodic progressions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic illustration of a user interface for a virtual musical instrument, according to an embodiment of the invention. 
         FIG. 2  illustrates a plurality of gestural legato articulations over a plurality of musically defined touch regions, according to an embodiment of the invention. 
         FIG. 3  illustrates a two-finger auto-legato gestural articulation over a plurality of musically defined touch regions, according to an embodiment of the invention. 
         FIG. 4  illustrates a pizzicato gestural articulation in a touch region of a touch-sensitive musical interface, according to an embodiment of the invention. 
         FIG. 5  illustrates a staccato gesture articulation in a touch region of a touch-sensitive musical interface, according to an embodiment of the invention. 
         FIG. 6  is a simplified flow diagram illustrating aspects of a method of determining a characteristic of a note outputted by a virtual instrument, according to an embodiment of the invention. 
         FIG. 7  shows an illustration of a user interface for a virtual musical instrument, according to an embodiment of the invention. 
         FIG. 8  illustrates a legato gestural articulation on a touch region of a virtual musical instrument interface, according to an embodiment of the invention. 
         FIG. 9  depicts a graph illustrating the cross-fading of two samples of a virtual instrument, according to an embodiment of the invention. 
         FIG. 10  illustrates a stereo field mapping configuration of a cross-faded composite sample, according to an embodiment of the invention. 
         FIG. 11  depicts a dual flywheel mass modeling system, according to an embodiment of the invention. 
         FIG. 12  illustrates an embodiment of a note lifecycle, according to an embodiment of the invention. 
         FIG. 13  is a simplified state machine illustrating aspects of modeling a lifecycle of a note played on a virtual instrument, according to an embodiment of the invention. 
         FIG. 14  illustrates an example of a musical performance system that can enable a user to compose and create music with a number of virtual instruments on a music application, according to an embodiment of the invention. 
         FIG. 15  illustrates a computer system according to an embodiment of the present invention. 
         FIG. 16  illustrates a chord touch region including four touch zones with chord configurations having successively wider harmonic ranges, according to an embodiment of the invention. 
         FIG. 17  illustrates a chord touch region including four touch zones with chord configurations having harmonic ranges successively higher in pitch, according to an embodiment of the invention. 
         FIG. 18  illustrates a chord touch region including four touch zones with chord configurations that change by a minimum number of notes between chord inversions, according to an embodiment of the invention. 
         FIG. 19  illustrates a chord touch region including four touch zones with chord configurations that change by a minimum number of notes between chords, according to an embodiment of the invention. 
         FIG. 20  is a simplified flow diagram illustrating aspects of a method of selecting a virtual instrument on a user interface, according to an embodiment of the invention. 
         FIG. 21  is a simplified flow diagram illustrating aspects of a method of modulating a lifecycle of a musical note or chord, according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure describes systems, methods, and products for musical performance and/or input. Various embodiments can include or communicatively couple with a wireless touch screen device. A wireless touch screen device including a processor can implements the methods of various embodiments. Many other examples and other characteristics will become apparent from the following description. 
     A musical performance system can accept user inputs and audibly sound one or more tones. User inputs can be accepted via a user interface. A musical performance system, therefore, bears similarities to a musical instrument. However, unlike most musical instruments, a musical performance system is not limited to one set of tones. For example, a classical violin or a classical cello can sound only one set of tones, because a musician&#39;s interaction with the physical characteristics of the instrument produces the tones. On the other hand, a musical performance system can allow a user to modify one or more tones in a set of tones or to switch between multiple sets of tones. A musical performance system can allow a user to modify one or more tones in a set of tones by employing one or more effects units. A musical performance system can allow a user to switch between multiple sets of tones. Each set of tones can be associated with a patch (e.g., channel strip (CST) file). 
       FIG. 14  illustrates an example of a musical performance system that can enable a user to compose and create music with a number of virtual instruments on a music application, according to an embodiment of the invention. Musical performance system  1400  can include multiple subsystems such as a display  1405 , one or more processing units  1410 , and a storage subsystem  1415 . One or more communication paths can be provided to enable one or more of the subsystems to communicate with and exchange data with one another. The various subsystems in  FIG. 14  can be implemented in software, in hardware, or combinations thereof. In some embodiments, the software can be stored on a transitory or non-transitory computer readable storage medium and can be executed by one or more processing units. 
     It should be appreciated that musical performance system  1400  as shown in  FIG. 14  can include more or fewer components than those shown in  FIG. 14 , can combine two or more components, or can have a different configuration or arrangement of components. In some embodiments, musical performance system  1400  can be a part of a portable computing device, such as a tablet computer, a mobile telephone, a smart phone, a desktop computer, a laptop computer, a kiosk, etc. 
     Display  1405  in some embodiments can provide an interface that allows a user to interact with musical performance system  1400 . Display  1405  can be a monitor or a screen in some embodiments. Through the interface, the user can view and interact with a GUI  1420  of a musical performance system  1400 . In some embodiments, display  1405  can include a touch-sensitive interface (also sometimes referred to as a touch screen) that can both display information to the user and receive inputs from the user. Processing unit(s)  1410  can include one or more processors that each have one or more cores. In some embodiments, processing unit(s)  1410  can execute instructions stored in storage subsystem  1415 . System  1400  may also include other types of user input and output mechanisms such as allowing a user to provide an input based on received accelerometer or gyroscope sensor readings (internal to system  1400 ) or provide output such as haptic output based on a desired musical characteristic. 
     Storage subsystem  1415  can include various memory units such as a system memory  1430 , a read-only memory (ROM)  1440 , and a permanent storage device  1450 . The system memory can be a read-and-write memory device or a volatile read-and-write memory, such as dynamic random access memory. The system memory can store some or all of the instructions and data that the processor needs at runtime. The ROM can store static data and instructions that are needed by processing unit(s)  1410  and other modules of system  1400 . The permanent storage device can be a read-and-write memory device. Some embodiments of the invention can use a mass-storage device (such as a magnetic or optical disk or flash memory) as a permanent storage device. Other embodiments can use a removable storage device (e.g., a floppy disk, a flash drive) as a permanent storage device. 
     Storage subsystem  1415  can store a touch gesture library  1415  that includes a number of system recognizable touch gestures  1432  on the GUI  1420 , MIDI-controlled audio samples  1434  for storing data relating to music played on the virtual instruments, and virtual instrument data  1436  for storing information about each virtual instrument. Further detail regarding system architecture and auxiliary components thereof are not discussed in detail so as not to obfuscate the focus on the invention and would be understood by those of ordinary skill in the art. 
     Chord View Interface 
       FIG. 1  shows a schematic illustration of a user interface  100  for a virtual musical instrument, according to an embodiment of the invention.  FIG. 1  shows the user interface displayed on a tablet computer such as the Apple iPad®; however the interface could be used on any touch screen or touch-sensitive computing device (e.g., smart phone, personal digital assistant, and the like). The interface  100  includes channel strip file (CST) sound browser  110 , a virtual instrument selector (VIS)  120 , an autoplay selector  130 , a chord player region  140 , and a chord/notes switch  170 . Alternatively, other sound files, or “patches” can be used instead of, or in addition to, the CST files, which can be accessed with the sound browser  110 . 
     The CST sound browser  110  can be configured to select a suite of virtual instruments, where each suite or ensemble is suited for a different musical preference. Selecting a given suite (e.g., Symphonic Strings, String Quartet, Baroque Strings, etc.) loads the appropriate CST and autoplay grooves specifically suited to the selected ensemble. For example, the Symphonic CST can include a first violin, a second violin, a viola, a cello, and a bass. In contrast, the String Quartet CST may include a first a second violin, a viola, and cello. Any number and configuration of virtual instruments can be grouped together in any preferred combination. In some embodiments, MIDI sequences can be used instead of, or in addition to, the autoplay grooves. 
     The interface  100  includes a number of chord touch regions (chord strips)  150 , shown for example as a set of eight adjacent columns or strips. Each touch region  150  can correspond to a pre-defined chord (base chord) within one or more particular keys, with adjacent regions configured to correspond to different chords and progressions within the key or keys. For example, the key of C major includes the diatonic chords of C major (I), D minor (ii), E minor (iii), F major (IV), G major (V), A minor (vi), and B diminished (vii), otherwise known as the Tonic, Supertonic, Mediant, Subdominant, Dominant, Submediant, and Leading Tone chords. In the example shown in  FIG. 1 , an additional non-diatonic chord of B-flat major is included with the key of C major, and the chords are arranged sequentially in accordance with the circle of fifths. This arrangement allows a user to easily create sonically pleasing sequences by exploring adjacent touch regions. A user can touch or initiate gestures (e.g., swipe gestures) on the various touch regions  150  to create sounds that are related to the chord that is assigned to the particular touch region. The types of touches, sounds, and/or gestures associated with the touch regions and features therein are further described below. It should be noted that any set of pitches (collectively known as a chord) can be defined for any chord touch region and is not limited to the conventional harmony described in this particular example. 
     Each chord touch region is divided into a number of touch zones referred to collectively as touch zones  160 . Referring to  FIG. 1 , each touch region  150  comprises four individual touch zones  162 ,  164 ,  166 ,  168 . Each of the touch zones  160  can correspond to various harmonic voicings (e.g., chord inversions) of the base chord assigned to the touch region  150  that it belongs to. The chords assigned to each touch zone  160  can be associated with MIDI-controlled audio samples (e.g., signals, streams, etc.). Touching or initiating touch gestures on any touch zone  160  in a region  150  plays the chord audio sample assigned to that touch zone. 
     MIDI (Musical Instrument Digital Interface) is an industry-standard protocol defined in  1982  that enables electronic musical instruments, such as keyboard controllers, computers, and other electronic equipment, to communicate, control, and synchronize with each other. For example, MIDI can be used to trigger playback of an audio sample to create a sound. In other words, MIDI is an instruction communications protocol typically used in electronic musical instruments. It should be noted that the term “audio signal” can also include a MIDI-controlled audio sample or digital signal processor (DSP) generated audio stream. Other audio processing systems and protocols may be used as would be known by one of ordinary skill in the art. 
     Virtual Instrument Selection in Chords View 
     A user can select a virtual instrument via the VIS  120 . In some cases, the virtual instruments can include one or more violins, violas, cellos, basses, or the like. When a user selects one or more instruments with the VIS  120 , the system loads the appropriate patch files (e.g., CST files) for the selected instruments. In some embodiments, the VIS  120  displays up to 5 instruments or more, which when selected (e.g., by tapping) will either activate or deactivate a note being played by that particular instrument. For example, while performing gestural inputs on the touch zone region corresponding to a given chord sequence, a user may select a violin and a bass to play (i.e., output an audible musical output) the given chord sequence. During the input of gestures to create the chord sequence, the user can deselect the bass in real-time, leaving only the violin sample to be voiced during the chords sequence. Likewise, reselecting the bass thereafter will reintroduce the bass sample&#39;s voice into the chord sequence. 
     Grooves/Auto-Play Selection 
     In certain embodiments, the interface  110  includes various chord sequence auto-play features. An auto-play selector  130  is configured to loop one or more MIDI-controlled audio samples (e.g., audio signals/streams, audio loops, etc.) that include a number of predefined accompanying rhythms when selected by a user. In response to a user selection, a groove plays for the chord being touched by the user. In some embodiments, the groove rhythm latches or stops when the user touches the same chord region again. The groove rhythm can switch to a new chord by selecting a different touch zone  150 . The playback tempo can be locked during playback or manually adjustable as required. Variations of the auto-play grooves can be selected by one or more taps on the auto-play selector  130 . In some cases, the auto-play grooves can be a MIDI sequence. Furthermore, the auto-play grooves (or MIDI sequences) will also correspond to the chord voicing selected for each touch zone  160 . 
     Chord Inversions 
     In certain embodiments, each of the touch zones  160  of a touch region  150  can include various inversions of the base chord. For example, the top most touch zone  162  can correspond to a root position of the base chord, such as a C Major triad C-E-G with C in the bass. The next adjacent touch zone  164  can correspond to a first inversion of the base chord, or the C Major triad C-E-G with E in the bass. The following adjacent touch zone  166  can correspond to a second inversion of the base chord, or the C Major triad C-E-G with G in the bass. The next adjacent touch zone  168  can correspond to a third inversion of the base chord, or the C Major triad C-E-G with an additional 7 th  in the bass (e.g., Cmaj7 with B in the bass or a C7 with a Bb in the bass). Alternatively, the touch zone  168  may include a 10 th  in the bass, or E-C-G, where the E note is one octave higher than the E note in the first inversion E. Optional adaptations of the first, second, and third inversions can be utilized. 
     In some embodiments, each of the touch zones  160  of a touch region  150  can include various inversions of the base chord that are successively higher in relative pitch than the base chord. To illustrate, the top most touch zone  162  may correspond to a root position of the base chord, such as a C Major triad C-E-G with C in the bass. The next adjacent touch zone  164  may correspond to a first inversion of the base chord, or the C Major triad C-E-G with E in the bass, where at least one note of the chord (e.g., the top note) is higher than any note of the base chord. Similarly, the following touch zone  166  may include a second inversion of the base chord, where at least one note of the second inversion chord (e.g., the top note) is higher in pitch than any note of the base chord or first inversions. Finally, the last adjacent touch zone  168  can include a third inversion of the base chord, where at least one note of the third inversion chord is higher in pitch than any note of the base chord or first inversions. Configuring inversions in this manner can make cycling through the touch zones  160  sounds as if the relative pitch is moving from one register to a higher register. 
     In further embodiments, each of the touch zones  160  of a touch region  150  can include various chord voicings (i.e., the harmonic arrangement of notes) that have successively wider harmonic ranges. To illustrate, the top most touch zone  162  may correspond to the base chord with a certain harmonic range. For example, touch zone  162  can include a C triad comprising C-E-G where the G can be 3.5 whole steps from the root C. The next touch zone  162  may correspond to a chord voicing (e.g., inversion) with a wider harmonic range than the base chord of touch zone  162 . For example, touch zone  162  can include a C triad comprising C-E-G with E in the bass and a C as the top note, where the top note C is 4 steps from the root E. The third and fourth touch zones  166  and  168  may follow in kind. 
       FIG. 16  illustrates a chord touch region  1600  including four touch zones with chord configurations having successively wider harmonic ranges, according to an embodiment of the invention. The chord touch region  1600  is assigned base chord C major and includes touch zones  1610 ,  1620 ,  1630 , and  1640 . In some embodiments, each touch zone has a different chord voicing of the base chord. As described above, the term “chord voicing” relates to a harmonic arrangement of notes. For example, a C major chord has the notes C-E-G. One chord configuration can have a C note as the bass note, while a second configuration may have the G note as the bass note. Each of these configurations can be referred to as a chord voicing. The distance between notes can be measured by half steps and whole steps, which can be thought of as musical units of measurement. Touch zone  1610  has a first voicing (i.e., harmonic arrangement)  1615  of a C major chord. The harmonic range of the first voicing is 3.5 whole steps. Touch zone  1620  has a second voicing  1625  of the C major chord with a wider harmonic range than the first voicing spanning 4 whole steps. Touch zone  1630  has a third voicing  1635  of the C major chord with a wider harmonic range than the second voicing spanning 8 whole steps. Touch zone  1640  has a fourth voicing  1645  of the C major chord with a wider harmonic range than the third voicing spanning 10 whole steps. Different chords, chord configurations, inversions, key signatures, and the like, can be customized to preference. 
       FIG. 17  illustrates a chord touch region  1700  including four touch zones with chord configurations having harmonic ranges successively higher in pitch, according to an embodiment of the invention. The chord touch region  1700  is assigned base chord C major and includes touch zones  1710 ,  1720 ,  1730 , and  1740 . In some embodiments, each touch zone has a different chord voicing of the base chord. As described above, the term “chord voicing” relates to a harmonic arrangement of notes. Touch zone  1710  has a first voicing (i.e., harmonic arrangement)  1715  of a C major chord. Touch zone  1720  has a second voicing  1725  of the C major chord with a higher pitch than the first voicing. In other words, both the E and C notes in chord voicing (“chord”)  1725  are higher in pitch than the highest note (G) of chord  1715 . In some cases, the audible effect of only one note in a first chord being higher in pitch than all notes of a second chord will make the first chord sound higher in pitch overall. Referring back to  FIG. 17 , touch zone  1730  has a third voicing  1735  of the C major chord with a higher pitch than the first ( 1715 ) and second voicings ( 1725 ). For example, the G note is higher in pitch than any of the notes of chord  1715  and  1725 . Touch zone  1740  has a fourth voicing  1745  of the C major chord with a higher pitch than the first ( 1715 ), second voicings ( 1725 ), and third voicings ( 1735 ). For example, the C note is higher in pitch than any of the notes of chords (i.e., voicings)  1715 ,  1725 , and  1735 . It should be noted that other chords, chord configurations, inversions, key signatures, and the like, can be customized to preference. 
     In some embodiments, touching or articulating gestures up or down through the touch zones  160  may cause the chord voicing to change by the minimum number of notes needed to switch to the nearest inversion from the chord voicing that was being played prior to the touch or gesture articulation. For example, if touch zone  162  includes a C major triad C-E-G with C in the bass, and touch zone  164  includes a C major triad E-G-C with an E in the bass, the E and G notes will not change between chords. Changing the minimum amount on notes in a chord change can result in a smoother or more harmonically pleasing sound, as further illustrated in  FIG. 18 . 
       FIG. 18  illustrates a chord touch region including four touch zones with chord configurations that change by a minimum number of notes between chord inversions, according to an embodiment of the invention. The chord touch region  1800  is assigned base chord C major and includes touch zones  1810 ,  1820 ,  1830 , and  1840 . Touch zone  1810  includes a C major triad C-E-G with C in the bass, and touch zone  1820  includes a C major triad E-G-C with an E in the bass. It should be noted that the E and G notes do not change during the chord change  1815 . As described above, changes between chord inversions (e.g.,  1810  to  1820 ) result in a change in the minimum amount of notes needed for the chord change. In some cases, this can be a dynamic process such that different chord voicings may be assigned to the same touch zone depending on the arrangement of the previous chord voicing. For example, the C chord inversion assigned to  1810  may produce (e.g., play back) a chord voicing of a different arrangement when approached from touch zone  1820  rather than from touch zone  1830  because the arrangement of the notes and the minimum changes required for the chord change is different. A first inversion C chord with an E in the bass may only require one note to shift positions when switching to a C chord with a C root note (e.g., C note moves to the bass position and other notes do not change). However, a second inversion chord with a G in the bass may require multiple notes to shift positions when switching to a C chord with a C root note. It should be noted that different chords, chord configurations, inversions, key signatures, and the like, can be customized to preference. 
     In other embodiments, touching or articulating gestures between chord regions may cause chord voicings to change by the minimum number of notes needed to switch between chords. For example, if one touch region includes a C major triad C-E-G as a base chord, and a second touch region includes an E minor triad E-G-B, the E and G notes does not change between the chord change. Changing the minimum number of notes between chord changes can result in more harmonically pleasing chord progressions. 
       FIG. 19  illustrates a chord touch region including four touch zones with chord configurations that change by a minimum number of notes between chords, according to an embodiment of the invention. The chord touch region  1900  is assigned base chord C major and includes touch zones  1910 ,  1920 ,  1930 , and  1940 . The chord touch region  1950  is assigned base chord E minor and includes touch zones  1960 ,  1970 ,  1980 , and  1990 . Touch zone  1910  includes a C major triad C-E-G with C in the bass, and touch zone  1960  includes a E minor triad E-G-B with an E in the bass. As described above, changes between chords result in a change in the minimum amount of notes needed for the chord change. In this case, the E and G notes do not change position during the chord change  1935 . In some cases, this can be a dynamic process such that different chord voicings may be assigned to the same touch zone depending on the arrangement of the previous chord voicing. For example, the E minor chord inversion assigned to  1970  may produce (e.g., play back) a chord voicing of a different when approached from touch zone  1910  rather than from touch zone  1920  because the arrangement of the notes and the minimum changes required for the chord change may be different. A described above, changing from a C chord with a C note in the bass to an E minor chord with an E in the bass results in only one note change (i.e., C changes to B), while both the E and G notes do not change. However, changing from a second inversion C chord ( 1930 ) with a G in the bass to an Em chord with E in the bass position may require multiple notes to shift position. It should be noted that different chords, chord configurations, inversions, key signatures, and the like, can be customized to preference. 
     Touch Gesture Articulations in Chord View 
     Each touch zone  160  in each touch region  150  is configured to detect one or more of a number of different touch gestural articulations, according to certain embodiments. The touch gesture articulations can include a legato articulation, a pizzicato articulation, and a staccato articulation. Each of the gesture articulations triggers playback (i.e., output) of a corresponding audio signal that simulates the selected articulation. In some cases, the audio signal can be a MIDI-controlled audio sample or a DSP generated audio stream. It should be noted that the term “audio signal” can also any suitable means of playing an audio signal. Playing an audio signal can include playback on an electro-acoustic transducer (e.g., a speaker), decoding the audio signal, saving in memory, coupling to other systems or devices, or other non-audible output scenarios that would be appreciated by one of ordinary skill in the art with the benefit of this disclosure. It should be noted that although three different gesture articulations are described, any number and/or type of gesture articulations (e.g., double taps, dual finger swipes, palm, circular motions, polygonal outline motions, multi-finger gestures, etc.) may be used in any combination or set as required. 
       FIG. 2  illustrates a plurality of legato articulations over a plurality of touch regions  250 , according to an embodiment of the invention. The touch regions  250  and touch zones  260  are similar to those which are described in  FIG. 1 . The legato articulation is initiated by a tap-to-swipe gesture on one of the touch zones  260 . In certain cases, the initial tap should be longer than a predetermined period of time before initiating the swipe gesture to differentiate the legato articulation from the staccato (i.e., tap gesture shorter than the predetermined time) or pizzicato articulation (i.e., swipe gesture with no initial tap period) further described below. A continuous up-and-down swiping motion of the legato articulation mimics the motion of a bow played across a set of strings on a stringed instrument (e.g., violin, cello, bass). Similarly, the corresponding audio signal (e.g., MIDI-controlled audio sample) plays the sound of a bowing action across a set of strings on a stringed instrument. In  FIG. 2 , gesture  270  illustrates a touch-and-swipe legato gesture on touch zone  264 A. The initial touch point on the touch zone  264 A determines the inversion of the base chord that is played. In this example, the legato articulation  270  plays a first inversion legato bowing sound in A minor. Gesture  280  illustrates a touch-and-continuous swipe legato gesture at touch zone  264 B. In this example, the legato articulation  280  plays a first inversion legato bowing sound in C major and continues to do so even if the swiping motion moves outside of the associated touch region. In some embodiments, as long as the finger is not removed from the touch surface of the interface  100 , the bowing action will continue to play the same inversion of the initial touch point regardless of the subsequent position of the finger on the interface. In some cases, changing base chords or chord inversions occurs by starting a new legato gesture articulation in a different zone by lifting the finger and placing it in the desired chord/inversion region. Alternatively, introducing a second finger in a different touch zone can begin a new legato gesture articulation. In other words, the interface recognizes the second touch input as a new legato gestural articulation and ignores the first finger. Furthermore, the speed or velocity of the legato articulation can determine various expression values of the corresponding audio signal, such as a volume of playback. Other expression values can include pitch, duration, equalization, filtering (e.g., high pass, low pass, band pass, notch filters, etc.), pan, effects (e.g., reverb, chorus, delay, distortion, presence, etc.), or any other parameters that can be modulated as described. In some embodiments, the direction and/or acceleration of the gesture articulation can also be used to modulate one or more expression values (e.g., variable of playback). 
     In certain embodiments, multi-finger gestures can allow the user to quickly transition from one chord zone to another. For example, if a second touch point is detected within the chord interface while a bowing gesture is active in another chord touch zone, the chord can quickly change to the new chord touch zone. Furthermore, if the second touch is a bow swipe (e.g., legato articulation), the expression (e.g., a volume) value generated from the first touch transfers to the new expression value for the second touch using the same swipe gesture ballistics that were present immediately prior to the change. To illustrate, if two consecutive swipes are occurring at substantially the same swipe gesture speed, then there should be substantially no change in expression value (volume) when the chord transition occurs. In contrast, if the second gesture occurs at a different speed than the first gesture, then the expression value (volume) transitions from the first expression value to the second expression value in a smooth and musical way, as further described below. 
       FIG. 3  illustrates a two-finger auto-legato gesture articulation, according to an embodiment of the invention.  FIG. 3  depicts a first touch  370  (tap-and-hold) and simultaneous second touch  372  (tap-and-hold) at touch zone  364 A of the touch region  350 A. Tapping and holding two fingers in a touch zone auto-plays a legato chord sound. In this example, the auto legato gesture articulation plays the inversion associated with touch zone  364 A of touch region  350 A (A minor). Two finger auto-legato gestures do not require a swipe gesture and can be configured to play instantly. In some cases, the volume of the auto-legato articulation is fixed and can be based on the initial velocity of the two finger-tap gesture. The initial velocity can be determined, for example, by an accelerometer. In certain cases where multiple touch zones are simultaneously touched, a resultant touch location can be selected as the initial touch zone. For example, a first touch  380  at touch zone  364 B and a simultaneous second touch  382  at touch zone  368 B results in an effective two finger-tap gesture  384  at touch zone  366 B of touch region  350 B. The note characteristics of the two-finger auto-legato touch gestures typically have relatively short attack times to support rapid transitions to different base chords or inversions. 
       FIG. 4  illustrates a pizzicato gesture articulation in a touch region  450  of a touch-sensitive interface, according to an embodiment of the invention. The pizzicato gesture articulation can be initiated by a quick tap gesture  480  including an initial tap in one of the plurality of separate touch zones  460  and a release prior to a predetermined period. The pizzicato gestural articulation triggers the playback of a second corresponding MIDI-controlled audio sample that simulates the sound of a plucking action on a stringed instrument. In one embodiment, the predetermined period is any suitably short period of time (e.g., less than 200 ms) that would be appreciated by one of ordinary skill in the art with the benefit of this disclosure. In some embodiments, if the touch is held longer than the predetermined period, the removal of the finger will not trigger the audio sample. Furthermore, the speed or velocity of the pizzicato gestural articulation (e.g., the speed at which the initial tap on the interface is received) can determine various expression values of the corresponding audio sample, such as a volume of playback. Other expression values can include pitch, duration, equalization, filtering (e.g., high pass, low pass, band pass, notch filters, etc.), pan, effects (e.g., reverb, chorus, delay, distortion, presence, etc.), or any other parameters that can be modulated as described. The initial velocity of the initial tap may be determined by an accelerometer. 
       FIG. 5  illustrates a staccato gesture articulation  580  in a touch region  550  of a touch-sensitive interface, according to an embodiment of the invention. The staccato gestural articulation is initiated by an in-motion swipe gesture beginning upon contact with a touch zone  560 . In other words, a quick swipe motion is initiated with no pause (e.g., tap) before beginning the swipe, where the swipe begins upon contact with the given touch zone  560 . The staccato gesture articulation triggers playback of a corresponding MIDI controlled audio signal that simulates the sound of a short burst of a bowing action on a stringed instrument. The speed or velocity of the staccato articulation can determine various expression values of the corresponding audio sample, such as a volume of playback. Other expression values can include pitch, duration, equalization, filtering (e.g., high pass, low pass, band pass, notch filters, etc.), pan, effects (e.g., reverb, chorus, delay, distortion, presence, etc.), or any other parameters that can be modulated as described. 
       FIG. 6  is a simplified flow diagram illustrating aspects of a method  600  of determining a characteristic of a note played on a virtual instrument, according to an embodiment of the invention. The method  600  is performed by processing logic that may comprise hardware (e.g., circuitry, dedicate logic, etc.), software (which as is run on a general purpose computing system or a dedicated machine), firmware (embedded software), or any combination thereof. In one embodiment, the method  600  is performed by the processor unit  1410  of  FIG. 14 . 
     Referring to  FIG. 6 , the method  600  begins with the receiving a user input on a touch zone of a touch sensitive interface ( 610 ). The user input can include one of a number of touch gesture articulations. At  620 , the processor determines if the touch gesture is a legato articulation. A legato articulation includes a touch-to-swipe gesture that imitates the motion of a bow played across a set of strings on a stringed instrument (e.g., violin, cello, bass). In certain cases, the initial touch should be longer than a predetermined period of time before initiating the swipe gesture to differentiate the legato articulation from the staccato (i.e., tap gesture shorter than the predetermined time) or pizzicato articulation (i.e., swipe gesture with no initial tap period). If the processor determines that the user input is a legato articulation, the processor plays (or outputs) one of a number of preselected audio signals (e.g., MIDI controlled audio samples, DSP generated audio streams, etc.) on an output device (e.g., speaker, headphones, or the like) corresponding to both the touch region selected (e.g., chord inversions) and the legato articulation ( 650 ). If the processor determines that the user input is not a legato articulation, the method proceeds to  630 . 
     At  630 , the processor determines if the touch gesture is a pizzicato articulation. A pizzicato gesture articulation can be initiated by a quick tap gesture including an initial tap in a touch zone and a release prior to a predetermined period. The pizzicato articulation imitates the sound of a plucking action on a stringed instrument. If the processor determines that the user input is a pizzicato articulation, the processor plays one of a number of preselected audio signals on an output device corresponding to both the touch region selected (e.g., chord inversions) and the pizzicato articulation ( 650 ). If the processor determines that the user input is not a pizzicato articulation, the method proceeds to  640 . 
     At  640 , the processor determines if the touch gesture is a staccato articulation. The staccato articulation can be initiated by an in-motion swipe gesture (i.e., no pause or tap) beginning upon contact with a touch zone. The staccato gesture articulation imitates the sound of a short burst of a bowing action on a stringed instrument. If the processor determines that the user input is a staccato articulation, the processor plays one of a number of preselected audio signals (e.g., MIDI-controlled audio samples) on an output device corresponding to both the touch region selected (e.g., chord inversions) and the staccato articulation ( 650 ). If the processor determines that the user input is not a staccato articulation, the method ends. 
     Differentiation between each of the legato and staccato gestural articulations is based on detecting a time interval associated with the initial touch which is followed by a consecutive (i.e., without a lift off of the touch) swiping gesture having a particular initial speed. A legato articulation is detected based on the detecting of an initial touch lasting a given time interval, which is followed by a swiping motion of any speed. The staccato gestural articulation is based on detecting that the initial touch is less than the given time interval. 
     It should be appreciated that the specific steps illustrated in  FIG. 6  provides a particular method of determining a characteristic of a note played on a virtual instrument, according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. In certain embodiments, the method  600  may perform the individual steps in a different order, at the same time, or any other sequence for a particular application. For example, the pizzicato gesture may be determined before the legato gesture. Moreover, the individual steps illustrated in  FIG. 6  may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize and appreciate many variation, modification, and alternatives of the method. 
     Notes View Interface 
       FIG. 7  shows an illustration of a user interface  700  for a virtual musical instrument, according to an embodiment of the invention.  FIG. 7  shows the user interface displayed on a tablet computer such as the Apple iPad®; however the interface could be used on any touch screen or touch-sensitive computing device (e.g., smart phone, personal digital assistant, and the like). The interface  700  includes CST sound browser  710 , a virtual instrument selector (VIS)  720 , a violin neck and headstock  730 , an Arco/Pizz badge  740 , and a chords/notes switch  770 . The violin neck  730  includes a plurality of virtual strings  750 , where each string corresponds to a base note (open string) and is divided into a plurality of touch zones  760  with notes corresponding to a predetermined scale (e.g., chromatic, pentatonic, harmonic minor, etc.). Each of the notes can correspond to one or more MIDI-controlled audio samples associated with the selected virtual instrument, the type of articulation performed, expression value, or any combination thereof, as further described below. 
     The chords/notes switch  770  functions as a virtual toggle switch configured to switch between the chords view (e.g.,  FIG. 1 ) and the notes view (e.g.,  FIG. 7 ). The CST sound browser  710  can be configured to select a suite of virtual instruments and their associated audio signals (e.g., MIDI-controlled audio sample, DSP generated audio stream, etc.), where each suite or ensemble is suited for a different musical preference. Selecting a given suite (e.g., Symphonic Strings, String Quartet, Baroque Strings, etc.) loads the appropriate CST files specifically suited to the selected ensemble. For example, the Symphonic CST can include a first violin, a second violin, a viola, a cello, and a bass. In contrast, the String Quartet CST may include a first a second violin, a viola, and cello. Any number of virtual instruments can be grouped together in any preferred configuration. 
     Virtual Instrument Selection in Notes View 
     The VIS  720  is used to select the virtual instruments desired by the user. In some cases, the virtual instruments can include one or more violins, violas, cellos, basses, or the like. When a user selects one or more instruments with the VIS  720 , the system will load the appropriate audio samples for the selected instruments. In some embodiments, one instrument can be played at a time in notes view. For example, a user may select a cello, thereby deactivating any basses, violins, and/or violas in that particular suite of virtual instruments. As shown in  FIG. 7 , only one virtual instrument is fully shown at one time to clearly identify the active virtual instrument. In alternative embodiments, multiple instruments can be selected for simultaneous playback in notes view. For example, the VIS  720  (“part selector”) can display up to 5 instruments or more, which when selected (e.g., by tapping) either activates or deactivates a note being played by the selected instruments. 
     In certain embodiments, the VIS  720  displays a number of virtual instruments (e.g., violins, violas, cellos, and basses) that are arranged on a virtual scrollable area on the user interface  700 . A swipe gesture on the virtual scrollable area can cause the area to slide in a direction relative to the swipe gesture, thereby changing an orientation of the virtual scrollable area. For example, as a user performs a swipe gesture on the VIS  720  from right-to-left, the virtual instruments cycle through or moves across the VIS  720  display area from right-to-left at a speed proportional to the speed of the swipe gesture. Selecting a virtual instrument can be based on the orientation of the virtual scrollable area. In addition, the virtual scrollable area can have a selection location  790  to select the virtual instrument located in the selection location  790 . In some cases, the selection location is located in the center position of the VIS  720 . Referring to  FIG. 7 , the cello is currently selected because it is positioned in the selection location  790 . It should be noted that the configuration of the VIS  720  can be customized to a user preference. For example, all virtual instruments may be visible on the VIS  720  or only a subset at any given time. The virtual scrollable area may slide linearly, or on a virtual axis of a suitable radius such that the virtual instruments appear to “rotate” into position at the selection location  790 . In some embodiments, a selection may be made automatically as a virtual instrument slides into the selection location  790 , or a selection may require an additional manual tap on the virtual instrument to confirm selection. 
       FIG. 20  is a simplified flow diagram illustrating aspects of a method  2000  of selecting a virtual instrument on a user interface, according to an embodiment of the invention. The method  2000  is performed by processing logic that may comprise hardware (e.g., circuitry, dedicate logic, etc.), software (which as is run on a general purpose computing system or a dedicated machine), firmware (embedded software), or any combination thereof. In one embodiment, the method  2000  is performed by the processor unit  1410  of  FIG. 14 . 
     Referring to  FIG. 20 , the method  2000  begins with displaying a plurality of virtual instruments on a scrollable area of a user interface ( 2010 ). In some cases, the user interface can be touch-sensitive, where the scrollable area is directly accessible by user touch (e.g., finger, stylus, etc.). In non-touch sensitive embodiments, the scrollable area may be accessible via input device (e.g., mouse, keyboard, etc.). At  2020 , the user interface (by way of a processor device) detects a swipe gesture on the virtual scrollable area. In response to the swipe gesture, the user interface scrolls the scrollable area (e.g., VIS  720 ) and virtual instruments (e.g., virtual violin, viola, cello, bass, etc.) thereon in a direction relative to the swipe gesture ( 2030 ). In certain embodiments, as a user performs a swipe gesture on the scrollable area (e.g., VIS  720 ) from right-to-left, the virtual instruments cycle through or moves across the VIS  720  display area from right-to-left at a speed proportional to the speed of the swipe gesture. The user interface then selects a virtual instrument based on an orientation of the virtual scrollable area ( 2040 ). The virtual scrollable area can have a selection location configured within the scrollable area to identify the virtual instrument to be selected. For example, if the user interface scrolls through the virtual instruments in response to a swipe gesture and a violin stops in the selection location, then the violin will be selected. In some cases, the selection location is located in the center position of the scrollable area. The selection location can optionally be configured in any suitable position on the scrollable area. At  2050 , the user interface assigns the selected virtual instrument to a first touch zone on the user interface. The user interface further assign one or more MIDI-controlled audio samples to the first touch zone where the audio samples correspond to the selected virtual instrument ( 2060 ) 
     It should be appreciated that the specific steps illustrated in  FIG. 20  provides a particular method of selecting a virtual instrument on a user interface, according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. In certain embodiments, the method  2000  may perform the individual steps in a different order, at the same time, or any other sequence for a particular application. Moreover, the individual steps illustrated in  FIG. 20  may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize and appreciate many variation, modification, and alternatives of the method. 
     Touch Gesture Articulation in Notes View 
     Each of the virtual strings  750  of a user interface  700  is configured to detect one or more of a plurality of different touch gesture articulations, according to certain embodiments. The touch gesture articulations can include a legato articulation, a pizzicato articulation, and a staccato articulation. Each of the gesture articulations triggers playback of a corresponding MIDI-controlled audio sample (or DSP generated audio stream) that simulates the selected articulation. As described above, the legato articulation mimics a bowing action across a set of strings, the pizzicato articulation mimics a sound of plucking a stringed instrument, and a staccato articulation mimics a sound of a short burst of a bowing action on a stringed instrument. 
     In some embodiments, the Arco|Pizz (i.e., bow/pizzicato) badge  740  is displayed at the far left of the notes view inside the headstock graphic. The arco/pizz badge (“button badge”)  740  is a toggle switch configured to select between a first mode (auto-bow mode) and second mode (arco|pizz mode) of operation. In some embodiments, the button badge  740  is a temporary switch where the badge remains in the second mode of operation as long as a user is pressing the badge  740 . In other embodiments, the badge  740  toggles between the two modes of operation and remains in the selected mode of operation until the badge  740  is depressed again. 
     In the auto-bow mode of operation (i.e., first mode), touching any point on one of the strings  750  can produce a corresponding legato note with a short attack. The legato note plays continuously as long as the touch object (e.g., finger, stylus, or any input device) remains on the strings. In certain embodiments, the first mode of operation interprets any gesture (e.g., tap, swipe, etc.) as a touch and plays the corresponding legato note, regardless of the initiating gesture. 
       FIG. 8  illustrates a legato articulation on the notes view interface  800 , according to an embodiment of the invention. When a user presses the button badge  840  and holds a point  880  on a string  864 , a graphic indicator  885  is shown prompting the user to perform an up and down swiping (bowing) motion. In some cases, the graphic indicator  885  is a note highlight radiating from the initial touch point  880  to indicate the bow motion gesture to the user. The interface  800  typically displays the graphic indicator  885  when the button badge  840  is active, a touch is detected on a string, and the touch is determined not to be a pizzicato tap (i.e., the tap is longer than a predetermined period). As the user swipes up and down, the instrument plays a legato note based on the start position of the bowing action. While the badge button  840  is held, the user can tap any string position for a corresponding pizzicato note. In certain embodiments, only one instrument can be played at a time (violin, viola, cello, bass) in the notes view. In alternative embodiments, multiple instruments can be selected in the notes view for simultaneous play. 
     With the button badge  840  pressed, both legato and staccato articulations can be performed in a similar way as in the chords view. The speed or velocity of the staccato articulation (i.e., how hard the note is tapped) can determine various expression values of the corresponding MIDI-controlled audio sample, such as a playback volume. A faster legato articulation can create a louder volume and slower legato articulations may create a softer volume. The notes view can be further configured to support multi-finger detection (e.g., double stop chords) with staccato, legato, and pizzicato articulations. In certain aspects, the instrument strings in the notes view are tuned to the standard open strings for the selected virtual instrument. In some embodiments, note sliding or pitch bending can be supported when the button badge  840  is not active. An accelerometer can optionally be configured to determine the velocity (i.e., how hard the note is touched) of any articulations in notes view. 
     Velocity Sample Cross Fades 
     According to certain embodiments, the virtual instruments described herein utilize a cross-fade of two channels comprising a loud sample and a soft sample to create notes of varying velocities (i.e., amplitudes, volume, etc.). Lower velocity audio signals created by the virtual instruments may have a higher weighted value for the soft sample, and higher velocity audio signals may have a higher weighted value for the loud sample. The resultant composite sample creates a more realistic sounding virtual instrument that more accurately reproduces the various aural characteristics and harmonic structures of notes played at varying degrees of amplitude. 
       FIG. 9  depicts a graph  900  illustrating the cross-fading of two samples of a virtual instrument, according to an embodiment of the invention. Graph  900  includes an envelope level axis  905 , a sample output level axis  910 , a soft sample  920 , and a loud sample  930 . As described above, a legato swipe gesture can trigger playback of two preselected MIDI-controlled audio samples for a virtual instrument, where the velocity of the swipe can control characteristics of the cross-fade and the amplitude of the composite signal. In some aspects, the soft sample  920  can be a first preselected audio sample that includes an audio sample of a bowed stringed instrument having a soft attack and a light timbre. The loud sample  930  can be a second preselected audio sample including an audio sample of a bowed stringed instrument having a strong attack and a heavy timbre. As the velocity of the swipe gesture increases, the playback (i.e., envelope level) of the two preselected audio samples cross-fades from the soft sample  920  (i.e., first preselected MIDI file) to the loud sample  930  (i.e., second preselected MIDI file). In contrast, as the velocity of the swipe gesture decreases, the playback of the two preselected audio samples cross-fades from the loud sample  930  to the soft sample  920 . In  FIG. 9 , at an envelope level (e.g., swipe velocity) of 75 out of 128 levels, the loud sample  930  cross-fades at a higher output level than the soft signal  920 . In some embodiments, the playback volume of the cross-faded composite of the two preselected audio samples increases in response to an increased velocity of a legato swipe gesture. In contrast, the playback volume of the cross-faded composite of the two preselected audio samples decreases in response to a decreased velocity of a legato swipe gesture. 
       FIG. 10  illustrates a stereo field mapping configuration  1000  of a cross-faded composite sample, according to an embodiment of the invention. In each MIDI channel (e.g., virtual instrument), the resulting composite mix of the soft sample and loud sample is summed to a mono signal and panned in a mixer to present a particular stereo spread of the five MIDI channel instruments (e.g., the violins, violas, cellos, and basses) in a stereo field. In other cases, a stereo signal is placed within the stereo field for the final output. In this particular example, the 1 st  violin, 2 nd  violin, violas, and basses are enabled and the cellos are disabled. The resulting mix of soft samples and loud samples for each instrument (i.e., channel) is summed, panned, and added to a master stereo output for playback. In alternative embodiments, additional expression data can be modulated with the cross-faded signals. For example, as low frequency notes are played, haptic feedback (i.e., vibration) can be cross-faded into the composite stereo signal to simulate the rumble of bass frequencies. Similarly, other expression data (e.g., equalization data) can be cross-faded into the output signal as would be appreciated by one of ordinary skill in the art with the benefit of this disclosure. 
     Dual Fly Wheel Aspect for Modeling a Physical System Response to a User Input 
     The legato gesture articulation (i.e., tap-and-swipe) provides a realistic sounding bowing action on a stringed instrument due, in part, to a virtual mass modeling system. The mass modeling system can include two series connected signal processors that can be analogized to a virtual dual flywheel system having mass and momentum to create smooth transitions between notes and chords by modeling a virtual momentum created by a user swiping gesture on the interface  100  and maintaining that momentum through subsequent harmonic or melodic progressions. 
       FIG. 11  depicts a mass modeling system (“flywheel system”)  1100 , according to an embodiment of the invention. The modeling system  1100  includes a first signal processing stage  1110  with a first input and a first output, and a second signal processing stage  1120  receiving the first output and having a second output. The first user input can include the legato gesture articulation (e.g., up-and-down swiping gesture) for a virtual instrument on the interface  100 . The first signal processing stage  1110  is coupled to the second signal processing stage  1120  such that the second input of the second signal processing stage  1120  receives the output of the first signal processing stage  1110 . The output of the second signal processing stage  1120  can determine an output expression level for the system  100  (e.g., a volume) in response to the first user input. In some embodiments, the output of the flywheel system  1100  is configured to modulate the lifecycle of a note  1130  played by a virtual instrument. The output of both the first and second signal processing stages  1110 ,  1120  can be modeled by equation (1):
 
 L =((( i *(1− h ))+( L′*h ))* F   s )+ C   (1)
 
     The various components of equation 1 are defined as follows: L is current level, speed, or momentum of the given flywheel, and L′ is the previous level, speed, or momentum of the given flywheel. The external input ‘i’ can be a physical user input (e.g., a legato swiping motion on a touch zone) for the first signal processing stage  1110 . Alternatively, the external input ‘i’ can be the output L of the first signal processing stage  1110  as the input to the second signal processing stage  1120 . The ‘h’ variable is a mass coefficient associated with each flywheel and can vary due to a state of the second flywheel (e.g., momentum). ‘F’ can be a scaling factor and ‘C’ can be an offset constant. The first term (i*(1−h)) corresponds to an influence of a new input momentum in the system. The second term (L′*h) corresponds to the existing momentum of the system. As can be seen in equation 1, a very high mass coefficient (e.g., 0.9) will require very high input levels to make any appreciable change to the existing momentum. Conversely, very small mass coefficients (e.g., 0.1) will only require small input levels to appreciably change the existing momentum of the particular flywheel. By coupling two virtual flywheels together, the output of the second signal processing stage  1120  depends on a combination of the input from the first signal processing stage  1110  and the current momentum of the second signal processing stage  1120 . In an alternative aspect, the momentum can be described as a rectification and low-pass filter of the input signal (e.g., up-and-down swipe gesture over time). 
     An aspect of the flywheel system  1100  is to determine a resulting expression level (e.g., volume) that produces a realistic sounding transition from one note or chord to the next by maintaining and modulating a virtual momentum developed in the flywheel system  1100 . To illustrate this concept, a user may initiate a very soft legato articulation on a touch zone by slowing swiping in an up-and-down motion (e.g., a slow first swipe speed) and gradually increasing speed until a relatively fast and constant rate is achieved (e.g., a fast second swipe speed). This can create the sound of a musical crescendo with the relative volume (i.e., expression data) of the virtual instrument increasing until the volume is commensurate with the momentum created by the constant rate of the input signal. Over a period of time, both signal processing stages  1110 ,  1120  (i.e., virtual flywheels) will be virtually rotating with the same momentum (e.g., the fast second swipe speed). As the user moves to a different touch zone (e.g., a different chord) and begins a swipe gesture at a slower constant swipe speed (e.g., a third medium swipe speed), the flywheel system  1100  would begin playback of the new chord with a strong volume corresponding to the second fast swipe speed and slowly reduce to a lower volume corresponding to the third medium swipe speed. The rate of change can be dictated by the various terms and parameters of equation 1 that define the virtual mass effect (i.e., momentum) of the flywheel system  1100 . It should be noted that the flywheel system  1100  can be applied to many applications other than modeling a lifecycle of a note or chord. For example, the flywheel system  1100  could control drawing/painting applications where the intensity (e.g., color, width of lines, straightness of lines, line characteristics) is modulated based on the output of the flywheel system  1100 . Alternatively, the flywheel system  1100  can be configured to control a smart drum machine that alters beat parameters (e.g., beat complexity, fill complexity, sample selection, volume, etc.) based on the output of the flywheel system  1100  to provide a continuously evolving drum track. It should be known that the application of the flywheel system  1100  is not limited to musical applications and may be adapted to many other systems well-suited for continuous modulation and control by a virtual momentum based system. 
       FIG. 12  illustrates an embodiment of a note lifecycle  1200 , according to an embodiment of the invention. The note lifecycle can be measured in a relative level  1210  (e.g., volume) versus time  1220 . The note life cycle  1200  can include an attack phase  1230 , a sustain phase  1240 , and a release phase  1250 . In some embodiments, the note lifecycle can include additional phases including a decay phase or any other characteristic that can be used to model the life cycle of a musical note. The foregoing note lifecycle phases would be readily understood by those of ordinary skill in the art. 
       FIG. 13  is a simplified flow diagram illustrating aspects of a method  1300  of modeling a lifecycle of a note played on a virtual instrument, according to an embodiment of the invention. The method  1300  is performed by processing logic that may comprise hardware (e.g., circuitry, dedicate logic, etc.), software (which as is run on a general purpose computing system or a dedicated machine), firmware (embedded software), or any combination thereof. The method  1300  is discussed in terms of the dual flywheel physical system model  1100  ( FIG. 11 ) as applied to the exemplary system  700  ( FIG. 7 ), which is configured to execute the method  1300 . 
     Referring to  FIG. 13 , the method  1300  includes receiving a touch down and motion touch gesture (e.g., a legato gesture) on a touch zone  760  of an interface  700  ( 1310 ). At  1320 , the attack phase of the note or chord begins. In some embodiments, the level (e.g., volume) and rate of attack (ΔL/Δt) is based, in part, on a mass coefficient (h a ) of the dual flywheel physical system model (“flywheel system”)  1100  and the input signal (e.g., legato articulation). After a first predetermined period of time ( 1330 ), the attack phase expires. The predetermined period of time for the attack phase  1330  can be a time constant of any suitable length as would be appreciated by one of ordinary skill in the art. At  1340 , the note enters the sustain phase. The length and rate of decay (ΔL/Δt) of the sustain phase is based, in part, on a mass coefficient (h s ) of the flywheel system  1100  and the input signal (e.g., speed of the legato articulation). If the input (e.g., legato gesture) stops for a second predetermined period of time ( 1350 ), the sustain phase  1240  expires and the note enters the release phase  1360 . The level (e.g., volume) of the release phase decays according to a predetermined time constant. The length and rate of decay (ΔL/Δt) of the release phase is based, in part, on a mass coefficient (h s ) of the flywheel system  1100 . At  1370 , a lift-off condition occurs and the method ends. 
     In certain embodiments, the flywheel system  1100  provides a mechanism to modulate the level and phase of a note lifecycle in real time. As described above, the sustain phase  1340  gradually decreases and eventually enters the release phase  1360  in response to a static input  1350 . For example, a level (e.g., output of the second virtual flywheel) of the sustain phase  1340  may decrease below a predetermined value and enter the release phase  1360 . In some embodiments, the sustain phase  1340  can be maintained indefinitely provided that there is a continuous input signal  1310 . Furthermore, increasing the speed of a swipe gesture can proportionally increase a current level of the sustain phase  1340  in accordance with the virtual momentum characteristics of the flywheel system  1100 . In some embodiments, the note lifecycle can return ( 1355 ) from the release phase  1360  back to the sustain phase  1340  in response to a sufficiently high input signal (e.g., swipe speed). It should be noted that the “level” described herein can refer to a volume. In addition, level can optionally refer to a relative “momentum” created by the flywheel system  1100 , where the different phases (e.g., attack, sustain, release) correspond to a level. 
     In alternative embodiments, other input signals may be used to modulate the note lifecycle, the flywheel system  1100 , or a combination of the two. Other user input signals can include the size of the touch point and/or the acceleration therewith (i.e., detect the pressure or downward force of the input gesture), the angle of the touch signal, multiple touch signals, or any other input signal that would be appreciated by one of ordinary skill in the art with the benefit of this disclosure. 
     It should be appreciated that the specific steps illustrated in  FIG. 13  provides a particular method of modeling a lifecycle of a note played on a virtual instrument, according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. In certain embodiments, the method  1300  may perform the individual steps in a different order, at the same time, or any other sequence for a particular application. Moreover, the individual steps illustrated in  FIG. 13  may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize and appreciate many variation, modification, and alternatives of the method. 
       FIG. 21  is a simplified flow diagram illustrating aspects of a method  2100  of modulating a lifecycle of a note, according to an embodiment of the invention. Method  2100  is performed by processing logic that may comprise hardware (e.g., circuitry, dedicate logic, etc.), software (which as is run on a general purpose computing system or a dedicated machine), firmware (embedded software), or any combination thereof. In one embodiment, method  2100  is performed by the processor unit  1410  of  FIG. 14 . 
     Referring to  FIG. 21 , method  2100  begins with generating a user interface on a touch sensitive display, at  2110 . The touch sensitive display can be similar to the user interface  100  for a virtual musical instrument, as shown in  FIG. 1 . The user interface can be implemented on any suitable touch sensitive device configured to detect touch input. At  2120 , method  2100  further includes generating a first virtual flywheel system (VFS)  1110 . The first virtual flywheel system  1110  can include an input and an output, where the input may include a user touch input on the user interface, or any suitable data input (e.g., input signal) from any desired source, as would be appreciated by one of ordinary skill in the art. The output of the first virtual flywheel system can include a data value that may be an expression value, virtual momentum, or other suitable data output that is based, at least in part, on the data input and a mass coefficient associated with the first virtual flywheel. The mass coefficient of the VFS  1110  may be predetermined, a fixed value, a dynamic value, or any suitable value as required. In some cases, the output of the first virtual flywheel  1110  is the product of a magnitude of the data input and the mass coefficient. 
     At  2130 , method  2100  further includes generating a second virtual flywheel system  1120 . The second virtual flywheel system (VFS)  1110  can include an input and an output. In some embodiments, the second virtual flywheel system  1120  is series connected with the first virtual flywheel system  1100 , such that the output of the first virtual flywheel system  1110  is coupled to the input of the second virtual flywheel system  1120 . In some cases, the input to the second virtual flywheel system  1120  may include additional sources (e.g., multiple inputs). The output of the second VFS  1120  can be based wholly, or in part, on the input to the VFS  1120  and a mass coefficient associated with the second VFS  1120 . The mass coefficient of the VFS  1120  may be predetermined, a fixed value, a dynamic value, or any suitable value as required. In some cases, the output of the first virtual flywheel  1120  is the product of a magnitude of the data input of the second VFS  1120  and its associated mass coefficient. 
     At  2140 , the user interface receives a user input. The user input can include a touch gesture, swipe gesture, or any suitable input from a user, stylus, or the like, on a touch-sensitive display. Alternatively, the input may be generated by systems other than the touch sensitive variety. At  2150 , method  2100  further includes determining a momentum of the first VFS  1110 . As described above, the momentum may be based on the input to the FVS  1110  and a mass coefficient associated with the VFS  1110 . At  2160 , method  2100  further includes determining a momentum of the second VFS  1110 . As described above, the momentum of the second VFS  1120  may be based on an input signal to the second VFS  1120  and its associated mass coefficient. The output of the VFS  1120  can be utilized as an expression level, an output level, a dynamic control signal, or the like, as would be appreciated by one of ordinary skill in the art. 
     At  2170 , method  2100  further includes applying the virtual momentum of the second VFS  1120  to modulate a lifecycle of a note or chord. In some embodiments, the output of the VFS  1120  (e.g., expression level) can be used to modulate one or more audio properties of an audio signal. For example, a volume of the audio signal can be controlled in real-time based on a magnitude of the expression level. The output of the VFS  1120  can be used to modulate any system including those not related to audio applications. For example, the expression level may be used to control or modulate visual properties (e.g., colors, intensity, contrast, tone) of a system. In broader contexts, the output of the VFS  1120  can be used to modulate in real-time any desired parameter in any suitable application. 
     It should be appreciated that the specific steps illustrated in  FIG. 21  provides a particular method of modulating a lifecycle of a note, according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. In certain embodiments, method  2100  may perform the individual steps in a different order, at the same time, or any other sequence for a particular application. Moreover, the individual steps illustrated in  FIG. 21  may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize and appreciate many variation, modification, and alternatives of the method. 
       FIG. 15  illustrates a computer system  1500  according to an embodiment of the present invention. The user interfaces described herein (e.g., interface  100  and  700 ) can be implemented within a computer system such as computer system  1500  shown here. Computer system  1500  can be implemented as any of various computing devices, including, e.g., a desktop or laptop computer, tablet computer, smart phone, personal data assistant (PDA), or any other type of computing device, not limited to any particular form factor. Computer system  1500  can include processing unit(s)  1505 , storage subsystem  1510 , input devices  1520 , output devices  1525 , network interface  1535 , and bus  1540 . 
     Processing unit(s)  1505  can include a single processor, which can have one or more cores, or multiple processors. In some embodiments, processing unit(s)  1505  can include a general purpose primary processor as well as one or more special purpose co-processors such as graphics processors, digital signal processors, or the like. In some embodiments, some or all processing units  1505  can be implemented using customized circuits, such as application specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs). In some embodiments, such integrated circuits execute instructions that are stored on the circuit itself. In other embodiments, processing unit(s)  1505  can execute instructions stored in storage subsystem  1510 . 
     Storage subsystem  1510  can include various memory units such as a system memory, a read-only memory (ROM), and a permanent storage device. The ROM can store static data and instructions that are needed by processing unit(s)  1505  and other modules of electronic device  1500 . The permanent storage device can be a read-and-write memory device. This permanent storage device can be a non-volatile memory unit that stores instructions and data even when computer system  1500  is powered down. Some embodiments of the invention can use a mass-storage device (such as a magnetic or optical disk or flash memory) as a permanent storage device. Other embodiments can use a removable storage device (e.g., a floppy disk, a flash drive) as a permanent storage device. The system memory can be a read-and-write memory device or a volatile read-and-write memory, such as dynamic random access memory. The system memory can store some or all of the instructions and data that the processor needs at runtime. 
     Storage subsystem  1510  can include any combination of computer readable storage media including semiconductor memory chips of various types (DRAM, SRAM, SDRAM, flash memory, programmable read-only memory) and so on. Magnetic and/or optical disks can also be used. In some embodiments, storage subsystem  1510  can include removable storage media that can be readable and/or writeable; examples of such media include compact disc (CD), read-only digital versatile disc (e.g., DVD-ROM, dual-layer DVD-ROM), read-only and recordable Blue-Ray® disks, ultra density optical disks, flash memory cards (e.g., SD cards, mini-SD cards, micro-SD cards, etc.), magnetic “floppy” disks, and so on. The computer readable storage media do not include carrier waves and transitory electronic signals passing wirelessly or over wired connections. 
     In some embodiments, storage subsystem  1510  can store one or more software programs to be executed by processing unit(s)  1505 , such as a user interface  1515 . As mentioned, “software” can refer to sequences of instructions that, when executed by processing unit(s)  1505  cause computer system  1500  to perform various operations, thus defining one or more specific machine implementations that execute and perform the operations of the software programs. The instructions can be stored as firmware residing in read-only memory and/or applications stored in magnetic storage that can be read into memory for processing by a processor. Software can be implemented as a single program or a collection of separate programs or program modules that interact as desired. Programs and/or data can be stored in non-volatile storage and copied in whole or in part to volatile working memory during program execution. From storage subsystem  1510 , processing unit(s)  1505  can retrieve program instructions to execute and data to process in order to execute various operations described herein. 
     A user interface can be provided by one or more user input devices  1520 , display device  1525 , and/or and one or more other user output devices (not shown). Input devices  1520  can include any device via which a user can provide signals to computing system  1500 ; computing system  1500  can interpret the signals as indicative of particular user requests or information. In various embodiments, input devices  1520  can include any or all of a keyboard touch pad, touch screen, mouse or other pointing device, scroll wheel, click wheel, dial, button, switch, keypad, microphone, and so on. 
     Output devices  1525  can display images generated by electronic device  1500 . Output devices  1525  can include various image generation technologies, e.g., a cathode ray tube (CRT), liquid crystal display (LCD), light-emitting diode (LED) including organic light-emitting diodes (OLED), projection system, or the like, together with supporting electronics (e.g., digital-to-analog or analog-to-digital converters, signal processors, or the like), indicator lights, speakers, tactile “display” devices, headphone jacks, printers, and so on. Some embodiments can include a device such as a touchscreen that function as both input and output device. 
     In some embodiments, output device  1525  can provide a graphical user interface, in which visible image elements in certain areas of output device  1525  are defined as active elements or control elements that the user selects using user input devices  1520 . For example, the user can manipulate a user input device to position an on-screen cursor or pointer over the control element, then click a button to indicate the selection. Alternatively, the user can touch the control element (e.g., with a finger or stylus) on a touchscreen device. In some embodiments, the user can speak one or more words associated with the control element (the word can be, e.g., a label on the element or a function associated with the element). In some embodiments, user gestures on a touch-sensitive device can be recognized and interpreted as input commands; these gestures can be but need not be associated with any particular array in output device  1525 . Other user interfaces can also be implemented. 
     Network interface  1535  can provide voice and/or data communication capability for electronic device  1500 . In some embodiments, network interface  1535  can include radio frequency (RF) transceiver components for accessing wireless voice and/or data networks (e.g., using cellular telephone technology, advanced data network technology such as 3G, 4G or EDGE, WiFi (IEEE 802.11 family standards, or other mobile communication technologies, or any combination thereof), GPS receiver components, and/or other components. In some embodiments, network interface  1535  can provide wired network connectivity (e.g., Ethernet) in addition to or instead of a wireless interface. Network interface  1535  can be implemented using a combination of hardware (e.g., antennas, modulators/demodulators, encoders/decoders, and other analog and/or digital signal processing circuits) and software components. 
     Bus  1540  can include various system, peripheral, and chipset buses that communicatively connect the numerous internal devices of electronic device  1500 . For example, bus  1540  can communicatively couple processing unit(s)  1505  with storage subsystem  1510 . Bus  1540  also connects to input devices  1520  and display  1525 . Bus  1540  also couples electronic device  1500  to a network through network interface  1535 . In this manner, electronic device  1500  can be a part of a network of multiple computer systems (e.g., a local area network (LAN), a wide area network (WAN), an Intranet, or a network of networks, such as the Internet. Any or all components of electronic device  1500  can be used in conjunction with the invention. 
     Some embodiments include electronic components, such as microprocessors, storage and memory that store computer program instructions in a computer readable storage medium. Many of the features described in this specification can be implemented as processes that are specified as a set of program instructions encoded on a computer readable storage medium. When these program instructions are executed by one or more processing units, they cause the processing unit(s) to perform various operation indicated in the program instructions. Examples of program instructions or computer code include machine code, such as is produced by a compiler, and files including higher-level code that are executed by a computer, an electronic component, or a microprocessor using an interpreter. 
     It will be appreciated that computer system  1500  is illustrative and that variations and modifications are possible. Computer system  1500  can have other capabilities not specifically described here (e.g., mobile phone, global positioning system (GPS), power management, one or more cameras, various connection ports for connecting external devices or accessories, etc.). Further, while computer system  1500  is described with reference to particular blocks, it is to be understood that these blocks are defined for convenience of description and are not intended to imply a particular physical arrangement of component parts. Further, the blocks need not correspond to physically distinct components. Blocks can be configured to perform various operations, e.g., by programming a processor or providing appropriate control circuitry, and various blocks might or might not be reconfigurable depending on how the initial configuration is obtained. Embodiments of the present invention can be realized in a variety of apparatus including electronic devices implemented using any combination of circuitry and software. 
     While the invention has been described with respect to specific embodiments, one skilled in the art will recognize that numerous modifications are possible: displaying the interface  100 / 700  and the configuration of the various elements therein, such as the position of touch regions and touch zones, the types of virtual instruments used, the types of chord inversions applied to the touch zones, which may be customizable, customizing aspects of the flywheel system, etc. Thus, although the invention has been described with respect to specific embodiments, it will be appreciated that the invention is intended to cover all modifications and equivalents within the scope of the following claims. 
     The above disclosure provides examples and aspects relating to various embodiments within the scope of claims, appended hereto or later added in accordance with applicable law. However, these examples are not limiting as to how any disclosed aspect may be implemented, 
     All the features disclosed in this specification (including any accompanying claims, abstract, and drawings) can be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     Any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C. §112, sixth paragraph. In particular, the use of “step of” in the claims herein is not intended to invoke the provisions of 35 U.S.C. §112, sixth paragraph.