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Augmenting the Acoustic Piano with ElectromagneticString Actuation and Continuous Key Position Sensing
Andrew McPhersonElectrical & Computer Engineering
Drexel University3141 Chestnut St.
Philadelphia, PA 19104
Youngmoo KimElectrical & Computer Engineering
Drexel University3141 Chestnut St.
Philadelphia, PA 19104
ABSTRACTThis paper presents the magnetic resonator piano, an aug-mented instrument enhancing the capabilities of the acous-tic grand piano. Electromagnetic actuators induce the stringsto vibration, allowing each note to be continuously con-trolled in amplitude, frequency, and timbre without exter-nal loudspeakers. Feedback from a single pickup on thepiano soundboard allows the actuator waveforms to remainlocked in phase with the natural motion of each string. Wealso present an augmented piano keyboard which reportsthe continuous position of every key. Time and spatial res-olution are sufficient to capture detailed data about keypress, release, pretouch, aftertouch, and other extended ges-tures. The system, which is designed with cost and setupconstraints in mind, seeks to give pianists continuous con-trol over the musical sound of their instrument. The in-strument has been used in concert performances, with theelectronically-actuated sounds blending with acoustic in-struments naturally and without amplification.
KeywordsAugmented instruments, piano, interfaces, electromagneticactuation, gesture measurement
1. INTRODUCTIONThe acoustic piano is among the most versatile of instru-
ments, capable of complex polyphony and rapid passage-work across an extremely wide register. Yet in comparisonwith most other acoustic instruments, the piano has a sur-prising limitation: once a note is struck, the performer hasvirtually no ability to modulate its sound before it is re-leased. Building a keyboard instrument with the ability tocontinuously shape each note is an age-old problem: In the15th century, Leonardo da Vinci devised an instrument us-ing rosined wheels to selectively sound a bank of strings;later, the late 18th and early 19th centuries saw a prolif-eration of new instruments attempting to bring indefinitesustain and continuous modulation to the keyboard .
With modern electronic synthesizers, infinite sustain andreal-time note shaping are no longer challenging. Yet de-spite decades of improvement, many performers find that
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electronic instruments still do not match the richness andnuance of their acoustic counterparts; very few pianistswould choose even the most sophisticated synthetic pianoover any acoustic grand of reasonable quality.
This paper presents the magnetic resonator piano, whichseeks to unify the flexibility of synthesis with the richness ofthe acoustic piano by electronically augmenting an acousticgrand piano. Electromagnetic actuators directly induce thestrings to vibration, bypassing the pianos percussive ham-mer mechanism and allowing continuous control over thesound of each note. Continuous position sensing of eachpiano key allows the performer to control parameters ofactuation in real time without impeding traditional pianotechnique, while an optional second keyboard can be usedto control the actuators without engaging the mechanicalaction. All sound is produced by the piano strings, with-out loudspeakers, facilitating integration with other acous-tic instruments in a concert hall. Vocabulary of this hybridacoustic-electronic instrument includes indefinite sustain,crescendos from silence, harmonics on each piano string,and new timbres which combine the warmth and resonanceof the acoustic piano with an ethereal purity often associ-ated with electronic synthesis.
2. PREVIOUS WORKElectromagnetic piano string actuation has been recently
explored by Bloland, Berdahl et al. [1, 3]. Their system, theElectromagnetically-Prepared Piano, uses twelve solenoidmagnets placed over selected strings. Signals are generatedin Max/MSP and can include periodic waveforms, filterednoise, and prerecorded samples. The resulting sounds arequite compelling, blending the warmth of the piano with auniquely electronic purity.
Electronic actuation has also been applied to the electricguitar, both in commercial technologies such as the EBow and using more comprehensive feedback approaches .Work by Boutin and Besnainou explores active control of aviolin bridge  and xylophone bar . Stable active con-trol of acoustic mechanisms requires very low processinglatency, and is often accomplished using specialized digitalsignal processing hardware. Dozio and Mantegazza  pro-vide guidance for the implementation of real-time controlsystems using general-purpose microprocessors; Lee et al. present one such system achieving latency as low as24s at a 40kHz sampling rate.
Separately, keyboard controllers have been developed whichreport the continuous position of each key. Freed and Avizie-nis  demonstrate a keyboard capable of continuous keyposition sensing, including high-speed communication witha host computer to transmit high-bandwidth position data.A keyboard controller by Moog  also permits horizontal
Proceedings of the 2010 Conference on New Interfaces for Musical Expression (NIME 2010), Sydney, Australia
motion of each key and touch sensitivity on the key surface.These interfaces build on established keyboard technique toallow control of more complex musical processes.
2.1 Comparison with Previous WorkOur electromagnetic actuation system operates on the
same principle as the Electromagnetically-Prepared Piano, but we present an implementation which allows cover-age of the entire range of the piano at reasonable cost. Wealso employ a feedback-based control strategy that can beimplemented without the requirement of ultra low latencyor separate pickups for each string. In comparison to effortswhich focus on actuation or keyboard sensing in isolation,we seek to tightly integrate both elements deeply into the pi-ano, fusing traditional (hammer-actuated) piano techniqueand electronic control into a single augmented instrumentthat acts as a natural extension of the acoustic piano.
3. ELECTROMAGNETIC ACTUATIONThe design of the actuator system is explained in detail
in an article currently in press . The major features ofthe hardware and software design are outlined below.
3.1 Hardware Design
Figure 1: The magnetic resonator piano. Top: com-plete system. Top inset: electromagnetic actuatorsabove the strings. Bottom: Brackets holding actu-ators for four octaves of strings.
Figure 1 shows a picture of the electromagnetic actuationsystem, which can be installed in any acoustic grand pianowithout modification to the instrument. A block diagramis shown in Figure 2. Actuation works on the principle offerromagnetic attraction, whose application to piano strings
is detailed in . One custom-wound solenoid electromag-net is used for each note of the piano, up to 88 notes total(48 in the current prototype). Actuators are suspendedabove the string by an aluminum bracket which rests onthe steel beams of the piano frame. Though the actuatorsmust initially be adjusted to the specific geometry of a pi-ano, removal and reinstallation are as simple as lifting andreplacing the aluminum brackets.
Each actuator is driven by a dedicated amplifier opti-mized for low cost and parts count. Amplifier input signalsare generated by computer, but it would be prohibitivelyexpensive to use a separate DAC channel for each note ofthe piano. Instead, each amplifier input is attached to a 16-channel multiplexer which dynamically selects an availableDAC channel. A microcontroller maintains a mapping fromDAC channels to amplifiers, receiving MIDI Control Changemessages to make and break connections. In this way, 88actuators can be covered using an inexpensive commercialaudio interface, with the maximum polyphony determinedby the number of DAC channels.
3.2 Signal ProcessingThe strongest tones are obtained when the actuator wave-
form remains locked in phase to the motion of the string.On the other hand, recording the motion of each string facesseveral obstacles, including EM interference from the elec-tromagnets and substantial digital buffering delays whichmake precise feedback control impossible1.
We have developed an intermediate approach which usesa single piezo pickup on the piano soundboard to recordthe sum of all string vibrations. Bandpass filters isolatethe fundamental frequency of each note (or, optionally, thefirst several harmonics). These filtered signals drive phase-locked loops which synthesize new waveforms that remainin phase with the filtered signal (Figure 3). Mechanical de-lays and digital buffering produce a phase lag between themotion of the string at the point of its interaction with theactuator and the pickup signal. This lag is unknown buttime-invariant; therefore, the PLL includes an adjustablephase offset , calibrated by ear, which allows the ac-tuator signal to remain precisely locked in phase with themotion of each string.
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High-Q bandpass Loop filter
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Figure 3: Input filter and PLL system for one ac