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Started by Elantric, January 23, 2008, 01:30:15 PM

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http://music.ece.drexel.edu/research/mrp



The magnetic resonator piano (MRP) is a hybrid acoustic-electronic instrument augmenting the grand piano. By using electromagnets to induce the strings to vibration, the MRP allows the performer to continuously shape the sound of every note.

Motivation
The design of the acoustic piano has been essentially unchanged for over a century. The instrument's ubiquity and versatility mask a surprising limitation: once a note is struck, the performer has no means to alter its sound before it is released. Consider the following simple gesture which is impossible on the piano:



Though some electronic keyboards allow the performer to shape a note after it has been sounded, few if any match the richness and expressivity of the acoustic piano. No concert pianist would choose even the most sophisticated digital piano over a moderate-quality acoustic grand. The MRP combines the rich sound of the acoustic piano with the flexibility of digital synthesis by augmenting, rather than replacing, an acoustic instrument.

Musical Capabilities


The MRP preserves all the sounds and techniques of the acoustic piano, while expanding its vocabulary to include:

Indefinite sustain
Crescendos (including crescendos from silence)
Harmonics on each string (8 to 16 harmonics are usable on the lower strings)
New timbres which can be shaped in real time
Subtle pitch bends
In contrast to the conventional (hammer-actuated) piano sound, the sounds of the MRP are pure and ethereal, emphasizing the fundamental frequency of each string over its high partials.

Comparison of spectra for traditional piano (left) and MRP (right). MRP example uses sinusoidal actuator waveform. Other spectra are possible by varying the signal to the actuators, though none exhibit the number of upper partials seen in the piano waveform.
The following video presents clips from a recent performance of Secrets of Antikythera by Andrew McPherson. In this video, the MRP is played both from the piano keyboard and a secondary keyboard atop the piano, each of which are configured to produce different sounds.


Electromagnetic String Actuation
Electromagnetic String Actuation
Electromagnetic actuators directly induce the strings to vibration, bypassing the piano's percussive hammer mechanism. One solenoid electromagnet is mounted above each note of the instrument, approximately 1/8" above the strings. (48 notes are covered in the present prototype, easily expanded to 88.) The electromagnetic actuators work on the principle of ferromagnetism, in which a magnetic field exerts a force on the ferromagnetic (steel) piano strings.

       
Left: Complete MRP system, showing amplifier at bottom and cables running to actuators inside instrument. Right: Electromagnetic actuators above the piano strings.
The generated magnetic field is directly proportional to the current through the actuator. By modulating the current at the natural frequency of the string (or one of its harmonics), the string is induced to vibration without any physical contact.

The actuator system can be installed in any acoustic grand piano. Because tuning varies slightly among instruments, a pickup is placed on the piano soundboard which measures the combined vibrations of every string. A digital phase-locked loop control system keeps the waveform through each actuator aligned in frequency and phase with the natural motion of the corresponding string, reinforcing and amplifying the string's vibration. The sound of the MRP is produced acoustically by the vibration of strings and soundboard; no external speakers are used.


Phase-locked loop system, implemented digitally for each note. The output is frequency-locked with the input, with a fixed phase offset determined by the control input Δφ.
Actuator waveforms are generated by a computer which responds to the actions of a human performer. A specialized amplifier and signal-routing system allows up to 88 notes to be controlled using many fewer audio output channels. For example, a six-channel audio interface will allow up to six simultaneous notes, which can be played on any of the piano strings. Modifying the amplitude, frequency and spectrum of each actuator waveform allows continuous shaping of the dynamics and timbre of the corresponding piano note.


Amplifier and signal routing system. One amplifier channel is used for each actuator, but any (smaller) number of audio output channels can be used.







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http://www.geofex.com/

https://web.archive.org/web/20121003094933/http://geofex.com/FX_images/Return%20of%20the%20More%20MOSFET%20Fol....pdf

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