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What is ASTRA?

The ASTRA projects aims to reconstruct the sound or timbre of ancient instruments (not existing anymore) using archaeological data as fragments from excavations, written descriptions, pictures...

The technique used is the physical modeling synthesis, a complex digital audio rendering technique which allows modeling the time-domain physics of the instrument.

In other words the basic idea is to recreate a model of the musical instrument and produce the sound by simulating its behavior as a mechanical system. The application would produce one or more sounds corresponding to different configurations of the instrument (i.e. the different notes).

What is physical modeling?

Physical modeling synthesis is the synthesis of sound by using a set of equations and algorithms to simulate a physical source of sound. Sound is then generated using parameters that describe the physical materials used in the instrument and the user's interaction with it, for example, by plucking a string, or covering tone holes, and so on. For example, to model the sound of a drum, there would be a formula for how striking the drumhead injects energy into a two dimensional membrane.

Thereafter the properties of the membrane (mass density, stiffness, etc.), its coupling with the resonance of the cylindrical body of the drum, and the conditions at its boundaries (a rigid termination to the drum's body) would describe its movement over time and thus its generation of sound. Similar stages to be modeled can be found in instruments such as a violin, though the energy excitation in this case is provided by the slip-stick behavior of the bow against the string, the width of the bow, the resonance and damping behavior of the strings, the transfer of string vibrations through the bridge, and finally, the resonance of the soundboard in response to those vibrations.

Although physical modeling was not a new concept in acoustics and synthesis, having been implemented using finite difference approximations of the wave equation by Hiller and Ruiz in 1971, it was not until the development of the Karplus-Strong algorithm, the subsequent refinement and generalization of the algorithm into the extremely efficient digital waveguide synthesis by Julius O. Smith III and others, and the increase in DSP power in the late 1980s that commercial implementations became feasible.

Reason for gridification

Physical modeling is a really computing intensive technique since the complex models of the musical instruments are solved by integrating numerical coupled differential equations. To have an idea of the needed time for simulation, on a Pentium IV1.6 Ghz, 512MB RAM Personal Computer to correctly reproduce a sound lasting for 30 seconds it could be required more than 4 hours.

ASTRA and GEANT2

ASTRA computations are quite demanding in terms of network and computing requirements. The extremely high reliability of the GEANT2 network, its performances in terms of number of institutions connected and thoughput, made the ASTRA project possible.

The project is running since 2006 thanks to the GEANT2 backbone, allowing researchers, musicians and historians to collaborate, communicate and share experiencies on lost instruments and sounds ASTRA brings again to life.