ï~~Proceedings of the International Computer Music Conference (ICMC 2009), Montreal, Canada
August 16-21, 2009
CALIBRATION METHOD TO MEASURE ACCURATE BOW FORCE
FOR REAL VIOLIN PERFORMANCES
Enric Guaus, Jordi Bonada, Esteban Maestre, Alfonso Perez, Merlijn Blaauw
Music Technology Group, Universitat Pompeu Fabra
enric guaus@upf edu
ABSTRACT
In this paper, we present a procedure to predict bow
pressing force in a violin from data acquired in real recordings. We focus on the calibration procedure that must be
robust to the bow tension changes in long recordings and
fast enough to not disturb the recording session. Because
of this last limitation, the calibration method here proposed does not exhaustively cover all the possible bow
conditions that potentially may appear in the recording.
We propose the use of Support Vector Regression to predict all these missing scenarios and compute the predicted force. On the other hand, bow tension variations
in long recordings produce decalibrations on the acquisition system. After analyzing their behavior, we propose
a solution to compensate this effect based on post processing and a specific behavior of the performer at the
beginning and the end of each phrase.
1. INTRODUCTION
Acquisition of bowing parameters in a violin is not a new
topic of research. Askenfelt[1, 2] presented a method
for measuring bow motion and bow force using diverse
custom electronic devices attached to the violin and the
bow. Paradiso[5] proposed to measure the bow force
by using a force-sensitive resistor below the forefinger.
More recently, Young[9, 10] measured downward and
lateral bow pressure with foil strain gages using the Hyperbow controller. Rasamimanana[6] used force sensitive resistors (FSRs) to obtain the strain of the bow hair
as a measure of bow pressure. Finally, Demoucron[4]
and Schoonderwaldt[7] presented, in their PhD theses,
an exhaustive and complete study on gesture acquisition
and bow parameterization, respectively. Let us remark
that the bow sensors we implemented are based on their
studies and recommendations.
Most of the state of the art acquisition systems here
presented have not been tested in a long real performance
environment focused on the audio quality instead of the
obtention of the bow motion parameters. Mechanical
properties of the bow may vary on time: temperature and
humidity are, among others, the most important extern
parameters that affect the ribbon hair tension and, as a
consequence, the bow force applied to the strings. This
paper describes the full process to obtain real force values (in Newtons) for long recordings in a studio. The
process is clearly divided in three parts. First, we describe the sensors and the corresponding signal conditioning. Second, we describe the calibration process and,
finally, we show the post processing operations to compensate the bow tension deviations
2. SENSING SYSTEM
Bow force acquisition is a very specific part in our violin
synthesizer design process. There exists many restrictions imposed by the other blocks that can be summarized as follows: a) Non intrusive to the player (dimensions and weight); b) Synchronized with other sensors;
c) Focused on audio quality instead of motion capture;
d) Time restrictions; e) Allow changes in bow tension by
the performer; f) Robust to intrinsic bow tension variations; g) Capture all of the possible scenarios presented
in the score using a short calibration procedure.
2.1. Motion Acquisition
Violin body and bow motions are recorded using the Polhemus Liberty system. It is a six degrees of freedom
electromagnetic tracker that provides information on localization and orientation of a sensor with respect to a
source. We use two sensors, one attached to the bow and
the other one attached to the violin, in order to obtain a
complete representation of their relative movement.
2.2. Strain Gages
Our sensing system for bow force prediction is based on
the work of Demoucron[4]. We mounted a dual strain
gage system attached to a steel foil (0.9x5cm), at the frog
of the bow. In order to capture the deformation of the ribbon hairs, the steel foil is forced to an initial bending with
no force applied, and this bending tends to zero as the
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