Music – Instruments – Electrical musical tone generation
Reexamination Certificate
2001-07-12
2002-09-10
Fletcher, Marlon T. (Department: 2837)
Music
Instruments
Electrical musical tone generation
C084S622000, C084S659000, C084S661000, C084S736000, C084SDIG009
Reexamination Certificate
active
06448488
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the measurement of stringed musical instrument vibrations and subsequent processing of these signals. More particularly, this invention relates to the reproduction of musical sounds characteristic of acoustic instruments into high fidelity electrical signals for amplification and reproduction of musical sounds, by uniquely exploiting, through measurements and subsequent signal processing the vector nature of string excitation forces (SEF) and body vibrations of stringed musical instruments (SMI's).
BACKGROUND OF THE INVENTION
Methods of amplifying (for purposes of both performance or recording) stringed musical instruments (SMI) employ sensors that measure acoustic pressure (i.e. microphones), force (i.e. piezo) and displacement (strain gauge, hall effect, laser), velocity (coil pickups) and acceleration (accelerometers). A common expectation in using techniques that combine sensors other than microphones is that the sensors will be mounted semi-permanently in a manner that mitigates sensor placement issues. While these sensors are obviously integral to electric guitars, players of acoustic SMI's have grown to rely on the convenience and consistency of “plugging in”. In fact, acceptance of this technique has grown to the point that approximately 20% of acoustic guitars sold in the US have factory installed embedded sensor (ES) systems. We will refer to these sensors, along with any subsequent processing (analog or digital), as an embedded sensor technique in contrast to a solely microphonic approach.
Although microphones are by definition the only objective, quantitative means to directly capture the true acoustic sound of a SMI, microphone measurements of SMI sound are affected by placement and, in amplified scenarios, there is the potential for unstable feedback among microphone, instrument and the amplifier. To avoid problems of placement and feedback, embedded sensors such as piezo force transducers are often placed under the saddle of an acoustic guitar or on the bridge of violins and/or cellos. The quality of this amplified sound (typically taken from either bridge or sound hole based signals) has heretofore fallen short of the true acoustical signal measured from a microphone.
In striving to improve the reproduction of acoustic SMI characteristics using embedded sensor techniques, prior efforts have focused primarily on either of two distinct mechanisms involved in SMI sound generation. The first SMI characteristic is that the string force applied to the witness point (the point of contact between saddle and string) can be resolved into a plurality (up to 3) of significant components. Prior art teaches methods to isolate, suppress or advantageously combine string excitation force (SEF) components by improved sensor means. These efforts include U.S. Pat. No. 3,453,920 issued to Scherer, (“Scherer1”) and U.S. Pat. No. 4,903,566 issued to Mcclish, (“Mcclish”).
Lazarus U.S. Pat. No. 3,624,264 issued to Lazarus, (“Lazarus”) aptly compares the motions of the bridge block of a guitar to those of a ship at sea; With the convention that the contact point of the guitar's low E string is port, and the high E string starboard, the three acoustically significant modes of bridge block vibration (BBV) are pitch, roll and heave, Through proper positioning of vibration sensors about the bridge, works such as Lazarus or its commercial descendent Trance-Audio's (“Acoustic Lens”) http://www.tranceaudio.com/manuals/lens.pdf and http://www.tranceaudio.com/lens.html), claim to effectively capture the tonal qualities of the SMI by indirectly measuring the multi-directional nature of SEF's through measurements of BBV's on the surface of the SMI. As shown below, while these sensors are responsive to the three vibration modes (pitch, roll and heave), the sound is primarily affected by repositioning the pickup on the body of the guitar and the ability to manipulate the sound is significantly constrained. Moreover, discussion below will describe the advantages of the present invention over limitations of sensor based component nulling techniques as represented by Scherer and McClish.
A second distinct SMI characteristic results from the structural features such as a resonant cavity that provide frequency responses unique to different classes of instruments. Embedded sensor approaches where sensors are directly responsive to the string excitation do not directly measure the characteristic colorations of an acoustic SMI. U.S. Pat. No. 4,819,537 issued to Hayes et al., (“Hayes”) teaches a post-processing methods that can reintroduce the characteristic Helmholtz resonance of a particular SMI. Other ES sensor approaches, such as Lazarus and Trance-Audio, claim to be uniquely responsive to vibrational modes due to and representative of these characteristic resonances, but are limited to the sound that can be measured on the surface of the guitar. In contrast the present invention provides a capability and theoretical framework for more flexible manipulation of embedded sensor (ES) signals.
Moreover, a body of work (fairly represented by “Plucked string models: from (Karplus-Strong) algorithm to digital waveguides and beyond”, by M. Karjalainen and V. Vlimki and T. Tolonen Vol 22, number 3, Computer Music Journal, 1998, or http://www.acoustics.hut.fi/~vpv/publications/cmj98.htm) has developed synthesis techniques that combine multiple polarization string models with models of guitar body resonances.
These works contain a sophisticated theoretical basis for synthesis of a guitar signal, but in contrast to the present invention, do not teach the processing of embedded sensor signals that can re-create the sound characteristics of a particular SMI.
SUMMARY OF THE INVENTION
For analysis purposes, SMI vibrations are decomposed into modes that can be generally defined as having monopole, dipole or even quadrapole physical interpretations of distinct surface plate modal patterns, for example as taught by fletcher ((“The Physics of Musical Instruments”) by Neville H. Fletcher and Thomas D. Rossing (Chapter 9) Springer Verlag ISBN: 0387983740). The representation of the SMI state by physical modes &PSgr;
i
(r) is advantageous in the study of SMI acoustics, but another modal representation that is particularly suited to the simulation and re-creation of SMI acoustic characteristics (an objective of the present invention) involves “PRISM” modes. PRISM modes will be introduced by way of a description of a standard physical mode model of SMI sound generating mechanisms.
The distribution of surface state of a SMI (e.g. a guitar) can be described via a summation of modes:
α
(
r
,
w
)
=
∑
i
λ
i
(
w
)
Ψ
i
(
r
)
,
(
1
)
where &PSgr;
i
(r) is the i
th
mode (r coordinates) linearly weighted and summed by the complex modal amplitude
&lgr;
i
(
w
)=
a
i
(
w
)
e
j&phgr;
i
(W)
(2)
(a
i
(w), &PHgr;
i
(w), magnitude and phase), to form the total state (displacement and or velocity) &agr;(r, w) as a function of frequency w and position r. The surface states &agr;(r, w) are then weighted and summed by the pointwise (with respect to r) acoustic transfer function C(r, w|R) to form the acoustic pressure
S
mic
(
w
)=∫
C
(
r,w|R
)&agr;(
r,w
)
dr
(3)
seen at a point R as a function of frequency w.
Equation 3 defines the relation between physical state &agr;(r,w) and the output S
mic
(w), but more importantly for the present invention is the relation between the output and the particular physical excitation of this system which is the SEF vector
F
=
[
V
T
L
]
,
(
4
)
whose vertical, transverse and longitudinal force components (all implicit functions of frequency) excite the heave, roll and pitch motions of the bridge block, which in turn excite unique combinations of the physical modes of an SMI &PSgr;
i
(r). These combinations of physical modes can be regrouped into “PRISM modes”, which serve the role of a transfer function between the vertical, transverse a
Ekhaus Ira
Fishman Lawrence
Chadbourne & Parke LLP
Fishman Transducers, Inc.
Fletcher Marlon T.
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