Composite pickup apparatus for stringed instruments

A pickup for a string under tension of a musical instrument having an improved planar response uses a pair of mechanically independent pressure transducers preferably adjacent to one another which provide independent support to transversely adjacent points of the string. The independent transducers allow the string to roll slightly on one transducer when applying pressure to the other as a result of the string vibration, thus increasing the sensitivity of the pickup in planes near the plane of minimum sensitivity. The transducers may rest on a common massive element which is resiliently coupled to the bridge of the instrument. The transducers preferably have rounded string-contacting surfaces in order to reduce the magnitude of the shear forces exerted on the pickup when the string tension is varied.

TECHNICAL FIELD 
The present invention relates to musical instruments and more specifically 
to electrically amplified stringed instruments. 
BACKGROUND ART 
Contact pickups are commonly used in connection with stringed musical 
instruments, especially in the bridge of electrically amplified acoustic 
guitars. The most versatile configuration for a bridge pickup is that 
which produces independent string signals. This allows the sound of each 
string to be individually processed and also allows the instrument thus 
monitored to control a second instrument such as a music synthesizer. 
U.S. Pat. No. 3,453,920 issued to Scherer exemplifies a contact pickup for 
an instrument string in which the string support is fixed to and supported 
by two transducer elements disposed to respond respectively to different 
planes of vibration. 
A first problem exists in such a pickup when the transducer elements do not 
lie in a common plane, that the bottom faces of the string support must 
mate well with the contacting faces of the supporting transducer elements 
in order to distribute the applied pressures evenly over the entire area 
of both transducer elements. Since it is not preferable to have thick bond 
lines between the parts for reasons of solidity as well as transduction 
preformance, this configuration imposes strict dimensional and angular 
tolerances upon the parts comprising the assembly, which tends to increase 
the cost of manufacture of such a pickup. 
A second problem exists in such an arrangement of parts, that when the 
vibrating string applies pressure to one transducer element, it must also 
exert shear forces on the other element. This reduces the transduction 
efficiency of the pickup especially when the transducer elements are 
relatively compliant or when the string support is made of a very hard and 
unyelding material. 
It is therefore a broad object of the present invention to provide a pickup 
for an instrument string having multiple transducer elements responding 
maximally in different planes of vibration and in which the shear forces 
applied to the transducer elements by the string vibrations are 
significantly reduced. 
It is a more specific object of the present invention to provide a pickup 
having high sensitivity in virtually all planes of vibration of the 
contacting string. 
It is a further object of the present invention to provide a pickup having 
the above-mentioned enhancements and which is more economical to 
manufacture than corresponding pickups in the prior art. 
SUMMARY OF THE INVENTION 
This invention is a pickup for the string of a stringed instrument that 
transduces the vibrations of the string to strong electrical signals that 
are characteristic of those vibrations both in amplitude and in frequency 
in virtually all planes of vibration. The pickup of this invention is 
composed of two separate and preferably identical halves juxtaposed 
symmetrically under the contacting string so as to define one end of the 
vibrating portion thereof. 
Each half of the pickup is preferably composed of a string contacting 
element which partially supports the string and distributes the contact 
pressures therefrom evenly over the entire area of an underlying pressure 
transducer element resting on a supporting base, so that the string 
vibrations will pass through the string contacting element to distort the 
pressure transducer element held between the contacting element and the 
base. 
Each half of the pickup is most sensitive to pressures which are applied in 
the direction of maximum sensitivity of the pressure transducer element. 
The pickup halves may be positioned at various angles between horizontal 
and almost vertical. For best results and for ease of manufacture, it has 
been found preferable to have the angle between the pressure transducer 
elements be either about 90 or 180 degrees. The 90 degree arrangement 
results in a better distribution of the pressures from the string to each 
pressure transducer element by reducing the shear forces applied to a 
first transducer element while exerting pressure on the other. This in 
turn increases the magnitude of the electrical signals produced and also 
increases the sensitivity of the pickup to planes of string vibration near 
the plane of minimum sensitivity. 
The two juxtaposed halves of the pickup jointly form a string receiving 
notch and provide approximately equal support to the contacting string. 
The string contacting surface of each half of the pickup is preferably 
rounded or bulged to sharply delimit the length of the vibrating portion 
of the string. 
Because the string contacting elements composing the string support do not 
have to match perfectly along their most proximate faces, both the 
dimensional and the angular tolerances of all parts of the pickup are 
significantly less stringent. This results in a pickup which is more 
economical to produce than would be the case if a single string support of 
the corresponding complex shape was used. This modular approach to the 
manufacture of the string support allows a first half of the pickup to be 
assembled and tested before the second half is added to the assembly, 
facilitating rework when it is required. 
The simple shape of each string contacting element of the pickup allows a 
choice of processes for its manufacture and also allows hard materials to 
be used economically in its construction, thus reducing the absorption of 
high frequencies in the contacting element, preventing permanent 
deformations of the string contacting surface and significantly reducing 
the magnitude of the shear forces applied to the pickup by the string when 
its tension is greatly varied either by tuning it or when a vibrato 
tailpiece is being used. 
In a first embodiment, a pair of semi-cylindrical pressure transducers of 
the piezoelectric type angled at about 90 degrees from each other directly 
support the contacting string. 
In a second embodiment, the pickup halves are also angled at 90 degrees 
from each other and each pressure transducer supports a string contacting 
element having a rounded string contacting surface. 
In a third embodiment, the two halves of the pickup rest on a massive 
element which is resiliently coupled to the supporting base. 
In a fourth embodiment, both halves of the pickup lie in the same plane and 
the pressure transducers support string contacting elements of truncated 
conical form which are retained sideways by abutments in order to hold the 
pickup halves together.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, FIG. 1 is a front view of a first embodiment 
of a pickup according to the present invention in which a pair of pressure 
transducers 2 and 3 having rounded string contacting surfaces 7 and 8, 
directly support the string 1 in a transversal manner and rest on the 
supporting faces 5 and 6 of a common base 4, also seen in cross-section. 
The string contacting surfaces 7 and 8 are rounded in order to sharply 
delimit the length of the vibrating portion of the string 1 and also allow 
the vibrating string to slip over them when its tension is varied. The 
angle between the pressure transducers 2 and 3 is approximately 90 degrees 
which corresponds to the angle between the supporting faces 5 and 6 of the 
base 4. The pressure transducers 2 and 3 must be positioned close enough 
to each other so that the string 1 will remain abutted on the string 
contacting surfaces 7 and 8. The string preferably contacts the center of 
each contacting surface so that the direction of maximum sensitivity of 
the corresponding pressure transducer will be an extension of the radius 
of the string. This positional relationship between the pressure 
transducers and the contacting string optimizes pickup sensitivity to the 
string vibrations. When the string 1 is brought to tension, the static 
pressure exerted by the string as it is streched over the pickup tends to 
compress the pressure transducers 2 and 3 in different directions and thus 
separate them slightly, depending on their compliance. 
The signal from a pressure transducer is proportional to the vector of the 
plane of vibration in the direction of maximum sensitivity of the 
transducer. Two transducers can therefore be oriented with respect to each 
other so that their combined output will be the same in more than two 
planes of vibration. For example, the joint vertical sensitivity of a pair 
of in-phase transducers depends on their orientation with respect to the 
string and also with respect to each other. If the transducers are almost 
vertical, very little output will be generated. As they are oriented 
gradually towards the horizontal plane, an angle will be found in the 
vicinity of 90 degrees, where the joint sensitivity to string vibrations 
will be the same in three planes of vibration. The planar response may be 
established in this way to best capture the planes of the desirable string 
vibrations encountered in a given instrument. 
When the string is vibrating, it tends to rock one transducer while 
exerting pressure on the other; the string also tends to rotate very 
slightly on itself as a result of the resistance of the rocked transducer 
to the rocking motions. This in turn slightly rocks the transducer to 
which the pressure is being applied, and the net effect of these 
interactions is a reduction in the magnitude of the shear forces applied 
to the transducer elements. Since the string 1 can apply pressures in 
different directions simultaneously and roll slightly on the contacting 
surfaces 7 and 8 of the pickup to accomodate for the slight compliance of 
the supporting elements 2, 3 and 4, a greater proportion of the string 
vibrations may be converted into electrical signals, increasing 
transduction efficiency. 
When a shear force is applied to a pressure transducer in a direction 
approximately perpendicular to its direction of maximum sensitivity, a 
voltage of opposite polarity is generated. That is, a transducer 
generating a positive voltage in response to a positive pressure in the 
direction of maximum sensitivity will generate a negative voltage in 
response to a shear force applied in a direction perpendicular to the 
direction of maximum sensitivity of the transducer. This occurs because 
the shear force tends to distend the entire element. 
A rocking motion, on the other hand, will generate approximately zero 
output since the transducer is distended over about one half of its active 
area while the remaining portion of the area is compressed. This 
characteristic of the pickup is desirable since it reduces the amount of 
signal cancellation between the pressure transducers. This tends to 
increase the pickup's response to vibrations in planes near the horizontal 
plane, where the rocking effect is greatest in both transducers 
simultaneously. The effect is noticeable mostly when the horizontal plane 
is the plane of minimum sensitivity. 
FIG. 2 shows a second embodiment of a pickup according to the present 
invention where pressure transducers 2 and 3 share a common conductor 
element 15 which contacts four piezoelectric pressure transducer elements 
11, 12, 13 and 14. Superposed piezoelectric elements 11 and 12 of pressure 
transducer 2 present an electrode of the same polarity to the conductor 
element 15. This is also the case of superposed elements 13 and 14 of 
pressure transducer 3. A lead 16 connected to the conductor element 
carries the pickup signal to utilisation means. This configuration has the 
advantage of being possibly self-shielding and also has some noise 
cancelling qualities in that it presents two edges of inverse polarity to 
interference fields. Although a single conductor element 15 is illustrated 
in the embodiment of FIG. 2, separate conductor elements may be used for 
each pair of superposed transducer elements in order to obtain separate 
signals therefrom, if desired. The conductor element 15 should be 
incompressible but it may be flexible, especially in the region between 
the transducer elements. In this embodiment, the pressure transducers 
generally designated 2 and 3 support string contacting elements 9 and 10 
which have rounded string contacting surfaces 17 and 18. FIG. 3 shows a 
detailed view of a string contacting element of the type used in the 
second and third embodiments of the present invention. The string 
contacting surfaces 17 and 18 may be electrically conductive and may be 
connected to an electrode or to a terminal of their respective underlying 
pressure transducers 2 and 3, and if a conductive string 1 is used, the 
string 1 may provide an electrical connection between the string 
contacting surfaces 17 and 18, possibly eliminating the need for separate 
electrical conductors therefrom. 
In the embodiment of FIG. 4, the two halves of the pickup are shown resting 
on a massive element 21 supported by resilient material 25 in order to 
mechanically insulate the pickup from the supporting base 4, and thus 
minimize string-to-string crosstalk. The massive element 21 may be 
conductive and it may be used as a conductor element. A lead 24 is used to 
connect the conductive massive element 21 to utilisation means. The lead 
24 may be stiff in order to determine the orientation of the pickup. If 
the massive element is to be at the same voltage potential as the surface 
supporting it, for example when using the transducer configuration of FIG. 
2 in conjunction with a massive element, the resilient material 25 or a 
part thereof may be electrically conductive and serve as a part of the 
conductive means to connect a portion of the pickup to utilisation means. 
String contacting elements 9 and 10 are prefereably hard and 
incompressible in order to prevent the string 1 from biting into the 
string contacting surfaces 17 and 18, and also for best pickup response 
and best string sustain in the high frequencies. 
FIGS. 5 and 6 respectively show a plan view and a side section view of a 
fourth embodiment of a pickup according to the present invention where the 
pressure transducers 2 and 3 lie in a common plane. The pressure 
tranducers 2 and 3 respectively support string contacting elements 26 and 
27 which are abutted respectively on retaining walls 30 and 31 created by 
a depression 33 practiced in the base 4. The depression 33 acts as a 
holding cavity which prevents the string 1 from applying shear forces 
which would tend to displace the two halves of the pickup. Pressure 
transducers 2 and 3 rest on the supporting face 32 of an insulator element 
34 which rests on the bottom face 35 of an electrically conductive base 4. 
String contacting elements 26 and 27 are also electrically conductive and 
they provide an electrical connection between the base 4 and one electrode 
of each transducer element. This simplifies the construction of the pickup 
and thus helps reduce its cost. The friction between the retaining walls 
30 and 31 and the string contacting elements 26 and 27 is preferably low 
to prevent the string contacting elements 26 and 27 from functioning as 
cantilevers, which may alter both the planar response and the magnitude of 
the electrical signal produced by the pickup. For this purpose, 
lubricating means or other friction reducing means may be used between the 
string contacting elements 26 and 27 and the walls 30 and 31. These 
friction reduction means may possibly be electrically conductive to ensure 
electrical continuity between the string contacting elements 26 and 27 and 
the walls 30 and 31, if desired. 
In all of the embodiments described herein, an electrically conductive 
string may serve as the ground lead for both string contacting elements 
simultaneously. This reduces the mass of the pickup by eliminating 
conductors normally connected thereto and makes the pickup more economical 
to manufacture. 
Whenever it was possible in this description, the conductive means to 
connect the pressure transducers to utilisation means have been omitted 
for clarity. 
While the present invention has been described with respect to the 
preferred embodiments in accordance therewith, it will be apparent to 
those skilled in the art that various modifications and improvements may 
be made without departing from the scope and spirit of the invention. For 
example, although the pressure transducers are herein described and 
illustrated in FIG. 2 as piezoelectric elements, other pressure sensitive 
devices such as piezoresistive elements, capacitive elements, pressure 
sensitive semiconductors or a combination thereof may be used in pickups 
of the present invention. Accordingly, it is to be understood that the 
invention is not to be limited by the specific illustrative embodiments, 
but only by the scope of the appended claims.