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Description  |
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FIELD OF THE INVENTION
The present invention relates to transducers for converting the vibration
of the strings of a musical instrument into electrical signals, and more
particularly to bridge pickups having a saddle directly contacting the
strings.
A number of pickup devices incorporated in an instrument bridge are known
in the art. Among these, U.S. Pat. No. 4,189,969 issued Feb. 26, 1980, for
an invention of S. Katayama et al., shows a pickup assembly with
individually potted piezoelectric sensor elements having T-slots for
receiving interchangeable crest elements for contacting the strings. U.S.
Pat. No. 4,314,495 issued Feb. 9, 1982, for an invention of L. Baggs shows
an elongated shielded piezoelectric crystal structure mounting within a
saddle member U.S. Pat. No. 3,712,951 issued Jan. 23, 1973, for an
invention of J. Rickard shows a bridge pickup with a separate movable
saddle element for each string. Various other structures are shown in U.S.
Pat. Nos. 3,154,701 of Evans; 4,252,990 of Sado; 4,278,000 of Saito et
al.; 4,290,331 of Izdebski; 4,378,721 of Kaneko et al.; 4,380,357 of Evans
et al.; and 4,160,401 of Tomioka.
As a rule, a central design problem of any pickup is that of obtaining both
a faithful signal and a good signal to noise ratio. In a piezoelectric
pickup this problem gains special dimensions because the pickup is
"contact" pickup and the signal is generated by the direct action of
compression waves transmitted through the piezo crystal element(s) via a
coupling structure such as the saddle element of a bridge. Both the
physical geometry and the mechanical/acoustic properties of the coupling
structure thus become quite important. Ideally the transfer of energy from
the string through the coupling structure to the sensor elements should
not color the sound, and should be relatively efficient independently of
the variations in construction or adjustment of the different strings
which may be used on the instrument.
SUMMARY OF THE INVENTION
The present invention provides a novel structure for a bridge pickup for a
string instrument in which a sensor assembly mounts in a base member. The
sensor assembly comprises a cast member encapsulating a holding means
holding a plurality of sensor elements. The cast member has an upper crest
portion for receiving the vibrations of the instrument strings and
transmitting them to the sensor elements, and extends to a lower portion
below the sensor elements. The lower portion has a groove extending
therethrough to an electrode contact surface of the sensor elements.
Conductive means extending through the groove provides an output from the
sensor assembly. Preferably the cast member is formed of a curable polymer
having a cured hardness in the range of 80-85 on the Shore D scale. In a
preferred embodiment, the holding means engages the upper surface of the
sensor elements and defines an air gap between that surface and the crest
portion. In a further preferred embodiment, the conductive means includes
a PC board held in registry with the the sensor elements between the cast
member and the base, and also includes a compressible unidirectionally
conductive medium, disposed in the groove and connecting the sensor
elements to the PC board. A method of assembling the pickup is shown.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will be more fully appreciated by
reference to the drawings, in which:
FIG. 1 shows an exploded view of a pickup according to the present
invention;
FIG. 2 shows a cross section of the pickup;
FIG. 3 shows an enlarged cross section of a preferred embodiment of the
sensor assembly;
FIG. 4 shows a perspective view of an alternative embodiment; and
FIG. 4A shows a top view of another alternative embodiment of the pickup.
DETAILED DESCRIPTION
FIG. 1 shows an exploded view of the pickup 1 of the present invention,
having a base 2, a sensor assembly 3 and conductive means 4. Base 2
comprises a U-shaped conductive channel which serves as a ground and a
partial shield for the pickup. The channel may be, for instance, an
aluminum extrusion. The sensor assembly 3 includes a cast polymer member
14 and a plurality of piezoelectric sensor elements 9 held in an array by
frame 12 encapsulated within the member. Sensor assembly 3 and a mating PC
board 18 fit within the U-shaped conductive channel 2. A ribbon cable 41
exits from an end of the base to carry the pickup signal to an external
device.
During manufacture of the preferred embodiment, member 14 is cast around
the array of sensors, with a groove 17 formed in the underside of the cast
member and extending to a lower contact face (11, FIG. 3) of the sensor
elements. A compressible conductor (19, FIGS. 2 and 3) is placed in the
groove 17. The sensor assembly 3 and the conductive means 4 are then
cemented in the channel 2, resulting in a pickup unit 1 that is closed on
all sides, having a high degree of protection from atmospheric degradation
of the enclosed sensor elements. This method of assembly involves no
injection molding or other pressurized manufacturing steps which might
introduce irregular stresses or unanticipated displacements of the sensors
during manufacture. It thus permits the consistent fault-free manufacture
of pickups having relatively uniform output and response characteristics.
As shown in FIG. 1, the sensors 9 are held in a generally longitudinal
spaced configuration by a pair of rails 12, which may be, for example,
L-shaped members formed of an ABS plastic and cemented to the sensor
elements. A common conductor 13 interconnects a face of each sensor. The
precise number of sensors and their spacing will correspond to the number
and spacing of the strings of the instrument for which the pickup is
intended. The pickup shown in FIG. 1 has 6 sensors arranged for a 6-string
guitar, or for a 12-string guitar with six similarly spaced pairs of
strings. For assembly, the array consisting of the sensors 9 and rails 12
is placed in a mold having a mold surface in the shape of the top of an
instrument bridge. A curable casting polymer is poured into the mold so as
to form cast member 14 with the array encapsulated therein. Cast member 14
is formed with a groove 17 therein extending to the electrode surfaces of
the embedded sensor elements. When cured, member 14 together with the
embedded array, which collectively form the sensor assembly 3, is removed
from the mold and its lower surface is filed or milled flat. A conductive
means accesses the sensor elements through the groove. Preferably the
conductive means includes a monodirectionally conductive strip 19, which
may be of the type formed of a compressible elastomer matrix having thin
conductive wires extending transversely therethrough from the top to the
bottom surface thereof, and also includes a PC board 18, in registry with
the sensor elements. As shown in FIG. 1, PC board 18 is a flexible PC
board, integrally formed with ribbon cable 41 which provides an output.
Board 18 includes conductive leads or paths 24 each terminating in a
conductive land 25 underlying a respective one of the sensors. A further
land 26 is provided for connection to the upper conductor 13 which serves
as a common or ground lead for the sensors.
The pickup unit is assembled by placing conductive strip 19 in groove 17,
and then placing the sensor assembly 3 over the PC board in channel 2. In
this manner a precisely aligned array of sensors is sealed in a
stress-free mounting to form the pickup. Preferably the sensor assembly 3
is cemented in the channel. An end cap 27 is then placed at each end of
the assembly and sealed thereto.
The completed pickup has a ridge 5 along its top surface upon which the
strings of the musical instrument rest. In use, the vibrational energy of
each string is transmitted from its point of contact with ridge 5, through
the upper or crest portion of the sensor assembly, to the sensor elements
within, which are located centrally below the strings. The present
invention provides a novel structure for supporting such sensor elements
within a saddle member in a manner to provide both a reasonable degree of
acoustic isolation from the instrument body and a good coupling to the
overlying strings, as will now be discussed in relation to FIG. 2.
FIG. 2 shows a cross section through the pickup 1 along the plane II--II of
FIG. 1, and passing through a piezo element. As shown, a piezo sensor
element 9 having upper electrode contact surface 10 and lower electrode
contact surface 11 is held in registration, with respect to a plurality of
other such sensor elements, by a holding means or frame 12. Frame 12 is
shown as a pair of I-shaped smembers orienting the sensors along a common
planar strip, but may alternatively comprise a molded ladder-shaped
structure having spaced apertures formed therein for receiving the sensor
elements and holding them at predetermined spacings or heights
corresponding to the positions of the strings of the instrument on which
the pickup is to be used. A conductor 13, which may be a wire, as shown,
or a conductive strip, electrically interconnects the upper electrode
surface of the sensor element with the other sensor elements of the
pickup. Finally cast member 14, having a crest portion 15 generally
overlying the piezo elements and extending down to a portion 16 below the
piezo elements, encapsulates the frame and piezo elements. Member 14
includes opposing side walls and is preferably formed by inverting the
frame/sensor element assembly in a jig and then casting member 14 in a
mold as a block around the frame. A removable strip is placed on the
sensors before pouring the member, so as to form, when removed, a groove
17 in the molded block, exposing the lower electrode contact surfaces 11
of the piezo elements. Cast member 14 together with the sensor array thus
forms a unitary sensor assembly 3.
As noted above, the upper region or crest portion 15 of the cast member has
an apex or ridge 5 along is upper surface serving as a bridge to transmit
the vibrations of the strings to the piezo elements. It will be observed
that frame 12 protrudes outwardly from the edges of the piezo elements 9,
so that crest portion 15 is quite thin at the sides thereof where it joins
to the side walls of the cast member. Moreover the crest portion
approaches a direct contact with the piezo element only in the central
region of the top electrode contact surface. This structure has been found
to result in good tonal pickup qualities, substantially free of adverse
tone coloration from the internal acoustics of cast member 14. FIG. 3
shows a cross section of a portion of a further preferred embodiment of
the invention, in which an air gap 22 is provided between the crest
portion and the piezo element to further enhance tonal quality.
FIG. 3 shows a section corresponding to FIG. 2 but of a preferred
embodiment of the cast member and conductor assembly. In this embodiment
piezo element 9, frame 12 and conductor 13 are substantially as shown in
FIG. 2. Upper and lower electrode contact surfaces 10, 11 are preferably
metallized, and conductor 13 is soldered to upper surface 10 by solder 20.
A sheet material or tape 21 is then wrapped around the frame/sensor
elements before pouring the cast member 14, so that when the casting
polymer is poured into the mold, an air gap 22 is created in the upper
central region of the electrode contact surface. The cast member 14 is
then formed with groove 17 therein, and cured, as before, and the tape 21
at the bottom of the groove is cut away to expose the lower electrode
surfaces.
It has been found that by forming air gap 22 between the crest portion and
the piezo element a truer tone is produced by the piezo elements, free of
harshness. In prototype pickups constructed according to this embodiment
of the invention, the air gap 22 has a depth, between the electrode
surface and the lower surface of the crest, in the range of 0.25-1.25 mm,
and extends for a width of 1-2 mm between the opposing rails of frame 12.
The taping produces other air gaps 23, of comparable dimensions, during
the casting of member 14, which serve to isolate the lateral portions of
the sensor element from the cast member, and which may also contribute to
this enhanced tonal quality. As before, connector 19 interconnects the
piezo elements to the PC board 18, providing an output from the pickup.
Also shown in FIG. 3 are conductive lines 24 on the surface of the PC
board, and a wider conductive land 25, located in registry with the
electrodes of piezo element 9, for contacting connector 19. In the
embodiment shown in FIG. 1 there are 7 conductive lines, one ground plus
one line for each of 6 conductive leads required for a 6-string guitar
pickup having 6 piezo elements. In the section of FIG. 3, corresponding to
the section II--II of FIG. 1, there are shown five of these lines 24,
corresponding to the sensors underlying strings 3-6, plus the ground wire.
In this embodiment PC board 18 is a flexible PC board, integrally formed
with a ribbon cable (41,FIG. 1) which exits from one end of the pickup
assembly to connect to a jack or a controller mounted on the guitar. The
base member (not shown) of this embodiment is similar to that of FIG. 2,
and accordingly further details thereof are omitted.
FIG. 4 shows a perspective view of another embodiment of the present
invention. As shown, ridge 5 in this embodiment is stepped, so as to
provide a bridge having a different height for different strings. It is
also possible to form the ridge portion for each string slightly offset
along the axis of the string to adjust the intonation of the instrument,
in a manner known in the art. FIG. 4A shows a top view of an embodiment of
the invention having such a ridge. As shown, ridge 5 includes a plurality
of adjacent ridge segments 5a, 5b . . . 5f each slightly offset along the
string axis.
In regard to the materials used in constructing the pickup, it is noted
that cast member 14 must be sufficiently hard to support the strings of
the instrument without having grooves worn or pressed into the bridge, yet
must not have the sort of brittle hardness which causes harsh internal
acoustic effects, and must to some extent dampen internal accoustic
vibrations emanating from the instrument body. I have found that by
forming cast member 14 from a curable epoxy resin having a cured Shore D
durometer of 75-90, and preferably in the range of 80-85, good results are
obtained. The cast assembly may be cemented to the base using a
cyanoacrylate or similar glue. End caps may be installed in the channel,
and casting imperfections may be filled or sealed with an epoxy, a curable
silicone sealer, or other compound.
It is also noted that the use of a conductive channel member for a base,
grounded to the upper electrode contact surface of the piezo elements,
provides, together with the upper electrode surfaces, a partial Faraday
cage shielding the lower electrodes and their conductors from extraneous
electrical or electromagnetic signals. Such partial shielding is enhanced
by the placement of conductor 19 centrally under the upper electrodes, and
further by the placement of the PC board 18 at the very base of the
channel, adjacent to floor 8. In this manner the preferred construction of
the present invention is substantially shielded from ambient noise.
It will be appreciated that while the invention has been described with
reference to certain preferred embodiments, such description is by way of
illustration, and the invention is not limited thereto. The invention
having been thus described, variations may be made therein by those
skilled in the art, and all such variations shall be within the spirit of
the invention, as limited only by the following claims.
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