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Claims  |
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What is claimed is:
1. A sustaining device for prolonging the vibration of a string of a
stringed musical instrument having magnetic pickup means responsive to a
change in the magnetic field caused by vibration of the string, the
sustaining device comprising
a magnetic string driver means in magnetic proximity to the pickup means,
an amplifier means coupled between the pickup means and the driver means
for amplifying current from the pickup means to the driver means to impart
sufficient magnetic drive energy to the driver means to produce sustained
vibration of the string, and
unbalancing means for creating a magnetic imbalance between the pickup
means and the driver means to minimize direct magnetic feedback between
the pickup means and the driver means.
2. The sustaining device of claim 1 wherein said pickup means includes a
first pickup coil and a second pickup coil, the second coil being placed
closer to the driver means than the first coil.
3. The sustaining device of claim 2 wherein each of the first and second
pickup coils includes a core portion having a cross-sectional area, and
the unbalancing means comprises the cross-sectional area of the first
pickup coil being greater than the cross-sectional area of the second
pickup coil.
4. The sustaining device of claim 3 wherein the unbalancing means
comprises, the material from which the core portions of the first and
second pickup coils are made, and the spacing between the pickup means and
the driver means being selected so that magnetic field sensed by each of
the first and second pickup coils, from the magnetic drive energy given
off by the driver means, is approximately equal in intensity and opposite
in polarity.
5. The sustaining device of claim 2 wherein each of the first and second
pickup coils includes a core portion comprised of a magnetically permeable
material, and the unbalancing means comprises the first pickup coil core
portion being comprised of a material having greater magnetic permeability
that the material from which the second pickup coil core portion is
comprised.
6. The sustaining device of claim 2 wherein the first pickup coil includes
a core portion and electrical conductor wrapped around the core portion,
and the second pickup coil includes a core portion and an electrical
conductor wrapped around the core portion, and the unbalancing means
comprises the electrical conductor being wrapped a greater number of times
around the first pickup coil core portion than the electrical conductor is
wrapped around the second pickup coil core portion.
7. The sustaining device of claim 2 wherein the unbalancing means comprises
an adder means coupled between the pickup means and the transducer means
for receiving the output of the first and second pickup coils, the adder
means being designed to have unequal input gains, the gain from the first
pickup coil being greater than the gain from the second pickup coil.
8. The sustaining device of claim 2 wherein the unbalancing means comprises
an adder means coupled between the pickup means and the transducer means
for receiving the output of the first and second pickup coils, and
an attenuator means coupled between the second coil and the adder means.
9. The sustaining device of claim 2 wherein the first and second pickup
coils are coupled in series, and wherein the unbalancing means comprises
an adder means coupled between the first and second pickup coils and the
driver means, the adder means including a first input and a second input,
the first input being coupled to the output from the first pickup coil,
and the second input being coupled to the combined outputs of the first
pickup coil and the second pickup coil.
10. The sustaining device of claim 2 wherein the first and second pickup
coils are coupled in series, and wherein the unbalancing means comprises
an adder means coupled between the first and second pickup coils and the
driver means, the adder means including a first input and a second input,
the first input being coupled to the combined outputs of the first pickup
coil and the second pickup coil, the second input being coupled to the
output from the second pickup coil.
11. The sustaining device of claim 2 wherein the first and second pickup
coils are coupled in series and wherein the unbalancing means comprises an
adder means coupled between the first and second pickup coils and the
magnetic string driver means, for combining the driver means magnetic
drive energy picked up by the first and second pickup coils, and
minimizing the driver means magnetic drive energy transmitted by the
pickup means to the driver means, when
A.sub.1 (V.sub.F1 -V.sub.F2) approximates A.sub.2 (V.sub.F2)
wherein
A.sub.1 =the gain of the adder means with respect to the difference voltage
V.sub.F1 -V.sub.F2.
A.sub.2 =the gain of the adder means with respect to the voltage of
V.sub.F2,
V.sub.F1 =the voltage produced by the first pickup coil in response to
changes in the magnetic field of the magnetic string driver means, and
V.sub.F2 =the voltage produced by second pickup coil in response to changes
in the magnetic field of the magnetic string driver means.
12. The sustaining device of claim 2 wherein the first and second pickup
coils are coupled in series and wherein the unbalancing means comprises an
adder means coupled between the first and second pickup coils and the
magnetic string driver means, for combining the driver means magnetic
drive energy picked by the first and second pickup coils, and minimizing
the driver means magnetic drive energy transmitted from the pickup means
to the driver means when
A.sub.1 (V.sub.F2 -V.sub.F1) approximates A.sub.2 (V.sub.F1),
wherein
A.sub.1 =the gain of the adder means with respect to the difference voltage
V.sub.F1 -V.sub.F2,
A.sub.2 =the gain of the adder means with respect to the voltage of
V.sub.F1,
V.sub.F1 =the voltage produced by the first pickup coil in response to
changes in the magnetic field of the magnetic string driver means, and
V.sub.F2 =the voltage produced by second pickup coil in response to changes
in the magnetic field of the magnetic string driver means.
13. The sustaining device of claim 2 wherein the first and second pickup
coils are coupled in series, and the unbalancing means comprises an
attenuator means coupled between the second pickup coil and the first
pickup coil.
14. The sustaining device of claim 13 wherein the attenuator means
comprises a resistor means for decreasing magnetic string driver means
magnetic drive energy picked up by the second pickup coil and transmitted
to the amplifier means.
15. The sustaining device of claim 2 wherein the stringed musical
instrument includes a plurality of generally parallel strings disposed in
a plane, and the unbalancing means comprises a magnetic shunt means
disposed between the second pickup coil and the transducer means, below
the plane in which the strings are disposed.
16. The sustaining device of claim 14 wherein the magnetic shunt means
comprises a generally planar metal plate disposed in a plane generally
parallel to a plane in which the second pickup coil is disposed.
17. The sustaining device of claim 2 wherein said magnetic string driver
means includes a first driver coil and a second driver coil.
18. The sustaining device of claim 1 wherein said magnetic string driver
means includes a first driver coil and a second driver coil, the first
driver coil being placed closer to the pickup means than the second driver
coil.
19. The sustaining device of claim 18 wherein each of the first and second
driver coils includes a core portion having a cross-sectional area, and
the unbalancing means comprises the cross-sectional area of the first
driver coil being less than the cross-sectional area of the second driver
coil.
20. The sustaining device of claim 18 wherein the unbalancing means
comprises, the material from which the core portions of the first and
second transducer coils are made, and the spacing between the pickup means
and the driver means being selected so that magnetic field sensed by the
pickup means from the magnetic drive energy given off by each of the first
and second driver coils, is approximately equal in intensity and opposite
in polarity in the proximity of the pickup means.
21. The sustaining device of claim 18 wherein each of the first and second
driver coils includes a core portion comprised of a magnetically permeable
material, and the unbalancing means comprises the first driver coil core
portion being comprised of a material having less magnetic permeability
than the material from which the second driver coil core portion is
comprised.
22. The sustaining device of claim 18 wherein the first and second driver
coils are coupled in series, the first driver coil includes a core portion
and electrical conductor wrapped around the core portion, and the second
driver coil includes a core portion and an electrical conductor wrapped
around the core portion, and the unbalancing means comprises the
electrical conductor being wrapped a greater number of times around the
second driver coil core portion than the electrical conductor is wrapped
around the first driver coil core portion.
23. The sustaining device of claim 18 wherein the first and second driver
coils are coupled in parallel, the first driver coil includes a core
portion, and the second driver coil includes a core portion and an
electrical conductor wrapped around the core portion, and the unbalancing
means comprises the electrical conductor being wrapped a greater number of
times around the first driver coil core portion than the electrical
conductor is wrapped around the second driver coil core portion.
24. The sustaining device of claim 18 wherein the amplifier means comprises
a first amplifier coupled between the pickup means and the first driver
coil, and a second amplifier coupled between the pickup means and the
second driver coil, and
the unbalancing means comprises the second amplifier having a higher gain
than the first amplifier.
25. The sustaining device of claim 18 wherein, the first and second driver
coils are coupled in series, and the amplifier means comprises a first
amplifier coupled between the pickup means and the first driver coil, and
a second amplifier coupled between the pickup means and the second driver
coil, and
the unbalancing means comprises the second amplifier having a gain greater
than twice the gain of the first amplifier.
26. The sustaining device of claim 18 wherein
the first and second driver coils are coupled in series, and
the amplifier means comprises a first amplifier coupled between the pickup
means and the first driver coil, and a second amplifier coupled between
the pickup means and the second driver coil, and
the unbalancing means comprising the first amplifier having a gain less
than twice the gain of the second amplifier.
27. The sustaining device of claim 18 wherein the first and second driver
coils are coupled in parallel, and the unbalancing means comprises an
attenuator coupled between the pickup means and the first driver coil.
28. The sustaining device of claim 18 wherein the first and second driver
coils are coupled in series, and the unbalancing means comprises an
attenuator means coupled to the first driver coil.
29. The sustaining device of claim 18 wherein the stringed musical
instrument includes a plurality of generally parallel strings disposed in
a plane, and the unbalancing means comprises a magnetic shunt means of
magnetically permeable material disposed between the first driver coil and
the pickup means, below the plane in which the stings are disposed.
30. The sustaining device of claim 17 wherein the magnetic shunt means
comprises a generally planar steel plate disposed in a plane generally
parallel to the plane in which the first driver coil is disposed.
31. The sustaining device of claim 1 wherein the stringed musical
instruments includes a plurality of generally parallel strings disposed in
a plane, and the unbalancing means comprises a magnetic shunt means
disposed between the pickup means and the transducer means, below the
plane in which the strings are disposed.
32. The sustaining device of claim 30 wherein the magnetic shunt means
comprises a generally planar metal plate having a longer dimension and a
shorter dimension, the longer dimension extending generally perpendicular
to the strings. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to an electronic device for use in connection
with a musical instrument, and more particularly to an electronic device
for sustaining the vibration of a string of a stringed musical instrument.
It has long been known that an amplifier could be coupled to a stringed
musical instrument to amplify the sound produced by the vibration of the
strings of the instrument. Probably the most popular example of such an
electrically amplified stringed musical instruments is an electric guitar.
An electric guitar typically includes a plurality of strings that extend
between the head of the guitar and the body of the guitar, with a fretted
neck interposed between the head and body of the guitar.
In an electric guitar, one or more magnetic pickups are placed on the body
of the guitar in magnetic proximity to the strings of the guitar. The
magnetic pickups are responsive to the change in magnetic flux caused by
the vibration of the strings. This magnetic energy picked up by the pickup
is then transmitted to a separate amplifier and speaker.
It has long been known that a pickup and external amplifier arrangement on
an electric guitar can not only adjust the volume of the sound produced by
the guitar, but can also be used by the musician to alter the nature of
the sound produced by the guitar. One means for altering this sound is to
introduce vibrational feedback into the system to prolong the vibration of
the strings of the guitar.
An early method for producing such sustained vibration was for the musician
to move the musical instrument in close proximity to the speaker of the
amplifier through which the guitar was being amplified. In such a
situation, the acoustic energy caused by the sound waves emanating from
the speaker of the amplifier would establish a sympathetic vibration of
the strings. The vibration of the strings induced by the speaker would
then be translated into magnetic flux energy picked up by the pickup
means. This magnetic flux energy would then be transmitted through the
external amplifier, through the separate amplifier, and would be
transformed into sound energy through the speaker of the amplifier.
Typically, this situation would result in a "feedback" loop which
sustained the vibration of the strings of a musical instrument, and hence
the duration of the sound produced by the plucking of the string.
One difficulty however with this method of introducing feedback is that it
is often difficult to control the amount and type of feedback produced.
Hence, it is difficult to control the sound produced through the use of
this feedback system. Several devices have been invented, to overcome the
problems discussed with the above method of sustaining string vibration.
A typical, prior art sustain device 8 is shown in FIG. 1 as including a
magnetic pickup 10, a magnetic driver 12, and an amplifier 14 interposed
in a circuit between the pickup 10 and driver 12. The pickup is typically
comprised of one or more pickup coils, such as pickup coil 11. The driver
12 is typically comprised of one or more of the driver coils, such as
driver coil 13.
The sustain system 8 may be used to sustain the vibration of a single
string, such as string 16, or a plurality of strings, such as the 4, 6, or
12 strings typically found on an electric guitar. The sustain system is
usually disposed on a counter-sunk portion of the upper surface of the
body of the electric guitar, so that the pickup 10 and driver 12 are in
magnetic proximity to the string 16 of the instrument.
The pickup 10 and driver 12 are constructed generally similarly. Both the
pickup 10 and driver 12 are constructed of a number of turns of a
conductor means, such as a wire 18, 20 which is wound around a magnetic
core 22, 24, respectively. The cores 22, 24 are generally either a
permanent magnet, or a ferrous material in contact with a permanent
magnet, to provide a permanent magnetic flux through the center of the
respective pickup coil 11 and driver coil 13.
For the purposes of this discussion as to the manner in which such a
sustain system works, the pickup coil 11 and driver coil 13 are modeled as
ideal inductors, L.sub.P and L.sub.D, respectively, having .sub.P, and
N.sub.D, respectively, turns of wire in series with a resistive element,
such as resistor R.sub.P 26 and resistor R.sub.D 28, respectively. The
amplifier 14 is modeled as having infinite input impedance, zero output
impedance, and a voltage gain of A. The string 16 is assumed to be under
tension, free to vibrate, and secured at both ends.
A condition exists in all prior sustain systems using a magnetic pickup and
driver in conjunction with an amplifier to sustain string vibration. When
the gain of the amplifier 14 is of a sufficiently high level to achieve
sustain of the string 16, a portion of the driver's 12 magnetic field F is
present at the pickup 10. This magnetic field induces the pickup 10 to
create a voltage. The pickup voltage is amplified and regenerated by the
driver 16, which then is picked up by the pickup 10, to induce the pickup
10 to create a greater voltage.
When the amplifier gain is increased to the point wherein the magnetic loop
gain is greater than or equal to unity, and the loop's phase angle is zero
degrees, 360 degrees, 720 degrees, or some whole multiple of 360 degrees,
the classical nyquist condition will be met, and the system will
oscillate. Since the frequency of oscillation is generally determined by
the self-resonant frequency of the pickup, the driver, and other phase and
amplitude characteristics of the amplifier, the oscillation frequency has
no musical relationship to the string vibration frequency. Oscillation is
therefore undesirable.
A second problem associated with direct magnetic feedback between the
driver and pickup is the contamination of the pickup signal with noise and
distortion produced by the amplifier means. The presence of amplifier
noise and distortion in the pickup signal produces an unnatural tone when
the pickup is used in conjunction with a loudspeaker to monitor the tone
produced by the vibrating string.
One common solution to the direct magnetic feedback problem is to decrease
amplifier gain. However, this decrease in amplifier gain also reduces the
ability of the system to pick up and sustain slight string vibrations.
Additionally, the amount of time required for the system to reach a steady
state sustain condition (where the maximum string vibration amplitude is
limited by the maximum dynamic range of the system) is lengthened.
A second, prior art solution to the problems of direct magnetic feedback is
to spatially separate the pickup and driver by a greater distance. One
example of a device which reduces direct magnetic feedback by such a
spatial separation is the SUSTAINIAC Model B sustain system, manufactured
by Maniac Music, Inc. of Indianapolis Ind., which is described in the
applicants' U.S. patent application Ser. No. 06/937,871, filed on Dec. 4,
1986.
In the SUSTAINIAC sustain device, the magnetic driver is a magnetic
vibrational transducer which attaches to the head, stock or body of the
musical instrument to provide an acoustic vibrational feedback to the
string through the string supports. Although this system performs its
function well, room for improvement exists. Particularly, room for
improvement exists in the area of providing a more predictable phase
relationship between the transducer drive current and the string
vibration, as the SUSTAINIAC sustain system transducer must act on the
string through the complex acoustic time delays and phase anomalies of the
musical instrument's body resonance.
Another variation on this second solution is to place the pickup and driver
at opposite ends of the strings. One difficulty with this method however
is that it precludes the use of frets on a musical instrument. Thus,
although this second method would adapt well to a piano, it adapts poorly
to a guitar.
A third method of overcoming direct magnetic feedback is to eliminate one
or both of the magnetic components. For example, the magnetic pickup may
be replaced with a piezoelectric device, or a strain gauge which can sense
string vibration while being insensitive to the driver's magnetic field.
A fourth method of overcoming the problem of direct magnetic feedback is to
provide the pickup and driver with a very small air gap between the
magnetic poles. The commercially available E-bow sustain system,
manufactured by Gregory A. Heet of Los Angeles, Calif., and described in
U.S. Pat. No. 4,075,921, embodies this type of approach. One difficulty
with this approach is that the strings must be in very close proximity to
the pickup and driver, and the string vibrational excursion must be
minimized to avoid direct contact between the strings and the pickup and
driver.
A fifth, prior art method for overcoming the problems caused by direct
magnetic feedback is to provide the pickup with a humbucking apparatus to
cancel the effects of uniform external magnetic fields. Such a humbucking
apparatus is described by Cohen in U.S. Pat. No. 3,742,113. Cohen
describes the humbucking apparatus as a "differential pickup of the type
well known in the state of the art" constituted by two coils wherein "both
coils respond to magnetic fields identically." One difficulty with such an
approach however, is that the humbucking pickup does not provide optimum
rejection of the non-uniform magnetic field generated by the driver due to
the balanced design of the pickup. As will be appreciated, the driver's
magnetic field is non-uniform in close proximity to the driver due to the
inverse square law of magnetic field intensity. This law provides that as
distance from the driver is increased, the magnetic field becomes more
uniform. It will be noticed that Cohen provides a shield, consisting of
layers of high and low permeability material around the perimeter of the
humbucking pickup, to lessen the effects of direct magnetic feedback. The
perimeter shield does not affect the magnetic balance of the humbucking
pickup due to the shield's symmetry.
A variation of this fifth method for overcoming the problems of direct
magnetic feedback is to provide the driver with a humbucking apparatus to
allow far field cancellation of the driver's generated magnetic field.
This is obvious since the driver is the electrical "dual" of the
humbucking pickup described in U.S. Pat. No. 3,742,113. One problem with
this approach, however is that the humbucking driver does not provide an
optimally cancelled magnetic field in the proximity of the pickup, due to
the balanced design of a humbucking driver.
A sixth prior art method for overcoming the problems caused by direct
magnetic feedback is to provide a magnetic shield to encase the pickup.
Such a shield is described in Holland's U.S. Pat. No. 4,236,433. One
difficulty with this method, however, is that it encases a portion of the
string and the encased string portion may not be plucked or struck.
A seventh prior art method for overcoming the problems caused by direct
magnetic feedback is to provide a device wherein the pickup is located
between identical drivers wired electrically out of phase. Such a device
is shown in Cohen's U.S. Pat. No. 3,742,113. One difficulty with this
device, however, is that it requires the drivers to be placed in "shields
of magnetic ingot iron" to minimize direct magnetic feedback. A second
difficulty with this device is that the driver cores must be "provided
with a concave figure to focus or concentrate the flux generated on a
string."
Although the above described attempts to solve the problem of direct
magnetic feedback all perform their intended function, to one extent or
another, room for improvement still exists.
Thus, it is one object of the present invention to provide a sustain device
which maximizes the ability to sustain the vibration of a string, while
minimizing the effects of direct magnetic feedback associated therewith.
SUMMARY OF THE INVENTION
In accordance with the present invention, a sustaining device is provided
for prolonging the vibration of a string of a stringed musical instrument
having magnetic pickup means responsive to a change in a magnetic field
caused by vibration of the string. The sustaining device comprises a
magnetic string driver means in magnetic proximity to the pickup means. An
amplifier means is coupled between the pickup means and the driver means
for amplifying current from the pickup means to the driver means to impart
sufficient magnetic driver energy to the driver means to produce sustained
vibration of the string. An unbalancing means is provided for creating a
magnetic imbalance between the pickup means and the driver means to
minimize direct magnetic feedback between the pickup means and the driver
means.
In one aspect of the present invention, a dual coil magnetic pickup is
unbalanced to create a cancellation effect of the electrical impulses
resulting from the pulsating magnetic field radiated by the magnetic
driver. The pickup's output may thus be amplified and delivered to the
driver to regenerate and sustain vibration of the string while the pickup
remains relatively insensitive to the driver's magnetic field.
In a second aspect of the present invention, a dual coil magnetic driver is
unbalanced to produce a cancellation of its emitted magnetic field in the
proximity of the magnetic pickup. The pickup's output may thus be
amplified and delivered to the driver to regenerate and sustain vibration
of the string while the driver's resultant magnetic field produces
relatively little effect on the pickup.
A variety of methods are disclosed to produce this unbalancing effect in
the pickup, the driver, or both.
In another embodiment of the invention, a shunt plate is used to
magnetically reduce direct magnetic feedback. In a fourth aspect of the
invention, two or more pickups are combined to be relatively insensitive
to direct magnetic feedback produced by a single driver being used to
regenerate and sustain string vibration. In a fifth aspect of the
invention, two or more drivers are used to regenerate and sustain string
vibration by being combined to produce relatively little direct magnetic
feedback to a single pickup.
These and other aspects of the present invention will become apparent to
those skilled in the art upon consideration of the following detailed
descriptions of the preferred embodiments exemplifying the best mode of
carrying out the invention as perceived presently.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 A schematic view of a prior art sustained circuit;
FIG. 1a A schematic view of another embodiment of a prior art sustain
circuit;
FIG. 2 A schematic view of the sustain circuit of the present invention;
FIG. 3 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 4 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 5 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 6 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 7 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 8 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 9 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 10 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 11 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 12 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 13 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 14 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 15 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 16 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 17 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 18 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 19 A schematic view of a circuit of another embodiment of the present
invention;
FIG. 20 A schematic view of a circuit of another embodiment of the present
invention; and
FIG. 21 A schematic view of a circuit of another embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
A description of the workings of a typical sustain device will be described
with reference to FIG. 1. As discussed above, FIG. 1 represents a prior
art sustain device.
A downward motion of string 16 causes an increase in the magnetic flux in
the core 30 of the pickup 10. This increase in the magnetic flux is
converted to voltage V.sub.S by Faraday's Law, V=N (DP/DT) where DP/DT is
the change in magnetic flux through the coil 11 with respect to time, and
N.sub.P is the number of turns of wire 18 around the core 30 of the
pickup. Amplifier 14 produces current I.sub.S which flows through the
driver coil 13 producing an increase in the driver's 12 magnetic field F
which further attracts the string 16, thereby reinforcing the downward
motion of the string 16.
During the next half cycle of the string's natural harmonic motion, the
upward motion of the string 16 causes a magnetic flux decrease in the
pickup producing voltage -V.sub.S which is opposite in polarity to voltage
V.sub.S. Amplifier 14 produces current -I.sub.S which lessens the driver's
12 magnetic field, thereby causing the string 16 to be attracted with a
lesser force. This reinforces the string's 16 upward motion.
The natural harmonic motion of the string 16 is regenerated and sustained
by positive feedback. However, there is an additional side effect.
A portion of the magnetic field F of the driver 12 passes through the core
22 of the pickup 10. Thus, an increase in the magnetic field of the driver
12 will be converted to voltage V.sub.F by the pickup 10. Amplifier 14
then produces current I.sub.F which further increases the driver's 12
magnetic field F. This positive magnetic feedback condition causes system
instability. In practice, this system instability frequently leads to an
uncontrolled oscillation whose frequency is typically musically unrelated
to the frequency of the vibrating string 16. The uncontrolled oscillation
is therefor undesirable.
A schematic view of another embodiment of a sustain device is shown in FIG.
1a which illustrates how positive string vibration feedback and negative
direct magnetic feedback are established by reversing the coil and
magnetic pole of the pickup.
Referring back to FIG. 1, it will be noticed that the pickup coil 11 and
driver coil 13 are arranged so that the south pole S of core 22 of pickup
coil 11 is disposed adjacent to the north pole N of core 24 of driver coil
13.
FIG. 1a shows a circuit 38 mounted adjacent to a string 16 of a stringed
musical instrument (not shown). The circuit 38 includes a pickup coil 40
having a core portion 42, having a north pole end and a south pole end
designated as N and S, respectively. A wire 44 is wrapped around the core
portion 42, a specified number of "turns". The circuit also includes a
driver coil 46 having a core portion 48. Core portion 48 includes a north
pole end and a south pole end, designated as N and S, respectively. A wire
50 is wrapped a predetermined amount of turns around the core portion 48
of the driver coil 46. It will be noticed that, unlike the circuit 8 shown
in FIG. 1, the circuit 38 shown in FIG. 1a is constructed so that the
respective North poles N of both the pickup coil 40 and the driver coil 46
are disposed adjacent to each other, with the South poles S of both the
pickup coil 40 and driver coil 46 also being disposed adjacent to each
other.
In operation of the embodiment shown in FIG. 1a, the string's natural
harmonic motion is sustained. A downward motion of the string creates
voltage V.sub.S. Amplifier 52 produces current I.sub.S which flows through
the driver coil 46 producing an increase in the driver's magnetic field,
which further attracts the string 16. Upward motion of the string 16
produces voltage -V.sub.S, and amplifier 52 produces current -I.sub.S
which lessens the driver's magnetic flux. This causes the string 16 to be
attracted with a lesser force. In circuit 38, magnetic feedback is
suppressed. An increase in the magnetic field F of the driver 46 will be
converted to voltage V.sub.F by the pickup. Amplifier 52 then produces
current I.sub.F which lessens the driver's magnetic field F, thereby
reducing voltage V.sub.F and so on.
The applicants have learned that several general rules exist relating to
string vibration feedback sustain systems, and the manner to exploit such
systems to reduce problems caused by oscillation.
The first general rule is that, for a positive string vibration feedback
sustain system having a pickup and a driver with one or more coils,
negative feedback is established when the closest pickup and driver coils
are of like magnetic polarity, such as is illustrated in FIG. 1a.
The second general rule is that, for a positive string vibration feedback
sustain system having a pickup and a driver with one or more coils,
positive magnetic feedback is established when the closest pickup and
driver coils are of opposite magnetic polarity, such as is illustrated in
FIG. 1. By shifting or inverting the phase response of the sustain system,
string harmonics can be selectively sustained. (A detailed discussion of
the use of negative string vibration feedback to selectively sustain
string harmonics is provided in U.S. patent application No. 06/937,871,
discussed above.)
Since the phase shift affects the resultant magnetic feedback signal as
well as the string feedback signal, two or mor | | |
