High asymmetry variable reluctance pickup system for steel string musical instruments

I claim:

1. In vibrating string musical devices which have a plurality of parallel strings composed of magnetically susceptible materials, said strings being oriented in a common string plane, a variable reluctance pickup for asymmetrically sensing vibrations of strings and generating corresponding electrical signals responsive thereto, comprising, in combination,

a plurality of means each for forming a separate magnetic circuit in combination with a linear segment of one string, each including

an individual means for shaping an individual magnetic field region encompassing one linear segment of one string, said magnetic field region having a magnetic flux gradient in a vertical direction (v) perpendicular to said string plane and perpendicular to said string segment (d.PHI. /dv) for producing large changes of reluctance in said respective magnetic circuit responsive to motion of said linear string segment in said vertical direction, and having a very small magnetic flux gradient in a horizontal direction (h) perpendicular to said string and parallel to said string plane (d.PHI./dh), where (d.PHI./dh)<<(d.PHI./dv) for producing very small changes of reluctance in said respective magnetic circuit responsive to motions of said linear string segment in said horizontal direction, and

means for sensing changes of reluctance in each of said magnetic circuits and producing an electrical signal responsive thereto, said means being adapted for electrical connection, whereby said produced electrical signals can be electronically amplified and converted into corresponding acoustic waves.

2. The variable reluctance pickup of claim 1 wherein said means for providing a separate magnetic circuit in combination with a linear segment of each string further includes

a magnetic element providing a magnetic field having a north and a south side and a corresponding north-south polarity axis, said magnetic element being oriented with its north-south polarity axis aligned parallel to said strings, said magnetic element being disposed proximate said string plane.

3. The variable reluctance pickup of claim 2 further including a pole piece for each string, each of said pole pieces being positioned contiguous with a side of said magnetic element with an end proximate one of said strings, said pole pieces being composed of a magnetically susceptible material.

4. The variable reluctance pickup of claim 3, wherein said individual means for shaping the magnetic field provided by said magnetic element each comprise a planar pole tip face contiguous with the end of one pole piece having a planar face parallel to said string plane, for shaping the magnetic field emanating from the end of said pole piece into a magnetic field region surrounding the linear segment of one string, said magnetic field region having said magnetic flux gradient in said vertical direction (v), (d.PHI./dv) and said very small magnetic flux gradient in said horizontal direction (h), (d.PHI./dh) where (d.PHI./dh)<<(d.PHI./dv), said planar pole tip face being composed of magnetically susceptible material.

5. The variable reluctance pickup of claim 4, wherein said pole pieces for adjacent strings are positioned contiguous opposite north and south sides of said magnetic element.

6. The variable reluctance pickup of claim 5, wherein said ends of said pole pieces have a cross-sectional area in a plane parallel the string plane and said planar poletip faces have an area greater than said cross-sectional area of said ends of said pole pieces.

7. The variable reluctance pickup of claim 6, wherein said planar poletip faces have a minimum linear dimension equal to a distance between adjacent strings, said minimum linear dimension measured in a perpendicular relationship to said linear portions of said strings.

8. The variable reluctance pickup of claim 1 wherein said means for providing a separate magnetic circuit in combination with a linear segment of each string further includes,

a magnetic element for each string having a north and a south end and a corresponding north-south polarity axis, said magnetic element being disposed in a parallel row with their north-south axes aligned in a common direction perpendicular to the string plane, each magnetic element providing a distinct magnetic field region for one string, which magnetic field bucks the magnetic field provided for adjacent strings, and

wherein each of said individual means for shaping the magnetic field, shapes the distinct magnetic field provided by one of said magnetic elements into a distinct magnetic field region encompassing the linear segment of one string said magnetic field region having said magnetic flux gradient in said vertical direction (v), (d .PHI./dv) and having said very small magnetic flux gradient in said horizontal direction (h), (d.PHI. /dh) where (d.PHI. /dh)<<(d.PHI. /dv).

9. The variable reluctance pickup of claim 8, wherein said individual means for shaping the magnetic fields provided by said magnetic elements comprises a plurality of planer pole tip faces each contiguous with an end of one magnetic element, which end is proximate the linear segment of said string, said planer pole tip faces each having a planer face lying in a plane parallel to the string plane, and said pole tip faces being composed of a magnetically-susceptible material.

10. The variable reluctance pickup of claim 9 wherein said planar poletip face has a minimum linear dimension equal to a distance between adjacent strings, said minimum linear dimension measured in a perpendicular relationship to said line segments of said strings whereby, upon a bending movement of any particular vibrating string from a normal quiescent position proximate to and above a first poletip face to a position proximate to and above a second poletip face adjacent to said first poletip face, a continuous resultant electrical signal is generated by said means for sensing changes of reluctance in the corresponding magnetic circuits.

11. The variable reluctance pickup of claim 10, wherein said means for sensing changes of reluctance in each of said magnetic circuits and for producing an electrical signal responsive thereto comprises a plurality of coils formed of insulative conductive wire, each disposed around one of said magnetic elements for generating an electrical signal responsive to a change of reluctance in a particular magnetic circuit, said coils being adapted for connection to suitable means for, amplifying and converting said electrical signals generated therein, into acoustic waves.

12. The variable reluctance pickup of claim 11, further defined in that said magnetic elements, said poletip faces, and said coils are potted in a rigid, insulative material to form a solid body and wherein said solid body is coated with a metallic paint.

13. In vibrating string musical devices which have a plurality of parallel strings composed of magnetically susceptible materials, said strings being oriented in a common string plane, a variable reluctance pickup for asymmetrically sensing vibrations of strings and generating corresponding electrical signals responsive thereto, comprising in combination,

a plurality of separate magnetic elements, said elements being disposed in a parallel row, each of said magnetic elements having a north and a south end and a corresponding north-south polarity axis, the north-south polarity axis of said separate magnetic element being aligned in a common direction parallel with said strings, each of said magnetic elements forming a separate magnetic circuit in combination with a linear segment of one string, whereby the magnetic field provided by each of the magnetic elements bucks the magnetic field provided by the magnetic elements adjacent thereto, and

a plurality of individual means each for shaping the magnetic field provided by one of said magnetic elements to provide a plurality of adjacent magnetic field regions, each magnetic field region encompassing the linear segment of one string and each magnetic field region having a magnetic flux gradient in a vertical direction (v), perpendicular to the string plane and perpendicular to said linear segment of said string, (d.PHI. /dv), and having a very small magnetic flux gradient in a horizontal direction (h), perpendicular to the strings and parallel to the string plane, (d.PHI. /dh), where (d.PHI. /dh)<<(d.PHI. /dv), for producing a maximum change of reluctance in the corresponding magnetic circuits responsive to motion of the linear string segment in said vertical direction, and for producing a very small change of reluctance in the corresponding magnetic circuits responsive to motion of the linear string segments in said horizontal direction, and

means for sensing changes of reluctance in each of said magnetic circuits and producing an electrical signal responsive thereto, said means being adapted for electrical connection, whereby said produced electrical signals can be electronically amplified and then converted into corresponding acoustical waves.

14. The variable reluctance pickup of claim 13 wherein said individual means for shaping the magnetic fields provided by said plurality of separate magnetic elements each comprise, in combination,

a primary pole piece having a longitudinal axis oriented at an angle with respect to said string plane and with respect to said polarity axes of said magnetic elements, said primary pole piece being positioned contiguous with one of said ends of one of said magnetic elements and extending toward said string plane for directing a distinct magnetic field encompassing said linear segment of one string, said primary pole piece being composed of a magnetically susceptible material and

a planer pole tip face contiguous with the end of the primary pole piece proximate the string plane having a planer face parallel to the string plane for shaping the magnetic field emanating from the end of the primary pole piece into a distinct magnetic field region surrounding the linear segment of one string providing said magnetic field region with said magnetic flux gradient in said vertical direction (v), (d.PHI. /dv) and with said very small magnetic flux gradient in said horizontal direction (h), (d.PHI. /dh) where (d.PHI. /dh)<<(d.PHI. /dv), said pole tip face being composed of a magnetically susceptible material.

15. The variable reluctance pickup of claim 14, wherein said primary pole pieces are positioned contiguous alternate north and south ends of adjacent magnetic elements.

16. The variable reluctance pickup of claim 15, wherein said ends of said primary pole pieces proximate the string plane have a cross-sectional area in a plane parallel to the string plane and said planar face of said poletip faces have an area greater than said cross-sectional area of said ends of said pole pieces.

17. The variable reluctance pickup of claim 16, wherein said planar poletip faces have a minimum linear dimension equal to a distance between adjacent strings, said minimum linear dimension measured in a perpendicular relationship to said linear portions of said strings.

18. The variable reluctance pickup of claim 13 wherein said individual means for shaping the magnetic field provided by each of said magnetic elements each comprises a primary pole piece having a rectangular end face proximate and parallel to the string plane and having a longitudinal axis oriented at an angle with respect to said string plane and with respect to said polarity axis of said magnetic elements, said primary pole pieces being composed of a magnetically susceptible material and being positioned contiguous with an end of a magnetic element of opposite polarity than the polarity of the ends of adjacent magnetic elements having primary pole pieces positioned contiguous the ends thereof to shape a plurality of distinc magnetic field regions, each encompassing a linear segment of one string, each magnetic field region having said magnetic flux gradient in said vertical direction (v), (d.PHI. /dv) and said very small magnetic flux gradient in said horizontal direction (h), ( d.PHI./dh), where (d.PHI. /dh).fwdarw.0, whereby lines of equal magnetic field strength of each magnetic field region in a reference plane perpendicular to the end faces of said primary pole pieces and perpendicular with respect to said strings have a rectangular-like configuration.

19. The variable reluctance pickup of claim 18, further including a plurality of secondary pole pieces each composed of a magnetically susceptible material and having a rectangular end-face parallel to the string plane and having a longitudinal axis oriented at an angle with respect to said string plane and with respect to said polarity axes of said magnetic elements, said secondary pole pieces being positioned contiguous alternate north-south ends of said magnetic elements opposite said primary pole pieces to provide a plurality of distinct magnetic field regions contiguous to said magnetic field regions provided by said primary pole pieces for further shaping said magnetic field regions provided by said primary pole pieces to enhance the magnitude of the magnetic flux gradient in said vertical direction, (v), (d.PHI. /dv) and to further decrease the magnitude of the very small magnetic flux gradient in said horizontal direction (h), ( d.PHI./dh).

20. The variable reluctance pickup of claim 19, wherein said means for sensing changes of reluctance in each of said magnetic circuits comprises a plurality of coils formed of insulated conductive wire, each of said coils being disposed around one of said primary pole pieces for generating an electrical signal primarily responsive to a change of reluctance in a particular magnetic circuit, said coils each being adapted for connection to a suitable means for amplifying and converting said electrical signals generated therein into acoustic waves.

21. The variable reluctance pickup of claim 20 further defined in that the longitudinal axes of said primary and secondary pole pieces are oriented perpendicularly with respect to the string plane.

22. The variable reluctance pickup of claim 21 further including means for minimizing the length of the linear segment of each string forming a part of each magnetic circuit, whereby high frequency oscillation modes of said linear segment of said string generate corresponding high frequency electrical signal oscillations in the particular sensing coil.

23. The variable reluctance pickup of claim 22 wherein said means for minimizing the length of the linear segments of each string forming said magnetic circuits comprise, in combination,

a. each primary poletip face having a length dimension (L.sub.p) perpendicular to the polarity axis of the corresponding magnetic element, and a heighth dimension (H.sub.p) parallel to the polarity axis of the magnetic element where the ratio of the heighth to the length (H.sub.p /L.sub.p) ranges between 0.05 and 1.0, and

b. each secondary poletip face having a length dimension (L.sub.s) equal to the length dimension (L.sub.p) of the primary pole piece perpendicular to the polarity axis of the corresponding magnetic element and a width dimension (W.sub.s) parallel to the polarity axis of the magnetic element where the length dimension (L.sub.s) is much greater than the width dimension (W.sub.s).

24. The variable reluctance pickup of claim 23, wherein the ratio of the width dimension and the length dimension of the secondary pole piece (W.sub.s /L.sub.s) ranges from 0 to 0.5.

25. The variable reluctance pickup of claim 21 further including means disposed on the end face of each primary pole piece further shaping the spatial configuration of the plurality of distinct contiguous magnetic field regions for generating a continuous resultant electrical signal in said sensing coils disposed around said first and second primary pole pieces upon a bending movement of any particular vibrating string from its normal quiescent position proximate and above a first primary pole piece to a position proximate and above a second primary pole piece adjacent to said first primary pole piece.

26. The variable reluctance pickup of claim 25, wherein said means disposed on the end face of each primary pole piece for further shaping the spatial configuration of the plurality of distinct, contiguous, magnetic field regions comprises a poletip composed of a magnetically susceptible material and having a planar face parallel to the string plane, said face having a configuration similar to an isosceles trapezoid, the parallel edges of said poletip face being oriented perpendicularly with respect to the linear segment of the corresponding string.

27. The variable reluctance pickup of claim 26, wherein the planar poletip has a unique long base dimension at least equal to a distance between the secondary pole pieces of the magnetic circuit elements adjacent to the primary pole piece on which said poletip is disposed.

28. The variable reluctance pickup of claim 26, wherein the planar poletip face has a maximum long base dimension equal to a distance between the secondary pole pieces of magnetic circuit element adjacent to the primary pole piece on which said poletip is disposed.

29. The variable reluctance pickup of claim 26, wherein each of said coils has an inside lead and an outside lead and wherein those coils disposed around the primary pole pieces positioned contiguous the north ends of said magnetic elements are referred to as a "north polarity series of coils" and those coils disposed around the primary pole pieces positioned contiguous the south ends of said magnetic elements are referred to as a "south polarity series of coils", said north polarity series of coils electrically connected with the inside lead of a first coil in said north polarity series being electrically connected to the outside lead of the adjacent coil in said north polarity series of coils which coil, in turn, has its inside lead connected to the outside lead of the adjacent coil thereto in said north polarity series, the last coil in said north polarity series of coils having its inside lead adapted for connection with a terminal of an amplifying means, said first coil in said north polarity series having its outside lead connected to the outside lead of a first coil in said south polarity series of coils, said inside lead of said first coil of the south polarity series of coils being connected to the outside lead of the adjacent coil thereto in the south polarity series of coils, which in turn has its inside lead connected to the outside lead of the adjacent coil thereto in said south polarity series of coils a last coil in said south polarity series of coils having its inside lead adapted for electrical connection to a second terminal of said amplifying means whereby said coils are electrically connected in series in a humbucking arrangement.

30. The variable reluctance pickup of claim 26 further defined in that said magnetic elements, said primary pole pieces, said secondary pole pieces, said pole tip faces and said coils are all potted in a rigid, insulative material to thereby form a solid body.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a variable reluctance pickup system for steel string musical instruments in which the vibrating strings cause variations of reluctance in a plurality of magnetic circuits generating electrical signals which, upon electronic amplification, are suitable for driving acoustic speaker systems.

2. Description of the Prior Art

Generally, variable reluctance pickup systems for steel string instruments comprise an arrangement of magnets and magnetically susceptible materials which establish a magnetic circuit in combination with the playing string. As the string vibrates, the changes in its position affect the reluctance and magnetic flux of the magnetic circuit. A sensing coil is inductively linked to the magnetic circuit for converting the variations in magnetic flux into a corresponding electrical signal. The electrical signal from the sensing coil is amplified electronically and fed into an acoustic speaker system for producing musical sounds.

There are many different configurations of the basic elements of variable reluctance pickup systems for steel string instruments. For example, U.S. Pat. No. 2,235,983 (Demuth) describes the basic elements of a magnetic pickup suitable for pianos and the like. U.S. Pat. No. 3,066,567 (Kelley) describes a magnetic pickup system having a single permanent magnetic element with a plurality of pedestals to provide a specific pickup zone for a given instrument string in combination with a single sensing coil. U.S. Pat. No. 3,483,303 (Warner) describes a variable reluctance transducer or pickup system for steel string musical instruments in which an attempt is made to isolate the magnetic circuits formed by adjacent strings so as to minimize "cross talk" between the various strings. U.S. Pat. No. 3,571,483 (Davidson) describes a variable reluctance pickup system having a plurality of isolated magnetic circuits, each specifically designed to be substantially insensitive to the plane of string vibration. Finally, U.S. Pat. No. 3,715,446 (Kozinski) describes a magnetic pickup system having a balanced coil assembly for each string wherein each assembly includes a bar magnet supporting two circular pole pieces and two sensing coils disposed around the pole pieces.

Before discussing the disadvantages of prior art variable reluctance pickup systems, it is instructive to review the fundamental properties of string instruments which give them their characteristic tones.

Basically, the tone of a plucked or struck string instrument is judged by the richness and complexity of the acoustic output in the "attack" or beginning portion of a note. In acoustic string instruments, the bridge structure constrains the motion of the sound board such that those components of string motion which are perpendicular to the plane of the sound board are well amplified, while those components of string motion which are parallel to the plane of the sound board are not. The path described by any arbitrarily small segment of a smoothly released plucked string is a precessing elliptical orbit of decreasing radius which rotates about the quiescent position of the string. Accordingly, the asymmetrical amplification of string motion provided by the bridge of an acoustic instrument yields a rich, full and complex tone of continuously varying harmonic content. The richness and complexity of the tone produced by acoustic string instruments is the primary criterion for judging the quality of such instruments.

Furthermore, the preferential or asymmetrical amplification provided by the bridge structure in acoustic string instruments enhances the expressive ability of the instrument. Specifically, the musician can control the initial motion of the string by plucking either parallel to the sound board for a "thin or nasal" tone or perpendicular to the sound board for a "full or rich" tone.

Steel string guitars and other similar instruments have a particular capability which distinguishes them from most other western musical instruments. This capability is referred to as "bending". Bending is accomplished after a string is fretted and plucked by moving the fretting finger with the string across the finger board stretching the string. The stretching of the string during bending can raise the pitch of the note played by as much as seven semitones, a factor which greatly enhances the expressive ability of the instrument. However, bending a note also results in a large displacement of the string from its quiescent position.

From the preceding discussion, it can be seen that for a variable reluctance pickup system to provide good tone (by acoustic instrument standards) it must be highly asymmetrical in converting string motion to electrical signal output. Further, such a pickup system must have a capability for high frequency response in order to preserve the richness and fullness of the varying harmonics in the attack portion of a note. Finally, for steel string guitars and similar instruments, the pickup system must be relatively insensitive to string displacements due to bending. Accordingly, variable reluctance pickup systems which are substantially insensitive to the plane of string vibration cannot generate a good acoustic tone.

Pickup systems with circular pole pieces have a similar disadvantage since the configuration of the magnetic field provided by such circular pole pieces is in the form of a symmetrical sinusoidal shell. Accordingly, the string vibrating within the magnetic field will generate equal magnitude electrical signals for string vibrations parallel to the string plane and string vibrations perpendicular to the string plane. The string plane is parallel to the sounding board.

Variable reluctance pickup systems which seek to eliminate cross talk between strings have two basic disadvantages: (1) elimination of cross talk also eliminates the possibility of bending; (2) isolation requires pole pieces having very narrow configurations aligned along the axis of the string, hence the pickup preferentially senses and generates electrical signals for string motions parallel to the string plane (parallel to the sounding board).

Other disadvantages of prior art variable reluctance pickup systems relate to their poor high frequency response characteristics. Specifically, the high frequency response of a magnetic circuit depends on two factors: (1) the aperture of the magnetic circuit (length of string sensed by the circuit); and (2) the impedance of the sensing coil.

Decreasing the aperture of the magnetic circuit increases its high frequency response capability. Specifically, the electrical signal response of a magnetic circuit reflects a summation of the changes of reluctance in the circuit which in turn are induced by movement of the linear portion of the string sensed by the circuit. If the length of string sensed is long (a large aperture), then different portions of the string within the aperture can move in opposite directions without changing the reluctance of the circuit. Hence, high frequency vibrational modes of the string may not be sensed for large aperture magnetic circuits. Accordingly, decreasing the aperture of the circuit increases the high frequency response of the circuit.

Variable reluctance pickup systems described in the prior art having "U-shaped" magnetic circuits have relatively large apertures. A U-shaped magnetic circuit comprises a bar magnet and two extending pole pieces positioned along an instrument string.

The impedance characteristics of variable reluctance pickup systems relate to the self-resonant nature of the sensing coils. Specifically, the impedance of the sensing coils increases with increasing frequency up to a maximum at the resonant frequency, whereupon the impedance decreases. Below the resonant frequency, the impedance is dominated by inductive effects. Specifically, the vibrating string causes variations in the magnetic flux of the magnetic circuit. The resulting variations in magnetic flux in the vicinity of the sensing coil induces an electrical signal in the coil which, in turn, creates another magnetic field which "bucks" or opposes the variations in flux induced by the string (Lenz's Law). This effect "impedes" the signal and increases with increasing frequency.

Above the resonant frequency, the impedance is influenced by the capacitive effects between turns of the coil and between layers in the coil winding. Specifically, the changing current in one turn of the coil influences the current in the neighboring turns of the coil. This effect becomes larger with increasing frequency such that the coil behaves as a capacitive reactance with turn-to-turn capacitive leakage to ground. Accordingly, the output signal from the sensing coils falls off rapidly above the self-resonant frequency.

Since both the inductance and capacitance of a sensing coil vary linearly with its mean radius, replacing one coil by multiple small coils can reduce the impedance of the pickup system by a factor equal to the number of coils and raise the self-resonant frequency by a factor equal to the square root of the number of coils.

From the above discussion, it can be seen that prior art variable reluctance pickup systems having a single coil for sensing variations in the reluctance of the magnetic circuit will have poor high frequency response. The desirability of utilizing multiple sensing coils to enhance the high frequency response characteristics of variable reluctance pickup systems, while recognized in the prior art, has not been extensively utilized. For example, in Kozinski, each pole piece has a separate sensing coil, which coils are connected in series to reduce background noise due to electromagnetic fields (a conventional humbucking arrangement). However, the Kozinski pickup is extremely sensitive to string position and cannot generate a sustained and continuous output upon bending a string from its quiescent position.

SUMMARY OF THE INVENTION

The invented variable reluctance pickup system for steel string musical instruments provides a highly asymmetrical magnetic field for preferentially sensing string vibrations perpendicular to the string plane and sounding board, and generating representative electrical signals which, upon electronic amplification and input into an acoustical speaker system, produce tones or notes analogous to those produced by purely acoustical string instruments. Specifically, the invented pickup system includes individual magnetic circuits for each string where each circuit includes pole pieces, a sensing coil and poletip faces. The pole pieces and the poletip faces provide a magnetic field region proximate each string which has a large magnitude magnetic flux gradient in a direction perpendicular to the string plane and a small magnitude magnetic flux gradient in a direction parallel to the string plane (parallel to the sound board).

A particular novel feature of the pickup system relates to special planar poletip faces which spread the magnetic fields emanating from the pole pieces to enhance the asymmetrical qualities of the field region and to render the pickup system insensitive to bending of a string from its normal quiescent position.

In a particular embodiment of the invented pickup system, a plurality of magnetic elements are arranged in a bucking configuration, a factor which further enhances the asymmetrical quality of the magnetic field regions created by the elements. Other embodiments of the invented pickup system include a single magnetic element with single pole pieces having rectangular cross sections for each string. In all the embodiments of the invented pickup system, a single sensing coil is provided for each string, a factor which serves to minimize the aperture of each magnetic circuit in the pickup and which maximizes the high frequency response capability of the pickup system.

Other novel features of the invented variable reluctance pickup system relate to the fact that the individual magnetic circuits are mounted on a printed circuit board with separate paired conductive paths for each sensing coil. The printed circuit board is modified for connection to a shielded multiple conductor cable which allows for easy connection for stereo output (bass strings to one channel, treble strings to another), multi-phonic output (each string to a separate amplifier), as well as many other combinations. Moreover, the separate conductor output allows individual electronic control over the volume and tone quality of the notes generated by each string.

The primary object of the invented high asymmetry variable reluctance pickup system is to produce an electronic signal which, upon amplification and input into an acoustical speaker system, generates a tone of continuously changing harmonic content at its leading edge, yielding the rich and complex attack normally expected of the best acoustic instruments.

A gradient of a scaler function is a vector whose components at any point are the rates of change (the derivatives) of the function along the directions of coordinate or reference axes at that point. In vector analysis a gradient of a scaler function f (x,y,z) is symbolized as grad f (x,y,z) and is defined as follows:

"The gradient of a scaler function is a vector whose magnitude is the maximum directional derivative at the point being considered and whose direction is the direction of a maximum directional derivative at the point." (See Reitz and Milford, Foundations of electromagnetic Theory (1964, page 7; and K. R. Symon, Mechanics QD, page 95.)

Magnetic flux is a scaler function and is typically symbolized by the Greek letter .PHI.. (See J. R. Reitz and F. J. Milford, Foundations of Electromagnetic Theory, (supra). Sections 8-9, page 166, and D. R. Corson and P. Lorraine, Introduction to Electromagnetic Fields and Waves, (supra), Section 5.2, page 197.

Further, an inherent feature of a vibrating string instrument limits the utility of vector quantity grad .PHI. in three-dimensional space for describing the magnetic fields provided by variable reluctance pickups. Specifically, the rate of change of magnetic flux along the direction of the string is not important, since an arbitrarily small string segment (ds) does not change its position along the string axis

The essence of the invention is that (d.PHI. /dh)<< (d.PHI. /dv). In language, (d.PHI. /dh) << (d.PHI. /dv) states that the rate of change of magnetic flux in the horizontal direction is much less than the rate of change of magnetic flux in the vertical direction. The rate of magnetic flux in the horizontal direction approaches zero, or (d.PHI. /dh).fwdarw.0.

Another object of the invented high asymmetry variable reluctance pickup system relates to providing some separation in the output from adjacent strings in order to minimize overlap in the stereo and separate amplifier configurations.

Still further objects, advantages and novel features of the invented high asymmetry variable reluctance pickup system will become apparent upon examination of the following detailed description of the basic elements of the invention, together with the accompanying figures.

DESCRIPTION OF THE FIGURES

FIG. 1a shows a top view of the basic elements of the invented high asymmetry variable reluctance pickup system.

FIG. 1b shows a top view of the printed circuit board on which the elements of the high asymmetry variable reluctance transducer are mounted.

FIG. 1c shows a cross-sectional view of the high asymmetry variable reluctance pickup system taken along line C--C of FIG. 1a.

FIG. 1d is another cross-sectional view of the high asymmetry variable reluctance pickup system taken along line D--D of FIG. 1a.

FIG. 2a is an enlarged partial top view of the high asymmetry variable reluctance pickup showing placement of the elements of the pickup in relation to the strings of a musical instrument.

FIG. 2b is a partial end view of the basic elements of the pickup system of FIG. 2a showing the configuration of the lines of equal magnetic field strength in relationship thereto.

FIG. 2c depicts the strength of the magnetic field emanating from the high asymmetry variable reluctance pickup system in a plane taken along line B--B of FIG. 2a.

FIG. 3 is a top view of the invented variable reluctance pickup system which illustrates the co-operation of the basic elements of the system upon bending of a string from its normal quiescent position.

FIG. 4 is a schematic representation of the electrical connection of the sensing coils in a conventional humbucking arrangement. FIG. 5a is a perspective view of another embodiment of a high asymmetry variable reluctance pickup system wherein the magnetic elements and poletip faces are arranged in a T-configuration.

FIG. 5b is a partial top view of a high asymmetry variable reluctance pickup system shown in FIG. 5a, illustrating the configuration of the poletip faces.

FIG. 5c is a cross-sectional view of the T-configuration pickup system and depicts the configuration of the lines of equal magnetic field strength.

FIG. 6 is a perspective view of the finished high asymmetry variable reluctance pickup system in which the components of the pickup system are potted in an insulative epoxy material.

FIG. 7 depicts still another embodiment of a high asymmetry variable reluctance pickup system.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1a, the invented high asymmetry variable reluctance pickup system includes a separate magnetic circuit element 10 for each string of the musical instrument (in this case a 6 string instrument). Each magnetic circuit element 10 has a permanent bar magnet 11 insulatively mounted on a printed circuit board 12 (see FIG. 1b). The bar magnets 11 are oriented parallel to each other in a row with their north-south polarity axes (indicated by the arrow 13) aligned in a common direction parallel to the longitudinal axes of the strings of the musical instrument. (See FIG. 2a). Each magnetic circuit element 10 further includes two pole pieces, a primary pole piece 14 and a secondary pole piece 16. The primary pole piece 14 has a thick rectangular cross section in a plane parallel to the string plane, while the secondary pole piece 16 has a thin rectangular cross section in the plane parallel to the string plane. The primary pole pieces 14 are located on alternate north-south ends of adjacent bar magnets 11. Planar poletip faces 17 are mounted on the end of each of the primary pole pieces. As shown in FIGS. 1a and 2a, the poletip faces 17 have a planar configuration of an isosceles trapezoid with a long base dimension approximately equal to the distance between the secondary pole pieces 16 of the adjacent magnetic circuit elements 10, and a short base dimension approximately equal to the width dimension of the primary pole piece 14.

The primary pole pieces 14, the secondary pole pieces 16 and the planar poletip faces 17 are composed of magnetically susceptible materials which serve to shape and direct the magnetic field provided by the bar magnets 11 into a plurality of asymmetrical magnetic field regions which encompass the strings.

Referring now to FIGS. 1c and 1d, sensing coils 21 are disposed around each of the primary pole pieces 14 between the planar poletip face 17 and the bar magnet 11. Each sensing coil has two leads, an inside lead 22 from the inside of the coil, and an outside lead 23 from the outside of the coil.

Referring now to FIG. 1b, the inside lead 22 and the outside lead 23 of each sensing coil 21 are connected to separate conductive strips 24 on the printed circuit board 12. A shielded multiple conductor cable 26 having a separate conductive wire 25 for each conductive strip 24 establishes the electrical connection between the sensing coils 21 and a suitable electronic amplification system. The amplification system amplifies the electrical signals produced in the coils 21 for driving a suitable acoustical speaker system.

Referring to FIG. 2a, the position of the magnetic circuit elements 10a, b, and c are shown relative to the quiescent position of the strings 27 of a musical instrument. Specifically, each magnetic circuit element 10 is centrally aligned beneath the quiescent position of a string 27 such that the string 27 is located centrally with respect to the magnetic field emanating from the pole pieces and poletip faces (See FIG. 2b). The magnetic fields emanating from the primary pole pieces 14, the secondary pole pieces 16, and the trapezoidal poletip faces 17 are illustrated by the lines of equal magnetic field strength 28 shown in FIG. 2b.

Specifically, since the polarity axes of the respective magnetic circuits are aligned in a common direction, the magnetic field of each circuit element 10 bucks the magnetic field of the adjacent magnetic circuit elements 10. In effect, each magnetic circuit element 10 has a separate magnetic field region which, in part, is shaped by the magnetic fields of the adjacent magnetic circuit elements 10.

The rectangular cross sections of the pole pieces 14 and 16 provide the magnetic field emanating therefrom with a large magnetic flux gradient in a direction perpendicular to the poletip face (perpendicular to the sounding board and string plane) and a small magnetic flux gradient in a direction parallel to the poletip face (parallel to the string plane and perpendicular to the string axes). From FIG. 2b, when the string 27 is moved perpendicular to the poletip face, it crosses a large number of lines of equal magnetic field strength 28 to generate a corresponding large change of reluctance in the magnetic circuit. However, when the string 27 is moved parallel to the poletip face, it crosses relatively few lines of equal magnetic field strength 28 and generates a correspondingly small change of reluctance in the magnetic circuit. The amplitude of the electrical signals generated in the sensing coils of the magnetic circuits 10 corresponds to the magnitude of the change of reluctance and, therefore, of magnetic flux in the circuits. Accordingly, it can be seen that the asymmetrical magnetic field regions provided by the magnetic circuit elements 10 tend to preferentially s