Abstract:
This invention relates to improvements to a stringed musical instrument, and more particularly to guitar design for use with transposing vibrato mechanisms. 
     Vibrato devices for guitars are known. The present device and method improve the ability to of a player to bend entire chords in a manner that maintains harmonic relationship between the individual strings.

Description:
[0001]    This invention claims priority benefit of provisional application 60/960,075 filed Sep. 14, 2007. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to devices which enhance the expressive qualities of stringed musical instrument by empowering the artist to “bend” notes and chords in a harmonic manner. 
       BACKGROUND 
       [0003]    Non-harmonic vibrato devices are known, typified by U.S. Pat. No. 2,741,146, which allows the musician to change the tension on all guitar stings in unison by activating a lever, without correcting for relative pitch between strings. 
         [0004]    Subsequent devices, typified by Jones, U.S. Pat. No. 3,411,394, correct pitch by varying the length of a crank arm or the radius of a string bearing cam. These devices suffer from one or more of the shortcomings of imprecise geometry, expressive difficulty, lack of range, tuning difficulty, tuning instability. 
         [0005]    Methods previously used to stabilize a vibrato, such as cam locks, or flats on activating cams, interfere with the smooth expressive motion of the vibrato. 
       SUMMARY 
       [0006]    The present invention improves the state of the art by utilizing tangential motion of string guides in a configuration that is significantly more accurate in pitch correction than the prior art. The guides are fixed relative to a pivoting tailpiece and cause the strings to be stretched or relaxed harmonically when the tailpiece is rotated. 
         [0007]    The enhanced accuracy allows the device to be made smaller than prior devices without loss of performance. When built at a larger scale, its geometric accuracy reduces required setup accuracy. Accuracy of the device is further enhanced by proper attention to string clamping and neck rigidity. 
         [0008]    The dual axis control allows a musician to sweep easily from “bend” to “dive” (sharp to flat) while using the muscles on only one side of the hand and wrist. A cam-enabled return spring maintains neutral tuning when the device is released without adversely affecting the action of the device. 
       OBJECTS OF THE INVENTION 
       [0000]    
       
         1) It is an object of the invention to provide an expressive vibrato device which bends chords while accurately maintaining relative pitch. 
         2) It is an object of the invention to provide a means of operating the device which allows smooth transitions from sharp to flat. 
         3) It is an object of the invention to provide a means of operating the device which provides tonal stability when the device is inactive. 
         4) It is an object of the invention to provide a means of operating the device which requires less effort and coordination than the prior art. 
         5) It is an object of the invention to provide a device which is easier to tune and maintains tune better than the prior art. 
       
     
     
    
     
       DRAWINGS 
         [0014]      FIG. 1  is a schematic showing geometric construction of string guide path. 
           [0015]      FIG. 2  is a top view of various embodiments of tuning head using zerofret and guide post improvements. 
           [0016]      FIG. 3  is a side view of a vibrato mechanism with rotational axis parallel to plane of strings. 
           [0017]      FIG. 4  is a top view of a vibrato mechanism with rotational axis perpendicular to plane of strings. 
           [0018]      FIG. 5  is a side view of a vibrato mechanism with rotational axis parallel to plane of strings, inverted with respect to  FIG. 3 . 
           [0019]      FIG. 6  is a cross sectional side view of a vibrato mechanism having variable length actuator cranks engaging ball receiver crank arms. 
           [0020]      FIG. 7  is a cross section and side view of a composite neck having adjustable zero fret. 
           [0021]      FIG. 8A through 8D  are side views of a flat plate tailpiece with axis perpendicular to string plane and body. 
           [0022]      FIG. 9A through 9E  are top views of various control cam embodiments on a flat plate vibrato tailpiece. 
           [0023]      FIG. 9G through 9I  are detail views of the cam means of  FIGS. 9A through 9C . 
           [0024]      FIG. 9J  is a side view of the tailpiece assembly of  FIG. 9D . 
           [0025]      FIGS. 10A and 10B  are top views of various control link arm embodiments on a flat plate vibrato tailpiece. 
           [0026]      FIG. 11A  is an exaggerated schematic top view of various improvements to a tuning head, including moveable tuning posts and tortured string paths. 
           [0027]      FIG. 11B  is an exaggerated schematic side view of a tuning post with eccentric mounting means. 
           [0028]      FIG. 12  is a top view of a vibrato assembly for retrofit to an existing guitar body. 
           [0029]      FIG. 13  is a top view of an arcuate guide path slot and guide having gear teeth means for adjustment. 
           [0030]      FIG. 14  is a top view of an multi component adjustable actuator cam assembly. 
           [0031]      FIG. 15  is a top view of an alternative adjustment means having a multitude of discrete anchor/guide holes  12   a  rather than moveably adjustable guides. 
       
    
    
     DESCRIPTION 
       [0032]    a) A main feature of the invention shown in  FIGS. 3 and 4  is a pivoting main vibrato member  8  (a moveable tail piece) holding in fixed relation to each other a group of string anchors  10 , and optionally a separate group of string guides  6 . The guides are preferably cylindrical rotating string rollers or posts with axes parallel to the pivot axis  1  of the main member, but may be any shape or construction which serves the purpose described, and the string anchors themselves may be incorporated into the guides, as illustrated in  FIGS. 3B and 8A . The radius of the guide preferably reduces the cyclic bending stress at the string anchor due to motion of the vibrato mechanism. 
         [0033]    String bearing means  3 , providing for a preferably slight change of string direction, may serve as the bridge, supporting one playable end of the string, as in  FIGS. 3 and 8F  Alternatively as in  FIGS. 4A and 8A , bridge means  9 , separate from string bearing means  3 , may be employed. 
         [0034]    Either the guides or the string bearing means may be notched or contoured to constrain the string axially, as illustrated in  FIGS. 8C and 8D . Of additional benefit, notches shaped to support the circumference of the string cross section will reduce sheer stresses on the string under tension. 
         [0035]    Referring to  FIG. 1 , the guides  6  are preferably positioned on the main member so that, at rest, any line  5  radiating from the pivot axis  1  to the center of curvature of any string&#39;s guide surface  6  will intersect the suspended string axis  4  at a substantially right angle. That right angle is assured at rest, regardless of adjustment, by constraining the guides to an arcuate path  7 , and fixed with respect to said main rotating member. The arc for any such arcuate path may be constructed through the centers of any three cylindrical guide surfaces meeting the foregoing requirement, as shown in  FIGS. 1A ,  1 B, and  1 C. If the guide surface radius is identical to the string bearing radius, and if the strings are routed to the outer surface of both string guide and string bearing, then the arc will pass through both the bearing axis and the main center of rotation, and be centered  2  on the mid chord between those two axes when the device is at rest, as shown in  FIG. 1A . 
         [0036]    Rotating the main member about its pivot axis  1  assures that the displacement of each guide is proportional to its distance from the pivot axis  1 . 
         [0037]    Because of the extremely accurate proportionality of the present invention with respect to the prior art, the unit may be made dimensionally very compact without losing tune. 
         [0038]    Because the pitch of a string varies with the square root of the string stretch, and the scale of the invention is large, the invention is robust enough to allow significant deviation from this optimal design without creating excessive transposing errors. Thus any configuration substantially equivalent to the preferred optimal configuration, for example  FIG. 5B , falls within the scope of the invention. 
         [0039]    The guides  6  may be constrained to the arcuate path, for example, by means of arcuate slots  12  (fitted with t-bolts or t-nuts, for example) or rails on a flat plate as in  FIG. 4 , or by crank arms  13  as in  FIG. 3 , rotationally adjustable about a path axis  2  fixed with relation to the main member, preferably resting on journal means (for instance a shaft or knife edge) with center of curvature at path axis  2 . 
         [0040]    The crank arm configuration of  FIG. 3  has the benefit of allowing any guide to be positioned with the string axis  4  near the main pivot axis  1 , such that rotating the main member  8  about its axis will have minimal effect on that string&#39;s tension. That feature may be achieved in the flat plate example by anchoring that string to the body of the instrument, or to the center of the rotating member  8 . Another benefit to the configuration of  FIG. 3  is that the rotation axis parallel to the plane of the strings eliminates conflict between strings which is avoided on the plate mechanism of  FIG. 8  by the differential notch height in string guides  6 . 
         [0041]    Rotating member  8  preferably has torsion resisting member  74  between opposed endplates, as in  FIGS. 3B ,  5 A, and  5 B, or torsion resisting shell structure  74 , as in  FIG. 5C . 
         [0042]    Adjustment of guide position along the arc in either configuration may be by linear adjusting screw  15 , an example of which is pictured in  FIG. 3 . Alternatively, the guides on a flat plate configuration may be manually positioned, or may have an adjustment aid in the form of a wrenchable pinion gear  6   a  preferably concentric with a string guide  6 , engaging teeth  12   b , preferably cut into the edge of the arcuate slot  12 , as in  FIG. 13 . 
         [0043]    Having anchor means  10  (for example slots in the edge of plate  8  as in  FIG. 4   b ) properly separated from guide means  6 , and correctly configured, has the advantage of allowing guide adjustment without an excessive change of string tension or pitch during setup. Additionally, the separate anchor means, as in  FIG. 5B , maintain constant direction of force on crank arms  13 , thus eliminating need for precision in component manufacture, and allowing adjustment by a simple unidirectional set screw. 
         [0044]    A plate (which may be flat, contoured, or ribbed, for example) rotating about an axis substantially perpendicular to a plane defined by the strings anchored thereto, as in  FIG. 4 , may be rigidly cantilevered from a rigid pivot shaft  11  in rigid bearing means, as in  FIG. 8A . Or, for example, it may pivot nonrigidly about a pin bearing  11 , constrained to a fixed plane by separate bearing means about its perimeter, for example one or more shafts  18  extending through 1 or more arcuate slots in the plate as in  FIGS. 4B and 4C , having bearing surfaces resisting axial motion of said plate. 
         [0045]    Graded markings on said plate, as in  FIG. 4A , allow quick setup according to prior records. Additional guides may be positioned for alternate tunings, allowing quick change between tunings without adjustment. 
         [0046]    The plate may be made of any material or mass, depending on desired properties, and the mass may be augmented by addition of weights, attached preferably by screw means to the unexposed face of plate. Rigid flat opposing washer means on guide and anchor means, and optionally on additional stiffening screws, in contact with preferably ground flat plate surfaces, may enhance the stiffness of a thin plate by reducing flex at arcuate slots. 
         [0000]    b) An alternative mechanism displayed in  FIG. 6  comprises for each string, bridge means  20 , string end anchor means  21  (preferably in the form of ball cups), fixed to ball crank means  22 , which pivot about a “ball crank axis”  23  preferably parallel to said string plane. 
         [0047]    Actuator crank means  8  rigidly supports a group of preferably cylindrical or spherical actuator surfaces  26 , preferably adjustable through a path substantially parallel to said force receiving surface  24  and essentially perpendicular to said ball crank axis  23 . 
         [0048]    An arm of each said ball crank includes a force receiving surface  24  oriented substantially parallel to a plane extending radially from and parallel to said ball crank axis, and separated from said plane by the radius of said actuators  26 . Said surface  24  is preferably substantially parallel to the plane of strings. 
         [0049]    Said bridge means  20 , with string bearing surface substantially arcuate about ball crank axis  23 , preferably includes vertical adjusting means providing for movement of bridge surface  9  in a direction normal to the plane of the strings  4  for adjustment of string “action”. Adjusting means is preferably provided by a single set screw  14  in a boss  17  on or rotating with said ball crank. Bridge component  20  is preferably supported at alternate end by action pivot pin  19 , preferably located in or near the plane of the strings. 
         [0050]    Adjustment of actuators is preferably from a line coaxial with the main axis of rotation  1 , in a direction toward or away from the ball crank axis  23 . That single adjustment affects both the effective length of the actuator crank arm and the effective length of the ball crank arm, thereby determining the displacement of the string anchors  21  when control arm  16  is moved. Adjustment means may be, for example, by linear adjusting screws  15  in  FIG. 6 , or by other means. 
         [0051]    The ball crank surface  24  is preferably cylindrically concave with its axis perpendicular to its axis of rotation  23 , and further is preferably slotted at the crank end to allow clearance for cantilevered actuator arms or adjusters  15 . 
         [0052]    The location of Bridge pivot support  28  is preferably adjustable in a direction parallel to the strings in order to adjust intonation. Intonation adjustment lock means  28  (preferably locking screw means extending through a slot in pivot support) locks support  28  in place after adjusting. The sliding of support  28  is preferably constrained to the by linear track means, preferably in the form of Track means, preferably in the form of a slot means  77  in pivot support  28  or base  76 , and corresponding pin means  78  extending into slot from the remaining component. 
         [0000]    c) For improved precision and to prevent losing tune after flat bends, the present invention may be implemented in combination with clamping of strings at the tuning head nut, as is known, or it may preferably be implemented using a zero fret  30  or fret roller, preferably in combination with string guide means  31  (preferably in the form of guide post bearings with axes substantially perpendicular to the plane of the strings, and having locking means beyond said guide means, for example, commercially available locking tuners  33  of the type that will tune a string in less than one full turn of the tuning post. 
         [0053]    Alternate locking devices include simple threaded post  39 , slotted or unslotted, preferably with keyed washer, as in  FIGS. 2   c  and  2   d . In  FIGS. 2C and 2I  clamping post  39 , has a small unthreaded guide surface at its root, allowing it to also serve as the guide post  31 . 
         [0054]    In  FIGS. 2   a,b,c,d  the guide post  31  preferably has adjustment means  32  for moving parallel to the zero fret, preferably by an eccentric having an axis substantially perpendicular to the string plane. Alternatively guide spacing may be adjusted by pivoting a multitude of guides about a single axis, for instance in the center or at one end of a gang casting  34  as in  FIG. 2E , where pivot and locking means may be a simple screw into the tuning head. 
         [0055]    The use of a guide post  31  beyond the zero fret  30  provides improved playability, allowing the “string bending” technique to be used with lower effort near the head end of the neck. Means for adjusting the position of guides in a direction parallel to the strings allows adjustment of “bendability”. Said adjustment may be, by multiple choice of mounting locations  31 . 1 , or by other means. 
         [0056]    Alternatively, precisely or adjustably located locking tuners of the type previously described could provide some of the benefits of said string guides when used in combination with a zero fret and other components of the present invention. In  FIGS. 11A and 11B  tuners  33  are preferably mounted with the post through an eccentric, preferably tapered bushing  36 , in a similarly tapered receiver hole in tuning head. An alternative adjustment uses a pivot pin or screw  34 . 1  perpendicular to face of tuning head, and an arcuate slot  34 . 2  about said pivot pin and through said tuning head perpendicular thereto. Loosening a lock nut on said tuner shaft and rotating said tuner in said arcuate slot allows variation of said string position, as in  FIG. 11A . 
         [0057]    The range of a flat plate vibrato device may be enhanced by locating tuning machines and guide posts on tuning head to define a tortured string path  37  for one or more minimally stretched strings (typically the lower pitched strings) as in  FIG. 11A , or by choosing strings with heavier windings, or thinner cores, or lower modulus. 
         [0000]    d) “Action height” adjustment, typically performed by cutting grooves into a nut and adjusting tension on a metal truss rod in prior art, may be improved by use of a zero fret  30  adjustable in a direction substantially perpendicular to the surface of the fingerboard. The zero fret is part of or joined to a support beam or flange  60 , preferably elastically cantilevered about a bending axis parallel to said zero fret, and is adjustably secured from motion and vibration by any of a) compressive set screws, b) tensile hold down screws  61 . 2 , flex modulus of flange  60 , string tension acting on string bearing  35 . 
         [0058]    If the neck and fingerboard are of suitably high modulus, as in  FIG. 7 , the cantilever may be the extreme end  62  of the fingerboard itself, preferably having interlaminar reinforcement  63  at the line of separation from the neck, for example with anchor screws substantially perpendicular to the fingerboard. If the fingerboard and neck are molded as a single unit, said reinforcement may be in the form of fibrous stitching or belting through or around the longitudinal fibrous reinforcement of the neck and fingerboard, or rigid anchor means, preferably flat plates or a plate assembly, of high modulus material inserted substantially parallel to the length of the neck and perpendicular to the plane of the fingerboard. 
         [0000]    e) The present vibrato invention may be made to retrofit onto an existing guitar, particularly one employing a removeable Gibson type bridge and tailpiece. Unit may be fabricated with anchor bolts  71  or bolt holes matching tailstock bolt pattern, and bridge height adjustment screws  72  either matching the existing threaded inserts, or riding on plate means  69  secured to said existing inserts, as in  FIGS. 8   e ,  8   f  and  12 . 
         [0059]    A preferred retrofit tuning head flange assembly in  FIG. 2B , for example to fit a typical Gibson tuning head, includes a flange  60 , preferably of flat metal or composite material extending substantially over the tuning head, to which is attached a combination of a zero fret  30 , string bearing means  35  to reduce string angle across zero fret, string guides  31  preferably having adjustment means  32  to adjust string spacing, vertical fret adjustment means  36  as previously described, and optionally string clamp means  39 . Alternative to string clamp means, unit may include locking tuners mounted through or external to the flange. Preferably set screw height adjusting means  36  project into nut cavity, preferably distinct from alignment pin means extending into nut cavity, which rest against wall or walls of nut cavity. 
         [0060]    For retrofit for flange  60  onto severely raked tuning heads, as in  FIGS. 2G and 2H , string bearing means  35  and string guide means  31  are preferably combined into a single roller  66  for each string, preferably having lateral adjusting means  36 . With a beveled flange on said string bearing  35 , boss  65  aligned with bearing axis may be normal to head face as in  FIG. 9H , or preferably canted, as in  FIG. 9G , with axis preferably normal to the plane of the string. Mounting of tuning machines  33  with axis normal to string plane at tuner, preferably on beveled bosses  67 , aligns tuning machine  33  to guide roller  66 . 
         [0000]    f) The position of vibrato mechanism at rest or “home” position may be determined by the force of a tensile or compressive counterspring  40  acting against the tension of the strings, each forcefully engaging the rotating member  8 , as is common in the prior art, and shown in  FIGS. 5   b  and  6   a.    
         [0061]    The control bar  16  may engage the main rotating member  8  directly as in  FIG. 6A , or it may engage the main rotating member through mechanical linkage, for example link arms  42  as in  FIG. 10A  or  10 B, or cam means  43  as in  FIGS. 9 and 5A , in order to achieve a desired purchase or direction of effort applied to the rotating member  8  for stretching or relaxing strings, or stability against drift and rebound. 
         [0062]    A counter spring  41  may maintain string tension alternatively by engaging the control bar  16 , rather than acting directly on the rotating member  8 , thus eliminating any backlash effect of imprecision in control linkage. 
         [0063]    Said counterspring or “balancing spring” force at rest is preferably adjustable using cam means  44  or adjusting screw means  45 . 
         [0000]    g) The preferred cam configuration in  FIG. 9   a  utilizes a cam  50 , preferably on an axis perpendicular to the plane of the strings, the force of said cam opposing the tension of the strings by acting on a cam follower  46 , and said cam having at least one rest area of constant radius  50 . 0 , with sharpening cam surface of increasing radius  50 . 1  on one side of rest, and flattening surface of decreasing radius  50 . 2  on other. Cam follower position, which determines resting pitch, is adjustable preferably by a lever  47  acting on an eccentric shaft. 
         [0064]    With string tension on main member  8  pressing cam follower  46  into first cam  50 , this first cam means creates increasing pitch when rotated in one direction from the rest and decreasing pitch when rotated in the other. 
         [0065]    An optional second cam and cam follower means  49  (between rotating vibrato member and instrument body) acts as a low pitch stop, so that when control bar is released below the rest position of the main cam, the rotating member will stop at a low key defined by the player using second adjustment means, preferably a lever rotating said cam means. (note: the cam itself may be a simple cam follower on an eccentric shaft) Lever shaft has friction means, preferably in the form of locking spring washers on a friction plate, resisting rotation except by manually applied torque. 
         [0066]    Said embodiment may be implemented with or without return spring means  56 , preferably pressing a follower  55 . 9  against return cam  55 , and preferably having adjusting means to allow precise return of cam to rest position when released. 
         [0067]    An optional “upper” cam  50 . 9  in  FIG. 14  includes a second sharpening surface  50 . 3  having higher slope and extending from first sharpening surface  50 . 1 . The tactile sensation provided by this surface contacting the cam follower  46  alerts the player when strings have been stretched a predetermined distance, preferably a tonal half step. 
         [0068]    Upper cam  50 . 9  and cam  50  may be combined into a single component, or they may preferably include angular adjustment means  50 . 7  to define the first tactile feedback point. 
         [0069]    Preferably upper cam  50 . 9  includes an upper rest  50 . 4  surface of constant radius extending from the peak of sharpening surface  50 . 3  over the remaining useable circumference, serving to prevent breaking strings, prevent breaking necks, and preferably create a transposing rest at a fixed tonal distance (for example a full step) form said first rest  50 . 0 . 
         [0070]    An optional “lower cam”  50 . 8  includes the rest surface main  50 . 0 , flattening surface of decreasing radius  50 . 2  of “center cam”  50 . 5 , and preferably a low limit surface of constant radius,  50 . 6 . Angular adjustment of lower cam with respect to sharpening cam surface  50 . 1 , by adjuster  50 . 6  or separate adjuster, adjusts or eliminates the size of the rest  50 . 0  exposed to follower  46 . 
         [0071]    Said stack of cams may be further subdivided with additional rests and/or adjustable cams as needed. 
         [0000]    h) A second preferred cam configuration in Figure utilizes preferably twin cam means (where second cam means may involve a separate cam or a second contact point on a first cam) each cam preferably rotating on a common axis. 
         [0072]    A first cam means  51  has a rest surface  51 . 2  of constant radius over much of its useable circumference, and sharpening surface means  51 . 1  of increasing radius extending from the meeting of the two surfaces at root  50 . 0 . 
         [0073]    With string tension on main member  8  pressing cam follower  46  into first cam  51 , this first cam means creates increasing pitch when rotated from the root  50 . 0  in the direction of increasing radius, and no tonal change when moved in the other. Cam means  51  may include the features of upper cam means  50 . 9 . 
         [0074]    A flattening cam  52  has a rest surface  52 . 2  of constant radius and flattening surface  52 . 1  of increasing radius extending from the meeting of two surfaces at root  52 . 0   
         [0075]    Sharpening spring means  53 , directly or indirectly forces cam follower  51  toward “home position” until further motion is prevented by contact of second cam  52  with home stop (or cam follower)  59  fixed rigidly with respect to instrument body. Cams  51  and  52  are each rotatable with respect to a common transport means  57  (preferably a flattening crank pivoting on axis  58  parallel to main cam axis). 
         [0076]    Said spring  53  is preferably of adequate spring rate and deflection to resist further deformation when cam  51  stretches strings to the maximum. 
         [0077]    Preferably, rotating control arm  16  in a second direction progressively reduces string pitch by engaging stop  59  with the flattening surface of increasing radius  52 . 1 , thus moving flattening transport means  57 , and thereby moving first cam  51  away from “home” position, allowing follower  46  to follow. 
         [0000]    i) The third cam embodiment in  FIG. 9C  varies from that in  FIG. 9B , in that flattening cam  52  includes a flattening surface  52 . 3  of decreasing radius from the root  52 . 0 . Rather than the sharpening spring  53  of the prior example, it is the rigidity of the combination of home stop cam  54  (substantially fixed with respect to instrument body) against the constant radius  52 . 2  of flattening cam  52  that holds the transport  57  (and actuator cam  51 ) in home position until the decreasing radius of flattening surface  52 . 3  is engaged, preferably by rotating control arm  16  in said second direction. 
         [0078]    Further, return spring  56  acting with mechanical advantage through flattening cam  52 , expends far less effort than sharpening spring  53  of  FIG. 9B . 
         [0000]    j) In a fourth embodiment using cam control, said second direction of rotation of control arm  16  for is in a different plane (preferably at right angles) from that used to sharpen string tone in  FIGS. 9B and 9C . 
         [0079]    This may be accomplished by simple linkage to the coaxial cam axes previously described, or it can be accomplished by rotating the flattening cam  52  of  FIGS. 9B and 9C  into a second plane preferably perpendicular to sharpening cam  51 . 
         [0080]      FIGS. 9   d  and  9   e  show further embodiments of the basic principle of  FIG. 9   c , wherein second control axis rotates with respect to the first. Transport means  57  includes sharpening cam means  51  in a single component, and is displaceable either linearly ( FIG. 9D ) or angularly ( FIG. 9E ) with respect to base means  70 , rotating about an axis fixed with respect to instrument body. Control arm  16  is fixed to base in preferably the first rotational plane and pivots with respect to base in the other. 
         [0081]    An advantage of sharpening and flattening motions being divided into two planes is that control arm  16  may be swung away from strings without effect on pitch, but may be pressed in a direction perpendicular to string plane to lower pitch, whereas pulling control arm toward strings about an axis perpendicular to sting plane increases pitch. Another advantage is that overshooting the root when returning from a bend will have no effect on string pitch as with other devices (unless the cam is specially cut for that effect, for example) 
         [0082]    In advantageous alternate setups, one of the two cams may be a full range cam  50 . 5  (as in  FIG. 9A ) while the other covers a similar range, but with no center rest  50 . 0 . 
         [0083]    Cams may act directly or indirectly through cranks and rockers. 
         [0084]    The large constant radius areas on certain cams help prevent audible mechanical shock at the end of a stroke, to allow overshoot without audible error, and to allow flexibility and tolerance during setup. 
         [0000]    k) In the preferred embodiment a combination of 2 or more springs would be used. The first spring (a balancing spring  40 ) is preferably adjustable, and preferably acts on the main rotating member, opposing the tension of the strings, in order to reduce the effort required for the performer to stretch the strings to a sharper pitch. Adjustment of said balancing spring will determine the amount of effort required to move rotating member  8  away from home position. Balancing spring  40  may be used in conjunction with sharpening spring  53  of  FIG. 9B  to further define the effort required in sharpening and flattening actions. 
         [0085]    One or more secondary springs acting on the control arm or on cams or linkage attached thereto compensate for string and first spring forces, particularly when the string pitch is bent flat, thereby allowing the control arm to return to home position or reducing the effort required for the user to return it to home position. 
         [0086]    One or more third spring means may act on the arm or on detents to assist in forcing the arm into or out of adjustable detents for selecting alternative arm positions. 
         [0087]    Preferably said first balancing spring may be adjusted to optionally completely balance the string tension at base tuning, thereby allowing main rotating member  8  to float freely without constraint by cams and stops. 
         [0088]    Note that, while coil springs are generally depicted here for schematic purposes, it is anticipated that any spring configuration fitting the application may be applied. In  FIG. 12 , a base plate  69  used to retrofit the current device to an existing body may be of spring steel material having a cantilevered balancing spring  40  cut into said plate and preferably rigidly or pivotably linked to rotating member  8 . 
         [0089]    In the prior configurations, the force exerted by balancing spring  40  is less than to total opposing force of the strings, and effort by the control arm is required to stretch the stings to a higher pitch. 
         [0090]    In alternative configuration shown conceptually in  FIG. 9F , balancing spring  40  is energizised to exert force adequate to stretch the strings to their highest allowable pitch, and the force of main control cam  50 , upon main cam follower  46  (the axis of one of which is fixed with respect to the rotating member, and the other with respect to the body) opposes the force of spring  40 . When cam  50  rotates to reduce its force on follower  46 , the balancing spring  40  moves the main rotating member to increase the tension on the strings. Return spring  41 , acting directly or through linkage or return cam  55  and return cam follower  55 . 9 , opposes the sharpening motion of the control arm  16  and returns it to neutral when it is released. The benefit of this configuration is that a broken string will have no effect on the pitch of the remaining strings or the as might another configuration if the force on balancing spring  40  were excessive. 
         [0000]    l) Because of the massive scale of the present invention and low angle of rotation as compared with prior art tremolo devices, string guide means may be visually placed by measurement or by index marks included on the device, and a small error in placement will be undetected acoustically. Further, because of the low angle of rotation, to resolve conflicts of space, a string may be wrapped about the geometrically wrong side of said guide or about a guide in a geometrically incorrect track without significant harm to acoustic accuracy. 
         [0091]    An embodiment of the invention taking advantage of said tolerance in a flat plate configuration may use fewer than the total complement of arcuate paths. It may also use additional (for example parallel to the high e) non converging paths to allow flexibility in setting up said device for multiple tuning. Where multiple paths converge near the main pivot axis, one may continue while the others terminate short of the convergence point. 
       m) Additional Notes: 
       [0092]    Previously described pitch adjusting lever means may be installed on either first cam follower or second cam follower, or both. 
         [0093]    Any alternative means of engaging vibrato device may be applied, for example a foot pedal with flexible cable coupled to the control cam, or coupled directly to the main rotating member. 
         [0094]    Rotation of control arm in two planes may be used to perform 2 differing tonal adjustments, for instance bending the b-string or some other subset of strings may be assigned to rotation in one plane, while rotation in the other plane affects the entire string complement. 
         [0095]    Alternatively, the two planes of rotation may serve similar functions, for instance similar cam operation, but with differing cam slopes and rests. 
         [0096]    Alternatively rotation in one plane may be used to set and release locking mechanism or brake for the rotation in the other plane. 
         [0097]    Likewise a foot pedal or other mechanism may operate in conjunction with one or more planes of control bar rotation, as may be required to perform any of the various functions. 
         [0098]    Control arm  16  preferably has control surfaces engageable by players fingertips substantially perpendicular to each major direction of motion, as in  FIGS. 9A and 9D . In an alternate embodiment, one or more projections  73  project substantially radially from an arcuate control arm  16 , providing a means to an improved playing technique, as in  FIG. 12A .  FIG. 12B  shows an alternative embodiment wherein control arm extends under pick guard or other solid surface means  79 . Control end  73  may extend in any direction. 
         [0099]    An advantage of the present invention is that transposing to an alternate key may be accomplished by adjusting the position of the cam follower  46  with respect to the main rotating member  8  (preferably by lever action as described), or by adjusting the position of the control arm cam pivot axis  60  (preferably by similar means). Thus the main control arm  16 , foot pedal, or other main control continues to be fully expressive. 
         [0100]    A preferred method of applying spring force to main control arm is by a sprung cam follower means acting on a separate cam mounted on main pivot axis, cut to provide counteracting torsion only when cam arm is rotated to lower string pitch, as in  FIG. 9A . Cam follower may be a simple low friction surface or rotating bearing surface. 
         [0101]    String bearing means may serve also as bridge saddle means. 
         [0102]    String guide means and string anchors may be combined in a single component. 
         [0103]    Note: Mechanical construction listed above is by way of example and conceptual schematic only. Any configuration functioning according to the described principles falls within the scope of this invention. In particular switching locations of cams and cam followers, rotating axes, and utilization of mechanical linkage in place of cams, or vice versa, falls under the scope of this invention. 
         [0104]    The “substantially accurate” adjusting path of string guides on a flat plate embodiment may extend to include slots or discrete holes having arcuate or linear configuration. 
         [0105]    The invention resides in the specification and claims and in those improvements and modifications which may become obvious to those skilled in the art.