Musical instruments equipped with sustainers

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The present invention relates to a device for providing a sustained sound from a musical instrument having a vibratory element such as a string.

BACKGROUND OF THE INVENTION

Musical instruments employing a vibrating mechanical element such as a string to produce sound have been provided heretofore with transducers commonly referred to as "pickups" for detecting the motion of the vibrating element and producing an electronic signal representing this vibration. This pickup signal may be amplified and converted to sound by a loudspeaker. The sound produced from the pickup signal supplements or replaces the sound produced by acoustical interaction of the string, the instrument body and the air. Typically, the instrument body has little or no acoustic response, so that the sound produced from the pickup signal constitutes essentially the entire sound of the instrument. This is the case in the common electric guitar, electric bass and the like.

The sound produced by instruments of this nature dies out progressively after the string is excited. This is particularly so in the case of instruments having little or no independent acoustic response. The sound can be prolonged somewhat by operating the amplification and loudspeaker system at extremely high power levels so that strong acoustic waves representing the original vibration impinge upon the string. Such "acoustic feedback" tends to sustain the vibration of the string, thereby prolonging the note. However, this approach is effective only when the sound produced by the amplification and loudspeaker system is extraordinarily loud. Moreover, the acoustic feedback effect depends upon the acoustic properties of the environment. Therefore, this effect will produce different results in different concert halls.

Various attempts have been made to provide a "sustainer" or device capable of prolonging the notes independently of acoustic feedback from the environment. U.S. Pat. No. 4,245,540 discloses a sustainer incorporating a loudspeaker mounted in close proximity to the strings. The amplified signal from the pickup is passed to the loudspeaker, so that acoustic vibrations produced by this loudspeaker will impinge directly upon the strings. U.S. Pat. No. 4,697,491 discloses a sustainer for a stringed instrument such as a guitar having a body and a neck projecting from the body. An electromechanical transducer is mounted to the neck, remote from the body. The pickup signal is passed to this electromechanical transducer. The transducer vibrates the neck and these vibrations are fed back into the strings. U.S. Pat. No. 3,813,473 discloses an instrument having a "bridge" or string support linked to an electromagnet. An electronic signal derived from the pickup signal is applied to this electromagnet, so as to vibrate the bridge and, hence vibrate the strings. U.S. Pat. No. 4,484,508 describes a generally similar sustainer having an electromechanical transducer adapted to shake the instrument body responsive to the pickup signal, and also having a circuit for progressively reducing the amplitude of the signal so as to provide a controlled fadeout. The fadeout circuit is arranged to provide a quicker fadeout for higher frequency signals.

U.S. Pat. Nos. 4,137,811 and 4,181,058 provide a sustain action utilizing magnetic interaction between a static magnetic field and electrical currents passing through the strings themselves. Thus, a magnet is mounted adjacent the strings, and both the strings and frets of the instrument are electrically conductive. Circuitry is provided for directing an alternating current feedback signal representing the pickup signal through the strings via the frets. The alternating current in each string interacts with the static magnetic field to produce an alternating magneto-motive drive force on the string. U.S. Pat. No. 4,236,433 discloses a sustainer employing an electromagnetically actuated tensioning device for each string, each such tensioning device being connected to a feedback circuit. The signal from a pickup associated with each string is applied through the feedback circuit to the tensioning device, so that the tensioning device will periodically stretch and release the string. The '433 patent also discloses an alternative arrangement wherein an electromagnet or "driver" is juxtaposed with each string so that flux from the electromagnet will impinge directly upon the string. Each such electromagnet is provided with a drive signal representing the signal from a pickup associated with the same string. Thus, variations in magnetic flux of the electromagnet will cause variations in the flux impinging upon the strings. This varying flux tends to excite the string in vibration, provided the string itself is ferromagnetic. U.S. Pat. No. 4,075,921 discloses a generally similar approach, employing a magnetic pickup and a magnetic driver arranged to directly excite a ferromagnetic string. The sustainer may be a hand held, battery-powered device incorporating both a pickup and a driver, and arranged so that the pickup and driver can be aligned with one string of the instrument. Alternately, the sustainer may be built into the instrument and may be provided with separate pickups and drivers for the various strings. U.S. Pat. No. 3,742,113 likewise employs a magnetic pickup and magnetic driver directly associated with each string, with a feedback and amplification circuit connected between the pickup and the driver. The ' 113 patent emphasizes that the feedback circuit or amplifier should have "zero phase shift" so as to provide a driving force "in phase with the string's fundamental frequency of oscillation as transduced by the pickup" so as to reinforce the fundamental mode vibration of the string.

The aforementioned '921, '433 and '113 patents utilize pickups and drivers having a separate ferromagnetic pole piece disposed beneath each string, so as to provide a substantially concentrated magnetic field from each pole piece at normal, undistorted position of the associated string. Separate coils may be provided for each pole piece. U.S. Pat. Nos. 4,580,481 and 4,535,668 disclose a pickup having a unitary, oblong coil and ferromagnetic core extending alterally across the string array. Movable permanent magnets are also provided. By repositioning the permanent magnets, the field direction can be varied so as to provide different phase relationships among the signals induced in the coil by the various strings. U.S. Pat. No. 3,983,777 suggests a pickup having a uniform magnetic field strength across the lateral extent of the string array to suppress variations in pickup response caused by lateral movement of the strings. Other unitary pickups having a single coil and a single ferromagnetic pole piece extending across the string array are shown in U.S. Pat. Nos. 4,364,295 and 4,151,776.

Despite the extensive efforts of the art heretofore, there have been substantial, unmet needs for further improvement. The sustainers available heretofore generally have been inefficient, in that they require substantial electrical power to the drive coil in order to produce an appreciable sustain effect. This high power consumption poses a significant problem where the sustainer draws its power from a battery mounted on the instrument.

Moreover, application of high power to an electromagnetic drive coil in a sustainer tends to produce substantial electromagnetic emissions. Electromagnetic fields radiated from the drive coils impinge upon the pickup and induce unwanted signals. Although the pickups used in electronic musical instruments typically incorporate features for suppressing the effect of stray electromagnetic radiation, these measures are not always perfectly effective. Radiation from the driver can be suppressed to some degree by shielding, but such shielding adds weight, bulk and cost. Thus, there has been a substantial need heretofore for an efficient sustainer capable of providing a powerful sustaining effect with only a modest power input to the driver. There has been a further need for a sustainer which would permit the musician to adjust the action of the sustainer to provide varied artistic effects.

SUMMARY OF THE INVENTION

The present invention addresses these needs.

Our own earlier U.S. Pat. No. 4,907,483 claims certain sustainers, and also claims musical instruments equipped with certain ones of these sustainers. The present application is directed to musical instruments equipped with the other according to one aspect of the present invention includes a structure and at least one vibratory element, which may be a string or the like. The instrument further includes a sustainer. The sustainer includes drive means for applying a drive force to a vibratory element of the instrument responsive to the drive signal so that the drive force bears a predetermined phase relationship to the drive signal. Feedback means are provided for accepting a pickup signal representing vibration of the vibratory element of the instrument and having a predetermined phase relationship to the vibration. The feedback means are arranged to provide a drive signal to the drive means such that the drive force applied by the drive means will be substantially in phase with the vibration of the vibratory element. The sustainer may further include a pickup for providing the pickup signal in response to the vibration of the string.

One or both of the pickup means and the drive means typically will have a non-zero phase shift. That is, the pickup signal produced by the pickup means may lag or lead the actual movement of the vibratory element, whereas the drive force applied by the drive means may lag or lead the drive signal. The feedback means preferably is arranged to provide a phase shift which is substantially inverse to the combined phase shift of the pickup means and the drive means, taken together. Thus, the combined overall phase shift of the entire sustainer will be approximately zero and the drive force will be applied in phase with the vibratory motion of the string itself, i.e., in phase with the sustainers according to this aspect of the invention can provide a powerful, sustaining action to prolong the fundamental mode vibration of a string or other vibratory element with only modest power input to the driver. Such sustainers according to the invention can provide sustaining action suitable for prolonged, continuous use, as in a concert environment, while employing only small, self-contained batteries as a power supply. Although the present invention is not limited by any theory of operation, it is believed that the enhanced results achieved arise at least in part from better phase matching of the force applied to the vibratory element and the actual, fundamental mode vibration of the vibratory element.

The feedback means may be arranged so that for at least some frequencies of the pickup signal, the drive signal differs in phase from the pickup signal and this phase difference varies with frequency. Most desirably, such variation in the phase difference between the pickup and drive signals is towards a drive signal leading phase difference with increasing frequency. Preferably, the feedback means is operative to provide the drive signal so that for at least some frequencies, the drive signal leads the pickup signal.

Control means may be provided for determining the frequency content of the pickup signal and altering the phase transfer function of the feedback means, the phase transfer function of the drive means or both depending upon this frequency content. Thus, the control means may include means for adjusting the phase transfer function of the feedback means towards a drive signal leading condition as the predominant or highest amplitude frequency of the pickup signal increases.

The drive means may include an inductive coil and means for applying the drive force to the vibratory element responsive to magnetic flux produced by the coil. The force applied by drive means employing an inductive coil tends to lag behind the drive signal or voltage applied to the coil. Moreover, this lag increases with the frequency of the signal. Thus, the phase difference and variation in phase difference with frequency provided by the feedback means according to this aspect of the present invention compensates for the characteristics of the drive means. A musical instrument according to a further aspect of the invention may include a plurality of taut, flexible strings extending in a lengthwise direction and disposed side-by-side in an array. The instrument according to this aspect of the invention includes means for providing a drive signal and drive means for applying drive forces to the strings responsive to the drive signal so that the drive force applied to each string is substantially independent of lateral displacement of the string. Therefore, the response of the sustainer is substantially unaffected by lateral bending of the strings.

Preferably, the drive means includes means for providing a magnetic field varying in accordance with the drive signal so that the varying magnetic field is substantially uniform throughout the range of lateral motion of each string. The means for providing a varying magnetic field may include a ferromagnetic element, means such as a coil juxtaposed with this element for directing magnetic flux through the ferromagnetic element and means for mounting the ferromagnetic element so that it extends laterally across the string array. The surface of the ferromagnetic element facing toward the strings may be substantially parallel to an imaginary surface defined by the strings when in their normal, undistorted position. The ferromagnetic element employed in this arrangement preferably includes a permanent magnet.

These and other objects, features and advantages of the present invention will be more readily understood from the detailed description of the preferred embodiment set forth below, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a sustainer in accordance with one embodiment of the present invention, in conjunction with a musical instrument.

FIGS. 2 and 3 are fragmentary, schematic sectional views taken along lines 2--2 and 3--3 respectively in FIG. 1.

FIG. 4 is a functional block diagram of the sustainer and instrument shown in FIG. 1.

FIGS. 5, 5A and 5B are a schematic circuit diagram showing a portion of the sustainer of FIGS. 1-4.

FIG. 6 is a graph of certain variables associated with the sustainer of FIGS. 1-5.

FIG. 7 is a fragmentary schematic circuit diagram depicting a portion of a sustainer according to a further embodiment of the invention.

FIG. 8 is a schematic, fragmentary perspective view depicting a portion of a sustainer in accordance with another alternate embodiment of the invention.

FIG. 9 is a fragmentary schematic sectional view taken along lines 9--9 in FIG. 8.

FIG. 10 is a fragmentary perspective view similar to FIG. 8 but depicting a sustainer in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A conventional electric guitar 20 has a structure including a body 22 and an elongated neck 24 projecting from the body. A conventional tailstock 26 and bridge 28 are secured to body 22, whereas a headstock 30 is secured to the end of neck 24 remote from head 22. Frets 25 are arranged along neck 24. Six ferromagnetic, typically steel strings 32 are held under tension by tailstock 26 and headstock 30, and engaged with bridge 28 so that each string extends generally in the same, longitudinal direction from the tailstock to the headstock, the strings being disposed side-by-side above the neck 24 and body 22. The strings thus define an array having a widthwise direction transverse to the longitudinal direction and generally parallel to the top or string-facing surfaces of the neck and body. As used in this disclosure the terms "widthwise" and "laterally" should be understood as referring to this widthwise direction of the string. Also, the terms "up" and "down" should be understood as referring to the directions from the strings away from and towards the surface of the guitar body, respectively. As seen in FIG. 2, the directions to the left and to the right are widthwise or lateral directions, whereas the directions towards and away from the top of the figure are upward and downward, respectively.

Guitar 20 incorporates a pickup 34 of the type known in the art as a "hum-bucking" pickup. mounted to body 22 adjacent bridge 28. Pickup 34 incorporates a permanent magnet 36 extending along the top surface of body 22, magnet 36 having its north-seeking pole facing rearwardly, towards headstock 30 and its south-seeking pole facing forwardly, towards tailstock 26. The pickup also includes six ferromagnetic prongs or projections 38 adjacent the north-seeking pole of magnet 36 and six similar prongs or projections 40 adjacent the south-seeking pole. These projections 38 and 40 are disposed in pairs. Each such pair includes one projection 38 adjacent the north-seeking pole and one projection 40 adjacent the south-seeking pole. Both projections of each pair are aligned with one string 32. The projections tend to concentrate the flux from the magnet on the strings.

As illustrated in FIG. 3, considering the generally accepted convention for magnetic flux direction, the flux emanates from each projection 38 upwardly through the aligned string 32 and returns, in the downward direction again through the string to the associated projection 40. A coil 42 wound in a first predetermined direction extends around all of the projections 38, whereas a coil 44 wound in the opposite direction extends around all of the projections 40. Coil 42 is in series with coil 44. Upward and downward motion of a string 32 associated with a particular pair of projections 38 and 40 will change the distance between the string and the projections 38 and 40 and hence will alter the magnetic reluctance between the projections. As the string approaches the projections (downward movement) the reluctance will decrease so that there will be an increase in upwardly directed flux through the projection 38 and an increase in downwardly directed flux through projection 40. The opposite will occur for upwardly directed movement of the string. For any particular upward or downward string movement, the voltages induced by the oppositely directed changes in flux in the oppositely wound coils will reinforce one another, and hence will produce an appreciable output voltage. As all of the strings cause similar flux changes, the output of pickup 34 will be a composite signal representing the upward and downward motions of all of strings 32. Stray electromagnetic signals will induce oppositely directed voltages in coils 42 and 44. Thus, stray electromagnetic fields produce little or no output signal.

The output or pickup signal may be sent to a conventional amplifier 46 and loudspeaker 48 (FIG. 4), desirably via a conventional free space communications link 50 such as a radio frequency link or the like. Preferably, the free space communication link and pickup are arranged to operate without any wired connection to either a fixed power supply or to the amplifier 46. Thus, those portions of the free space communication link 50 mounted to guitar 20 may be powered by a battery likewise mounted to the guitar. Pickup 34 desirably is connected to free space communications system 50 via the preamplifier 74 of the sustainer, further discussed hereinbelow.

The sustainer includes a driver 52. Driver 52 incorporates an elongated generally rectangular ferromagnetic element 54 (FIG. 3). Element 54 is a permanent magnet composed of a ceramic ferromagnetic material such as the material commonly available in the magnet trade under the designation "Ceramic-B". The magnetization of element 54 is directed so that the north-seeking pole of the element extends along one relatively long, narrow face 56 of the element and the south-seeking pole extends along the opposite face 58. Driver 52 also includes a drive coil 60 encircling element 54. Coil 60 is generally helical, the shape of the helix being distorted to fit closely around element 54. The axis of helical coil 60 extends in the pole to pole direction of element 54, i.e., between faces 56 and 58. Drive coil 60 has a ground connection 62, an end connection 64 opposite from the ground connection, and a center tap 66.

Appropriate means such as screws 66 or other conventional securement devices are provided for mounting driver 52 to the structure of instrument 20 at a preselected drive location along the longitudinal extent of strings 32. The drive location is preferably remote from bridge 28 and from headstock 30, and may be approximately midway between the bridge and the headstock. Thus, the drive location may be adjacent the juncture between body 22 and neck 24. The mounting means are arranged to secure driver 52 to the instrument structure so that the long dimension Z (FIG. 2) of element 54 extends in the lateral direction of the string array, and so that the north-seeking pole face 56 of element 54 faces upwardly towards the array of strings 32. As the long dimension Z of ferromagnetic element 54 is greater than the lateral extent W of the string array 32, the ferromagnetic element protrudes laterally beyond both edges of the string array.

With driver 52 is secured to the body, magnetic flux resulting from the permanent magnetism of element 54 impinges on strings 32. As best seen in FIG. 3, the permanent flux from ferromagnetic element 54 is generally co-directional with the flux in each rearward projection 38 on pickup 34. The flux in element 54 and in each projection 38 is upwardly directed. Stated another way, the flux in the driver ferromagnetic element is co-directional with the flux in the closest active portion or projection of the pickup. As best seen with reference to FIG. 2, the upwardly facing, north-seeking pole face 56 of ferromagnetic element 54 extends substantially parallel to an imaginary surface 68 defined by strings 32 at the driver location. Thus, the upper or string-facing surface 56 of element 54 has a slight upward bow adjacent its midpoint. This slight curvature matches the curvature of the imaginary surface 68 defined by strings 32 at the drive location, also visible in FIG. 2. Thus, the distance between the string facing surface 56 and the imaginary surface 68 defined by the strings is substantially constant across the entire lateral extent of the string array. Surface 56 of the ferromagnetic element is substantially devoid of appreciable projections extending towards the strings or notches extending away from the strings, at least within the lateral extent of the string array, and preferably beyond this extent as well. Thus, the permanent magnetic flux from element 54 impinging on strings 32 is substantially uniform across the entire width of the string array, and this uniform flux extends laterally beyond the string array.

As strings 32 are ferromagnetic, the flux from element 54 produces a constant attractive force on the strings. Magnetic flux generated by coil 60 will either oppose or reinforce the flux due to the permanent magnetism of element 54, depending upon the direction of current flow in the wi