05/065619

01/16/1973

08/20/1970

Download PDF 3711617       PDF help

Click for automatic bibliography generation

Columbia Broadcasting System, Inc. (New York, NY)

84/730

84/733, 84/738, 84/236, 84/735, 84/736, 984/320, 84/DIG.024

G10H1/055; G10C3/12

84/1.01,1.04,1.14,1.16,1.26,1.13,DIG.24,1.16,1.15 310/8.5,8.1

3366808	Keyboard key transducer	January 1968	Steward
3464531	MANUAL ELECTRONIC KEYBOARD	September 1969	Herr et al.

Wilkinson, Richard B.

Weldon U.

I claim

1. An electric musical instrument of the keyboard type, which comprises:

2. The invention as claimed in claim 1, in which said thump circuit includes a low-pass filter means, and further includes amplifier means additional to the amplifier portion of said amplifier and loudspeaker means.

3. A percussion musical instrument, which comprises:

4. The invention as claimed in claim 3, in which said thump circuit means includes a low-pass filter, and in which said thump circuit means does not include an inductor.

5. The invention as claimed in claim 3, in which said thump circuit means includes an amplifier additional to the one in said amplifier and loudspeaker means.

6. The invention as claimed in claim 3, in which envelope control means are interposed between said transducer means and said magnitude-determining means, said envelope control means including capacitor means charged in response to said voltage generated by said transducer means, and further including means to effect progressive discharge of said capacitor means to thus progressively decrease the magnitude of the musical sound signal received by said amplifier and loudspeaker means from said tone generator.

7. The invention as claimed in claim 3, in which a key is provided, and in which means are provided to effect striking of said blow when said key is depressed by the musician, thereby effecting said stressing of said transducer means.

8. The invention as claimed in claim 3, in which said transducer means is a monomorph transducer which is compressed upon a striking of said blow.

9. An electronic musical instrument of the keyboard type, which comprises:

10. An electronic musical instrument, which comprises:

11. The invention as claimed in claim 10, in which a diode is interposed in said circuit means between said capacitor and the output of said first Darlington.

12. The invention as claimed in claim 10, in which switch and circuit means are associated with said key to effect shunting of said resistor means in response to release of said key, whereby to rapidly discharge said capacitor.

13. The invention as claimed in claim 10, in which threshold circuit means are provided to effect charging of said capacitor to a value adapted to result in generation of a barely audible sound by said amplifier and loudspeaker means, even during those occasions when said key is struck so softly that said capacitor is not charged in response to generation of voltage by said piezoelectric transducer.

14. The invention as claimed in claim 10, in which a thump circuit is connected between the output of said modulator and the portion of said circuit means which connects said capacitor to the output of said first Darlington.

15. The invention as claimed in claim 14, in which said thump circuit comprises a series-related resistor and capacitor, said resistor and capacitor being in series with an additional resistor, and in which third and fourth capacitors are connected, respectively, to opposite sides of said additional resistor to pass the relatively high frequencies to the ground terminal of said power supply.

16. The invention as claimed in claim 10, in which means are provided to bias each of said first and second Darlington circuits to near cut-off.

17. An electronic keyboard instrument, which comprises:

18. The invention as claimed in claim 17, in which said thump circuits include low-pass filters, and further include amplifier means.

19. The invention as claimed in claim 17, in which said transducer support means is a mounting member adapted to shake in response to striking of each of said keys, and in which each of said transducers is oriented in such manner that it will generate a signal in response to such shaking.

20. The invention as claimed in claim 19, in which said mounting member is a wooden board, supported at the ends thereof.

21. The invention as claimed in claim 19, in which each of said transducers is a monomorph piezoelectric ceramic wafer the sides of which are adhesively secured, by non-conductive adhesive, to metal layers.

22. An electric musical instrument, which comprises:

23. The invention as claimed in claim 22, in which the series combination of a damper switch and a low-value resistor are connected in shunt with said high-value decay resistor, said damper switch being adapted to be operated by the musician.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of electronic musical instruments, and more particularly to the field of electronic musical instruments of the keyboard type. Stated more specifically, the invention relates to an electronic piano capable of producing both realistic piano tones and various other tonal effects.

2. Description of Prior Art

As stated in Markowitz U.S. Pat. No. 3,445,579, the problem of generating a "thump" or "knock" sound, simulative of the sound produced by a conventional piano when the hammer strikes the strings, is a major one. U.S. Pat. No. 3,445,579 teaches the use of soundboard and acoustic rod means for creating knock, and also teaches electronic knock-producing means including an inductor. Both of such means, however, are very much more expensive, and less satisfactory, than is the thump-producing means of the present invention.

Relative to the switching aspects of the present disclosure, large numbers of switch types have been employed in association with piano or organ keys. Switch means are shown, for example, in the above-indicated Markowitz patent. Reference is also made to Lund U.S. Pat. No. 3,251,923 for an illustration of the type of switch illustrated in the present specification, but which is associated with the key in a much more complex manner than that illustrated herein.

SUMMARY OF THE INVENTION:

A piezoelectric transducer is mounted on a somewhat flexible support, such as an elongated wooden board, in order to permit generation of knock or thump sounds without the need for an inductor. A threshold circuit is associated with the piano key in order to cause generation of a tone even when the pianist does not strike the key sufficiently hard to actuate the piezoelectric transducer. The switch portion of the threshold circuit, and the damper switch of the piano, are operated by a rod which projects inwardly from the end of the key through an opening in the hammer support. The control of the envelope of the generated tone is achieved by capacitor and switch means, in conjunction with impedance transformers which are provided on both sides of the capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view, partially in side elevation and partially in vertical section, illustrating one key of an electronic piano incorporating the present invention;

FIG. 2 is an enlarged view of a portion of the showing of FIG. 1, showing the key in the same position (prior to striking thereof by the pianist) as in FIG. 1;

FIG. 3 is a view corresponding to FIG. 2 but illustrating the positions of the parts while the piano key is being maintained fully depressed by the pianist;

FIG. 4 is a fragmentary view showing the transducers and the support therefor, as viewed from station 4--4 illustrated in FIG. 1;

FIG. 5 is a greatly enlarged cross-sectional view of the transducer and associated layers;

FIG. 6 is a block diagram of the circuitry of the present invention; and

FIG. 7 is a detailed showing of the impedance transformers, the envelope control circuit and the thump circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Although the present drawings and disclosure relate primarily to a single key, action, transducer, etc., it is to be understood that this is merely exemplary of all of the eighty-eight keys (or greater or lesser number of keys) in the complete piano. Stated otherwise, each key, action, transducer and circuit disclosed in detail herein is repeated 88 times in the completed instrument.

Each piano action, including the piano key and hammer but not the damper and not the supports, is preferably of the type disclosed in U.S. Pat. No. 3,270,608, for Piano Action, inventor Harold B. Rhodes. Such patent is hereby incorporated by reference as though set forth in full herein.

The apparatus shown in the present drawings includes fixed supports 10-14, inclusive, which are preferably formed of wood. Key means 16 are pivotally mounted on one support, number 13, in order to actuate hammer means 17. Each key means 16 includes a forward portion 18 adapted to be pressed or struck by the finger of a pianist in order to pivot the key means 16 about a fulcrum 19 to thereby elevate a rear portion 21 of the key means and which constitutes the actuating and brake portion thereof. The elevation of rear portion 21 operates through a felt 22 to pivot the hammer means 17 about pivot pin or axis 23. Such hammer means includes a head 24 bearing felt 25, and also includes a shank 26 and foot 27.

When the key is at rest, as shown in FIGS. 1 and 2, the rear key portion 21 rests on a stop 28 adapted to prevent excessive downward pivotal movement of such key portion and of the hammer means. Upon striking of the forward key portion 18 by the pianist, rear key portion 21 moves upwardly until the inner end of felt 22 engages a stop and brake region 29 of the foot 27 of the hammer means. A secondary stop 30, provided below forward key portion 18, does not become operative unless excessive downward force is applied to the key. Thus, the thrust is absorbed by the pivot pin 23, such pin 23 being fixed in position and being connected to a support 31 which, in turn, is suitably mounted on support 14.

The tine or reed 10, damper 20, inertia bar 13, pickup or transducer 11, and associated fastener and support means of Rhodes Pat. No. 3,270,608, are eliminated, being replaced by piezoelectric transducer means 32 and associated switching means and electronics. Thus, felt 25 on hammer 24 strikes the transducer 32 instead of striking a string or tine.

THE PIEZOELECTRIC TRANSDUCER MEANS, AND MANNER OF ASSOCIATION THEREOF WITH THE PIANO KEY

Each piezoelectric transducer means 32 is generally rectangular in shape, and is mounted (by a suitable epoxy or other adhesive) in a corresponding rectangular recess formed in the underside of a support means 11 (described hereinafter). Other shapes (such as discs) could be used.

Referring next to FIG. 5, there is illustrated a greatly enlarged cross-section view of the piezoelectric transducer means 32. Such means comprises a thin wafer 35 of a suitable piezoelectric material, for example, piezoelectric ceramics such as barium titanate, lead zirconate titanate, or lead zirconate. Wafer 35 is polarized in a direction (generally vertical) perpendicular to its own plane. In the illustrated embodiment, the wafer is so polarized that the upper surface thereof is positive.

The piezoelectric transducer wafer 35 is sometimes referred to, in the present specification and claims, as being "monomorph," thus distinguishing it from bimorph piezoelectric transducers. "Monomorph" is hereby defined as denoting a homogeneous or unitary construction of the ceramic, in contrast to the two ceramic layers employed in bimorph transducers. It is, however, pointed out that monomorph piezoelectric transducers may be employed in a stack or laminate (for example, to increase the generated voltage), without being bimorph. A primary distinction between monomorph and bimorph is that a monomorph transducer is employed by being compressed, expanded or sheared, whereas a bimorph transducer is employed by being flexed (bent or twisted).

Provided above and below wafer 35, in a laminated arrangement, are layers 36 and 37 of a suitable plastic such as that employed to form certain circuit boards, namely epoxy-impregnated fiberglass cloth. Layers 38 and 39 of copper or other electrically conductive material are provided, respectively, on the inner faces of layers 36 and 37 (for example, by electrodeposition). Such conductive layers 38 and 39 serve as terminals for the piezoelectric wafer 35.

Metal layers 38 and 39 need not be in electrical contact with wafer 35, but are instead secured thereto by epoxy or other non-conductive adhesive. This is because the capacitive coupling between the upper surface of wafer 35 and the layer 38, and between the lower surface of wafer 35 and the layer 39, provide the electrical "connections" required.

Conductive layers 38 and 39 are connected, respectively, to leads 41 and 42 illustrated in FIGS. 1 and 7, and which lead to the electronics portion 43 of the apparatus. Such electronics portion 43, described in detail below relative to FIGS. 6 and 7, may be mounted on a card (circuit board) disposed in vertical relationship on support 12 and secured in place by suitable means, not shown.

An additional layer of copper or other conductor, numbered 45, is provided (as by electrodeposition) on the upper surface of upper insulating layer 36 and is connected to provide a shielding function (against electrical interference) relative to the ceramic wafer 35 and associated conductors. The upper surface of such layer 43 is provided with adhesive and thus secured to support 11 in a recess therein as above indicated.

Preferably, layer 45 is (like 39) connected to lead 42, which is grounded. Such lead 42 is the sleeve portion of a coaxial cable, whereas lead 41 is the central wire thereof.

The lower surface 44 of the lower insulating layer 37 is planar and is the one which is struck by the felt 25 on hammer 24. The felt thus strikes a smooth surface, as distinguished from strings or a tine, and therefore will not become grooved or otherwise wear excessively.

With the described arrangements, the present monomorph piezoelectric transducer means 32 operates in compression, being squeezed when struck to thereby generate voltage.

Assuming that the action is initially at rest, as shown in FIGS. 1 and 2, striking of the forward key portion 18 by the pianist operates through felt 22 and foot 27 to throw the hammer means 17 upwardly until hammer felt 25 strikes transducer means 32. If the pianist then holds the forward key portion down, the hammer 24 and felt 25 thereon immediately stop in the position shown in FIG. 3, at the escapement distance E.D. from transducer means 32, due to the action between felt 22 and foot 27 as described in detail in the cited Rhodes patent. If desired, the escapement distance E.D. (FIG. 3) may be reduced to zero.

SWITCH MEANS

Proceeding next to a description of an exceedingly simple and economical form of switch means associated with the key means 16, this comprises an elongated pin 46 mounted on the inner end of the key means 16 generally coaxially thereof. Pin 46 projects through a clearance opening 47 in fixed support 14, and has a ball-shaped end 48 snapped through an opening in an insulating plastic plate 49 of the general type indicated in U.S. Pat. No. 3,251,923 cited above. Such plate is vertically oriented, being mounted on two flexible coil springs 51 and 52 formed of electrically conductive material. The bases of such springs are anchored on an insulating plate 54 (FIGS. 2 and 3) which, in turn, is mounted on the rear (inner) surface of fixed support 14.

The relationship is such that when the action is at rest, as shown in FIGS. 1 and 2, spring 51 engages a fixed conductor or terminal 56 whereas spring 52 engages a fixed conductor or terminal 57. Striking or pressing of the forward portion 18 of key means 16 causes upward shifting of plate 49 and springs 51 and 52 until such fixed conductors 56 and 57 are disengaged, the spring 52 then engaging another fixed conductor or terminal 58. Spring 51 and fixed conductor 56 thus constitute a single-throw switch, whereas spring 52 and fixed conductors 57 and 58 constitute a double-throw switch.

As indicated in FIG. 1, suitable conductors schematically represented at C (there being several conductors) connect the switches to the electronics 43. It is to be understood that there are suitable connections between the springs 51 and 52 and the conductors C, and between the respective fixed conductors 56-58 and the conductors C.

With the described switch construction, the key means 16 may be readily removed by merely snapping the ball through plate 49, lifting the key means off fulcrum 19 (and over the fulcrum pin associated therewith), and then sliding the key means outwardly.

DESCRIPTION OF THE ELECTRONICS

Referring to FIG. 6, the electronics (represented collectively at 43 in FIG. 1) are illustrated in block form as comprising the piezoelectric transducer means 32 (including the wafer 35, FIG. 5, of piezoelectric ceramic or other material) and which connects through an impedance transformer circuit 61 to an envelope control (or envelope generating) circuit 62. Such envelope control circuit, in turn, connects through another impedance transformer circuit 63 to the control (amplitude controlling) terminal 71 of an amplitude modulator circuit 64. The output of such modulator feeds through a mixing junction 72 to a power amplifier 66 and thence to a loudspeaker means 67.

A filter F and phase-shifter S may be interposed between junction 72 and amplifier 66 in the circuitry for all but the highest two octaves of the piano.

The relationship is such that the voltage at the control (amplitude controlling) terminal 71 of modulator 64 (supplied by the envelope control signal passing through impedance transformer 63) determines the amplitude (envelope) of the signal fed through such modulator 64 to amplifier 66 from a tone generator circuit 68.

Various types of tone generator circuits 68, and various types of modulators 64, may be employed in the present invention. Therefore, such circuits 64 and 68 will not be described in detail herein. One such circuit is described in detail in my co-pending patent application Ser. No. 58,239, filed July 27, 1970, for a Tone Generator. More specifically, element 16 shown in FIG. 1 of such co-pending patent application performs the functions of elements 64 and 68 of the present application. The amplitude modulator 64 may be in the form of a linear gate.

Of course, the tone generator 68 produces a differently-pitched tone for each key of the piano. Thus, all of the elements shown in FIG. 6, except the tone generator 68, are identical for each key. Such tone generator 68 changes from key to key only to the extent necessary to alter the pitch of the generated tone (from C to C sharp, etc.). As above-indicated, elements F and S of FIG. 6 may be omitted in the top two octaves of the piano.

The remaining circuit illustrated in FIG. 6 is the "thump" or "knock" circuit, numbered 69. Such circuit is connected at one end to the junction 106 between impedance transformer circuit 61 and envelope control circuit 62, and at the other end to the mixing junction 72 between modulator 64 and amplifier 66.

Stated briefly, the tone generator 68 supplies the musical tone which may, for example, be highly simulative of that generated by one of the keys (and associated action and strings) of a traditional (conventional) piano. Modulator 64, which receives the envelope signal fed from envelope control circuit 62 through impedance transformer 63, controls the envelope of the tone supplied from tone generator 68, again in a manner which may closely simulate that of a traditional piano. The thump circuit 69 (in conjunction with the mounting means described below) creates the initial striking, "thump" or "knock" sound which results in a traditional piano when a hammer strikes the string and thus shocks the sound board. The signal fed through power amplifier 66 to loudspeaker means 67 is thus a combination of that from tone generator 68 (controlled in amplitude at modulator 64) and that received from thump circuit 69. The result is the generation, in loudspeaker means 67, of a sound very closely simulative of that of a traditional piano.

By making appropriate circuitry changes, the sound produced may be that of a celeste, harpsichord, etc. Certain aspects of the present invention may also be employed, for example, to simulate non-keyboard types of percussive musical instruments including those adapted to generate bongo sounds, drum sounds, marimba sounds, vibraphone sounds, etc.

Proceeding next to a detailed description of FIG. 7, which shows the internal components of elements 61-63 and 69 (but not of elements 64 and 68) shown in FIG. 6, the impedance transformers 61 and 63 preferably comprise, respectively, first and second Darlington circuits 73 and 74. Such Darlington circuits effectively isolate the envelope control circuit 62, and assure that there will be fed into such envelope control circuit 62 a pulse the height of which is highly representative of the force with which the felt 25 on hammer head 24 strikes the piezoelectric transducer means 32. The Darlington circuits 73 and 74 are biased to near cut-off condition, since this permits a low power supply voltage to be employed and, furthermore, eliminates the necessity of providing offset voltages in the output of the second Darlington 74.

The positive terminal of the piezoelectric transducer means 32 connects through lead 41 (FIGS. 1 and 7) to the base of the first transistor in Darlington 73. Such lead 41 also connects through a resistor 76 to the junction between two additional resistors 77 and 78. The remaining terminal of resistor 78 connects at a junction 79 with the lead 42 (FIGS. 1 and 7) to the negative terminal of transducer 32. Junction 79 connects also through a resistor 81 to the emitter of the second transistor in Darlington 73.

A.D.C. power supply is provided, being schematically represented as a battery 82 the negative terminal of which connects to ground and the positive terminal of which connects to a positive lead 83. Such lead, in turn, connects to the remaining terminal of resistor 77 and, also, to both collectors of Darlington 73. A negative lead, numbered 84, connects to junction 79 and also to ground (whereby there is a circuit through ground to the negative terminal of battery 82).

Resistor 76 is a high-value isolating resistor having a resistance of a substantial number of megohms, thereby preventing any undesired loading of the piezoelectric transducer means 32. For example, the value of resistor 76 may be 10 megohms. Resistors 77 and 78 are bias resistors, whereas resistor 81 is a load resistor for the Darlington 73.

Relative to the second or output Darlington 74 (impedance transformer 63), the positive lead 83 connects (as in the case of Darlington 73) to both collectors. Negative lead 84 is connected through a load resistor 86 to the output emitter of the Darlington 74. The junction between such emitter and the resistor 86 connects to terminal 71 which is the control terminal of the amplitude modulator or linear gate 64 of FIG. 6.

The power supply 82 (schematically represented as a battery but which normally is not) is caused to have a very low internal impedance. Furthermore, the Darlington circuit 73 has a very low output impedance when a pulse is supplied thereto from the transducer. Both of these factors operate, as stated hereinafter, to cause a capacitor incorporated in envelope control circuit 62 to charge extremely rapidly upon striking of the transducer means 32 by the hammer. The sudden percussive response, created when the hammer of a conventional or traditional piano strikes the strings thereof, is thus closely simulated.

The Darlington circuit 73 has a very high input impedance, which input impedance (like the high resistance of resistor 76) prevents excessive loading of the transducer 32. Such transducer generates a relatively high voltage (for example, 10 volts) upon striking thereof by the hammer.

For reasons indicated below, the voltage supplied by the power supply 82 is caused to be higher than the voltage generated upon striking of the key portion 18 with the maximum force exerted by the pianist during normal playing. Stated otherwise, the pulse from transducer means 32 cannot be higher than the voltage supplied by power supply 82.

The value of resistor 76 may be made extremely high, such as 100 megohms. This high value, when it is employed, permits the pianist, when the escapement distance E.D. (FIG. 3) is zero, to create vibrato and other effects by varying the pressure on key portion 18 while it is in fully depressed condition.

The first Darlington 73 is connected to the envelope control circuit 62, the connection including a diode 87. The anode of such diode connects to the Darlington 73. Thus, diode 87 is oriented to pass a sharp positive pulse to the capacitor contained in envelope control circuit 62. The cathode of diode 87 connects to such capacitor, numbered 89, and also to a lead 88 extending to the input base of the second Darlington 74. The remaining capacitor terminal connects to a lead 91 which, in turn, connects through a resistor 92 to ground lead 84.

Another resistor, numbered 93, is connected between positive lead 83 and lead 91, whereas an additional resistor 94 is connected between lead 88 and lead 91. A further resistor 95 is connected between lead 88 and the fixed conductor (terminal) 56 which cooperates with flexible spring 51 in forming the single throw switch described relative to FIGS. 2 and 3. Such switch 51, 56 is the "damper" switch, there being one such switch for each key of the instrument.

There is, however, only one "sustaining pedal" or "loud pedal" switch for the entire instrument, such one switch being in series-circuit relationship with each of the damper switches 51, 56. The sustaining pedal switch is represented at 96 in FIG. 7 and is operated through a connection 97 by the sustaining pedal schematically represented at 98.

Both of the switches 51, 56 and 96 are shown in open condition, which is the condition which occurs when the piano action has been actuated by the pianist to the depressed condition of FIG. 3 and when the pianist has his foot on the sustaining pedal 98. The double-throw switch 52, 57, 58 (described above relative to FIGS. 2-3) is also shown in FIG. 7 in the position corresponding to the depressed piano key, namely the position of FIG. 3. The purpose of such switch 52, 57, 58 is described below.

Resistor 94 has a high value, for example 10 megohms, and may (if desired) be variable in nature in order to adjust the duration of the dwell period which follows striking of a key. Resistor 92 has a low value, in the range of hundreds of ohms, and may (if desired) be replaced by one or more diodes in series. Such resistor 92 is a biasing resistor, as is the resistor 93 which has a value in the kilohm range. As in the case of the resistors which bias the input Darlington circuit 73 to a low value near cut-off (for example, to about 1.7 volts), the relationship between resistors 92 and 93 is such that the output Darlington circuit 74 is biased to (for example) about 1 volt, again near cut-off.

The resistor 95 has a low value but one which is sufficiently high to prevent a "click" sound from being heard from the loudspeaker when the damper switch 51, 56 closes. Such value is not, however, so high as to prevent the listener from hearing a sound simulative of the engagement between a damper and a piano string.

In the operation of the envelope control circuit 62 and associated circuits, as thus far described, striking of the piano key means 16 causes the transducer means 32 to generate a control pulse. As previously stated, the relationships are such that this pulse cannot be higher than the voltage produced by the power supply 82. The pulse is isolated by the high value resistor 76 and by the high input impedance of the first Darlington 73. However, the pulse causes the voltage at the output emitter of Darlington 73 to be approximately equal to the peak value of the pulse.

Stated otherwise, the transducer (control) pulse applied to the input base of the Darlington 73 causes such Darlington to have a very low impedance between power supply 82 and diode 87, so that such power supply (which has a low internal resistance as stated above) very rapidly effects charging of capacitor 89 through a charging circuit including diode 87 and resistor 92. Such extremely rapid charging of capacitor 89 is highly simulative of the initial portion of the envelope of the sound produced by a traditional piano. Capacitor 89 is thus charged generally in proportion to the force applied to the piano key by the pianist, and thus to the force of striking of the hammer 24 against transducer means 32. It is a feature of the circuit that the very rapid charging of capacitor 89, to a high voltage, masks and renders harmless any subsequent and undesired pulses or transients.

The charge on capacitor 89 operates through the second Darlington 74 to determine the amplitude of the signal passed through modulator 64 (FIG. 6) from tone generator 68. Thus, when the capacitor 89 is charged to a relatively high value, exceedingly rapidly, a loud sound is suddenly passed through modulator 64 from generator 68. Such sound decays with the charge on the capacitor 89, and at a rate determined by the magnitude of resistor 94.

The magnitude of the voltage present on capacitor 89 is reflected at the control terminal 71 (FIGS. 6 and 7) of modulator 64 because, as previously mentioned, the output impedance of the Darlington 74 varies in general proportion to the magnitude of the voltage applied to the input base of such Darlington. The capacitor 89 being connected across such input base (in series relationship with low-value resistor 92), it follows that the Darlington will permit supply from power supply 82 to terminal 71 of a voltage representative of that present on capacitor 89.

After the hammer releases from the transducer means 32, an opposite-polarity pulse is created due to expansion of the compressed ceramic. However, this has no effect since the diode 87 prevents any discharge of capacitor 89 through the Darlington or any associated circuit. Instead, assuming that one or both of switches 95 and 51, 56 is open as shown, the voltage of capacitor 89 decays (as stated above) at a rate determined by the magnitude of high-value resistor 94. Such decay is normally selected to be representative of that present in a traditional piano. However, the decay may be made exceedingly slow, or fast, if desired. It is emphasized that the input impedance of the second Darlington 74 is so extremely high that the rate of decay of the voltage on capacitor 89 is determined substantially entirely by resistor 94.

As soon as the pianist releases his finger from the key, the damper switch 51, 56 closes to the position of FIGS. 1 and 2 to thus shunt (assuming that sustaining pedal 98 is not being pressed) the high-value resistor 94 and short the capacitor 89 through low-value resistor 95. The charge on capacitor 89 is therefore rapidly reduced to zero, so that the sound ceases.

As previously indicated, the Darlington 73 is biased to near cut-off in order to eliminate the necessity for offset voltages, and to minimize the drain on the power supply 82. This produces the additional beneficial action that, when the piano key is only relatively lightly pressed by the pianist, the Darlington 73 departs from linearity in a direction producing a somewhat smaller degree of charging of capacitor 89 (for a given low-force striking of the piano key) than would be the case if the Darlington were perfectly linear in this range. It follows that the pianist may strike the key harder in order to produce a lower-magnitude charging of the capacitor and consequent lower-volume sound. This permits the pianist to simulate a very light and feathery touch without striking the key extremely softly.

THE THRESHOLD CIRCUIT

In classical or traditional pianos, it sometimes occurs that the pianist, in attempting to achieve an extremely light or feathery touch, presses the keys so softly that no sound at all results. Even in the present piano, as thus far described, no sound will result if the key portion 18 is bottomed with a very high degree of softness. Because of the presence of the threshold circuit, such a condition cannot occur. Such circuit insures the presence at terminal 71 of a voltage sufficiently great to permit passage through modulator 64 of a signal from generator 68 which causes a barely audible tone to be generated in the loudspeaker 67, upon each and every bottoming (however soft) of the piano key portion 18.

The threshold circuit means comprises a capacitor 101 which is connected between ground lead 84 and the flexible spring or pole 52 of the double-throw switch 52, 57, 58 (FIGS. 2-3 and 7). Fixed conductor (terminal) 58 of such switch is connected through a lead 102 to lead 88, whereas fixed conductor (terminal) 57 of such switch is connected through a lead 103 and a battery 104 to ground.

The threshold circuit effects charging of capacitor 101 through a circuit including battery 104, lead 103, terminal 57, flexible spring (pole) 52, capacitor 101 and ground, every time the key is in rest position (FIGS. 1 and 2). Upon striking of the key, the spring or pole 52 engages contact 58 (as shown in FIG. 7) and places capacitor 101 in parallel with the series combination of capacitor 89 and low-value resistor 92. The polarity of battery 104 is such (terminal 57 being connected to the positive battery terminal) that the charge on capacitor 101 aids that on capacitor 89.

The magnitude of the voltage produced by power supply 104 is so selected that, during the great majority of time that the instrument is being played, the voltage present as the result of connection of the capacitor 101 to capacitor 89 will be of little or no significance. Stated otherwise, the voltage on the capacitor 101 is normally, in effect, overpowered by that on capacitor 89.

However, in those instances when the key is pressed so very softly that no voltage is impressed across capacitor 89 from elements 32 and 61, the closing of pole 52 to terminal 58 causes capacitor 101 to charge the capacitor 89. The charging is to such value that the parallel combination of capacitors 89 and 101 creates sufficient voltage at the input base of Darlington 74 that modulator 64 will transmit a barely audible tone from tone generator 68 to loudspeaker 67. It thus becomes impossible for the pianist to press a key completely down without creating a sound in the loudspeaker 67. The resulting soft sound decays at a desired rate which is dependent upon the magnitude of resistor 94.

Switching means may also be provided to cause generation of organ sounds with sustained tones, as distinguished from decaying tones. This may be done in various ways, including (for example) providing switch means to connect an organ tone generator to amplifier 66 (independently of modulator 64) each time the key is depressed.

THE KNOCK OR THUMP PRODUCING MEANS

Although the control pulse generated upon striking of the transducer means 32 by hammer head 24 is extremely sharp, as is desired, this does not completely simulate a traditional piano because such a piano generates certain mechanical "thump" or "knock" sounds upon striking of the strings by the hammers. Such sounds, in a conventional piano, result from "shocking" of the sound board, by the strings, when the strings are struck. Applicant has discovered that very realistic thump sounds may be generated, in an inexpensive, controllable and simple manner, by deriving from the piezoelectric transducer means 32, and the associated mounting means, a damped oscillation, and filtering out the relatively high-frequency signals. Such damped oscillation is amplified and then combined with the musical signal on the output side of modulator 64 to thereby generated in loudspeaker means 67 a very realistic knock sound of a traditional piano.

The piezoelectric transducer means 32 is, in accordance with the present invention, mounted on a support 11 which is slightly flexible in the direction of hammer movement. The result is that there is a slight amount of movement or "shaking" of the piezoelectric transducer means 32 when the hammer strikes it and thereafter. Because such piezoelectric transducer has mass, the inertia of the transducer causes generation of an oscillatory "pulse" by the transducer when it is mechanically shaken with the flexible support 11.

Stated otherwise, the transducer means 32 reacts differently (when struck by the hammer 24) when it is mounted on a slightly flexible support 11 than would be the case if the support 11 were a very large and heavy mass. Even after the hammer disengages from the transducer means 32, the mechanical vibration present in the support 11 produces a pulse-generating effect because the transducer means has mass and the support 11 "pushes" on the transducer means and therefore generates a voltage signal therein. The pulse or signal generated in response to shaking of support 11 is very weak in comparison to the voltage generated by the transducer during the instant of striking by the hammer.

Referring to FIGS. 1 and 4, the slightly flexible support 11 preferably comprises an elongated wooden board (a preferred wood being maple) the ends of which are secured to upstanding supports 10 provided at opposite ends of the keyboard. Such mounting of the board may, for example, be by means of angle brackets 99 which are secured by screws 100 to board 11 and to end supports 10.

As an example, the board 11 may have a dimension of about one inch in a direction parallel to the hammer-striking direction, and a dimension of about two inches perpendicular to such striking direction.

The damped "thump" oscillation generated in the transducer means 32 (due to shaking thereof) is, as shown in FIGS. 6 and 7, fed through the first impedance transformer circuit 61 (including Darlington 73) to the junction region 106 between such impedance transformer 61 and the envelope control 62. It is then passed through a lead 107 to the thump circuit 69, and thence to the junction terminal 72 at which the thump signal is combined with the output of modulator 64 and thus fed through amplifier 66 to loudspeaker means 67.

The thump circuit 69 comprises a capacitor 108 and a resistor 109, the former being adapted to decouple any direct current and the latter being adapted to prevent excessive loading of (diminution of) the pulse supplied from elements 32 and 61 to the envelope control circuit 62.

Circuit 69 additionally comprises a low-pass filter, such as the illustrated pi filter. Such filter includes a resistor 111 which is connected between terminal 72 and the series combination of capacitor 108 and resistor 109. Connected to the opposite sides of resistor 111 are capacitors 112 and 113, the remaining terminals of such capacitors being grounded. Such capacitors are adapted to pass to ground the relatively high frequencies present in the pulse transmitted from elements 32 and 61 as the transducer means is struck by the hammer.

An amplifier A is interposed between the pi filter and junction terminal 72.

The magnitude of resistors 109 and 111, or the degree of amplification, may be varied to increase or decrease the "thump" sound supplied to terminal 72 and thus to the loudspeaker means 67. Resistors 109 and 111 may be made so large (or the amplification caused to be so small) that the listener is not conscious that there is a thump sound, even though the generated tone is more simulative of a traditional piano than would otherwise be the case. The resistors 109 and 111 may also be made sufficiently small (or the amplification caused to be so great) that the listener is definitely conscious of the thump sound. Preferably, resistors 109 and 111 (and the degree of amplification) are set to such values that the traditional piano is most closely simulated. It is to be noted that, in a traditional piano, the degree of thump is different at different portions of the scale.

The described mechanical vibration of the mounting board 11 may create thump or knock signals not only in the transducer 32 actually being hammered, but also in adjacent transducers. This, however, is of no importance to the listener, who is unaware whether the thump sound is coming through the electronics associated with one key, or the electronics associated with adjacent keys. Of extreme importance, however, is the fact that there is no cross-talk relative to the signals produced by envelope-control circuits 62, from one key to the next. This is because the high voltage pulse is very much larger than the thump voltage, and the high voltage pulse is generated only by a direct hit. The threshold of amplitude modulator 64 is so adjusted that no sound will pass therethrough unless (a) the associated transducer is actually struck, or (b) the associated key is fully depressed to thus operate the threshold circuit described above. Thus, striking of one key does not cause the generation of a musical tone by the electronic circuitry associated with any other key.

The thump or knock characteristics may be varied not only by means of thump circuit 69, but in other ways. Thus, for example, the characteristics of mounting board 11 may be altered. The felt 25 may be made hard or soft, or covered or replaced by rubber. In some types of instruments the felt 25 may be replaced by wood.

The described thump or knock producing means is not only very simple and satisfactory, but also very economical. One reason for the economy is that there is no requirement for any inductor, which is a relatively expensive component.

In another form of the circuitry, not shown in the drawings, the output of thump circuit 69 may be coupled through an amplifier and several coupling capacitors (one on each side of the amplifier) to the control terminal 71 to the amplitude modulator 64. The initial percussive effect is thus further augmented. In such a circuit, there is still a connection from the thump circuit to junction terminal 72.

PHASE-SHIFT VIBRATO

There will next be given a description of the elements F and S shown at the lower portion of FIG. 6. Element F is a suitable filter adapted to separate the fundamental frequency from the various overtones or harmonics. The fundamental is passed directly to amplifier 66, through the lower lead L. The overtones or harmonics, on the other hand, are fed through a phase-shift vibrato circuit S before being supplied to the amplifier 66.

The phase-shift vibrato S is set to operate at a very low frequency, such as one-third cycle per second.

The described circuitry produces a slow variation of the frequencies of the overtones or harmonics, but no variation in fundamental frequency. In this manner, the sound produced by the present instrument is more closely simulative of that produced by a traditional piano.

In a traditional string-type piano, the overtones are frequently not true harmonics, that is to say, that the overtone frequencies are not exact multiples of the fundamental frequency. On the other hand, the tone generator 68 is frequently such that all overtones are true harmonics. Reference is made to my above-cited patent application for Tone Generator.

By slowly varying the frequencies of the harmonics from tone generator 68, relative to the fundamental therefrom, the somewhat off-pitch effect of traditional piano overtones is simulated.

It is to be noted that the filter F is not necessarily a separate element, but may be built into the circuitry described in my co-pending patent application for a Tone Generator. Also, as above noted, the elements F and S are not required in the top two octaves of the piano.

SPECIFIC EXAMPLE

The following specific example is provided for illustrative purposes only, and is not to be interpreted as limiting the scope of the appended claims.

Resistor 76 10 megohms Resistor 77 10 kilohms Resistor 78 800 ohms Resistor 81 10 kilohms Resistor 86 10 kilohms Resistor 92 600 ohms Resistor 93 22 kilohms Resistor 94 100 kilohms to 20 megohms Resistor 95 2.2 kilohms Resistor 109 100 kilohms Resistor 111 50 kilohms Capacitor 89 0.47 microfarads Capacitor 101 0.02 microfarads Capacitor 108 0.1 microfarads Capacitor 112 0.01 microfarads Capacitor 113 0.01 microfarads

Each of Darlington circuits 73 and 74 is a 2N5305, made by General Electric.

Diode 87 is a 400 volt silicon diode.

Power supply 82 is a Hewlett Packard Model HP-721-A supplying a voltage of 25 volts D.C.

The transducer 32 includes a ceramic wafer formed of barium titanate. Such wafer has the following dimensions: length 3/8 inch, width 3/8 inch, height 1/16 inch.

The foregoing detailed description is to be clearly understood as given by way of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.