US2314496A - Electrical musical instrument - Google Patents

Description

L. HAMMOND ELECTRICAL MUSICAL INSTRUMENT March 23, 1943.

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ELECTRICAL MUSICAL INSTRUMENTL" Filed June 2e, 193s 4 sheets-sheet 2 6o 50 zA 6o 56 78 76 /oo 73 76 /oo 741s, 74152741537415t 7415,4 5 74;; 7417 7413 74E, M1; F5

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ELECTRICAL MUSICAL INSTRUMENT` Filed June'z'e, 1959 4 sheets-sheet 4 Patented Mar. 23, 1943 UNITED STATES' PATENT OFFICE ELECTRICAL MUSICAL INSTRUMENT Laurens Hammond, Chicago, Ill. A Application Junev26, 1939, Serial No. 281,077

(Cl. SLi-1.25)

20 Claims.

My invention relates generally to electrical musical instruments, and more particularly to an improved instrument of this character in which the tone signals are electrically produced by rotating tone Wheels used either as generators, or as modulators of a super-sonic carrier frequency.

It is a comparatively dioult and expensive operation to form tone wheels of rotary electromagneticor electrostatic generators or modulators with such accuracythatthe signal produced contains only the desired fundamental frequency and a `series of predetermined harmonic frequencies, and which vis free-'from objectionable noise. In endeavoring toform-the tone wheel-in such manner as to generate not only-the fundamental, but also-the higher harmonic frequencies to produce a tone of complex quality, it is found to be-nearly impossible to eliminate from the signal various -noises and unwanted frequenciesby any means which can be used practically in the quantity production of-suchinstruments.'V y i' It is of course desirable, inth'e rendition of certain kinds of music,` that the tones be rich lin harmonics, and unless the rotating generator -or modulator which supplies the fundamental-'frequency can also be made to supply'the higher harmonic frequencies, it' is necessary to provide a relatively expensive switching mechanism whereby the harmonics may be derived from other generators and combined with the fundamental frequency to produce a note of the desired'tone quality. In order to provide a full range of liarmonics in an instrument of the latter character, a relatively large number of generators are lre'- uuired, more than the'numher required for'supplying only the fundamental frequencies. To re'- duce thenurnber of generators requiredpractical considerations dictate the use' of harmonics which are of the tempered musical scale.

' In the present invention, I have provided means whereby a rotating generator may bemade to supply a signal which is rich in harmonics, and which, nevertheless, can be very easilyand iriexpensively manufactured; Although th'einstrument may be made without the use of' switching mechanism for adding harmonics, I hav'eillustrated the invention as' including ccuplerswitching means. When the' latterA are used, the tone wheels may be designed to' generate less-harmonically developed tones,l and the coupler'switche's used to build up the desired degreepfbrightness in the-tone quality, thereby increasing'lthe range of tone coloring available.

It is thus an object of my invention to provide an improved electrical musical instrument in which rotary generators are employed to provide tone signalseach consisting of a fundamental frequency and a large Anumber of partiels, but whichy is free from .noise and extraneous frequencies. n v

`A further object is to provide an improved electrical musical-"instrument incorporating rotary signal generatorsv in which all of. the generators rotate ata speed in the order of seven revolutions persecond. -v-

A -further...objectis to provide an vimproved electrical musical instrument .employing :rotary signal generators whichV are utilized to modulate a;radio frequency carrier. '.wave, and in which a radio .receiving set* mayibe employed to detect and amplifythe modulated carrier wave.

A further object .is toprovide.. an improved electricall musical instrument in which the attack and decay of the notes is controlled by the operation ofthe playing keys.' l A-further object is .to'provide an improved electrical musical instrument 'playable by keys, wherein the intensityof the notes sounded may be ldetermined `bythe extent of depression of the keys.:

A furthenobject is to provide an improved electricalmusical' instrument in-Which the tone quality 'may .be varied throughoutV a considerable range, and vby which .tones 1 of high harmonic development may be produced'.l Y

A further object is to provide an improved tone wheel signal generating system inwhich-certain inaccuracies'in-the form and speed of rotation.` of the tone wheels are permissible, andin fact, are

desirable. f i

A further .object is to provide an improved electrical musical instrulment which is touch responsive..4 -l' A further electrical musical instrument which may be manufactured atvery-fsmall expense and which will produce beautifull rich :vibrato tones,i and by which the player may adequatelyfrender the great works of-classical and modern composers.

A'A furtherif'object isl to" provide animproved means for producing avibrato effect.

Other objects will appear from the following description, reference-being had to the accompanyingdrawings in which: Figure l is asche'matic diagram of representative portions --of the instrument,a portion of the keybo'ardand theI generating system being shown in transverse section;

Figure 1a is a fragmentary sectional view taken on the line Ia--la of Figure l; I

'object is to provide an improved4 Figure 6 is a wiringdiagram showingreplesentative portions of the key switch circuits;

Figure 7 is a fragmentary sectional View showing a vibrato producing apparatus;

Figure 8 is a transverse sectional view takenon the line 8--8 of Figure 7;

Figure 9 is a fragmentary sectional viewv of a modified form of vibrato producing apparatus;

and Figure 10 is a transverse sectional view tak on the line llll0 of Figure 9, and showing a plurality of the modified vibrato producing devices together with a control .circuit therefor.

In general, the instrument comprises a tunable radio frequency oscillator which supplies a carrier wave to a key controlled variable condenser of unique construction,A the radio frequency carrier-wave passing through this condenser upon operation of the key, to be modulated by passingv through a circuit having a capacity varied by a rotating tone wheel. The modulated carrier. wave thus produced is demodulated, amplified, and translated into sound.

One of Athe `distinctive-features .of .the invention is that all of the-rotary modulators, or tone wheels, rotate at a speed near seven revolutions per second, so that any irregularities in the tone wheel form will result in introducing vibrato frequencies rather than dissonant and noise frequencies. y

Referring to Figure 1, the -instrument comprises a shielded radio frequency oscillator I0 which may be of any suitable construction, shown as comprising an electron discharge device l2 having a tuning mesh consisting of an inductance I4 and variable condenser IE in its output circuit. The output of the oscillator l0 is connected by a conductor I8 with a metallic bus bar 20, so that the potential of the latter with respect to ground is varied. at the radio frequency of oscillation, The bus bar forms a -plate common to a plurality of variable condensers, and for this reason, the oscillator l0 should be of a type well known in the art, whose frequency of oscillation is not appreciably affected by such variation in the impedance of its output circuit.

A wedge-shaped sheet 22 of mica, micarta or the like, is secured to the bus bar 2B and is adapted to be engaged by exible metal strips 24 which are attached to each of the keys of the manual. In Figure 1, but two keys, E1 and Ez are illustrated as representative of the '.72 or more keys of the complete instrument. These keys are provided for controlling the notes E1 and Ez respectively of the tempered musical scale which have frequencies 82.407 and 164.81 respectively. The keys are diagrammatically indicated as vp ivoted upon a common rod 26 and are normally held in uppermost position by compression coil springs 28. A suitable felt stop strip 29 attached to a rail 3U limits upward movement of the keys, while their downward movement is limited by a felt strip 3|. The exible metal strips 24 are secured to the keys by insulatingvpads 32, being thereby suitably insulated from any metal portion of the keys.

When the keys are in their uppermost positions, the iiexible metal strips 24 make contact with a grounded conductor 34. As the key is depressed, the ground connection with the strip 24 is broken and the latter is pressed against the insulating sheet 22 so that it is progressively flexed and flattened against said sheet. The extent of depression of the key thus controls the capacity of the condenser formed by the iiexible strip 24 and the bus bar 20, and thereby determines the amplitude of the radio frequency signal transmitted through the flexible strip 24. The exible strips 24, as shown in Fig. la, may be tapered, or may be made of any other shape necessary to secure the desired variation incapacity as the key is depressed. Because the loudness of a sound does not vary directly with its amplitude, but instead varies substantially logarithmically, it will generally be desirable that the capacitance between the strip 24 and the bus 20`increase at an accelerating rate as the key isl depressed. This may be accomplished in part, or-wholly, either by .having the insulating sheet 22- vary in thickness, as indicated in Fig. 1, or by having the strip 24 vary in width, .or by both of these methods. Y

The iexible strips 24 may be made of sufficiently resilient material so that they are capable of returning -the keys, thus eliminating the necessity of providing other key return means, such as the springs 28.

ABy this arrangement the keys are touch responsive, that is, if the key is struck a sharp blow, thenote will commence with a percussive attack, while if the key is vpressed down rather slowly, vthe attack of the tone will be correspondingly slow. Furthermore, the intensity of the tones may be maintained at any desired level by holding the keys partially depressed. For the latter function it is desirable .that the key-return springs preferably be relatively strong and short, so as to afford a suiiiciently variable resistance to depression of the keys to make it feasible to hold the keys partially depressed.

The exible strips 24 of the keys Ei and E2 are Aconnected respectively with groups of switch contacts 36 and 38 by conductors 31 and 39 respectively, A similar set of switch contacts 40 are adapted to be connected by a conductor 4I with a flexible strip 24 associated with some other key such as A0. Each of the conductors such as 31, 39 and` 4| is connected to ground by a resistor 42 shunted by a condenser 43, thus `forming a shunt impedance load for `the key completed circuit. The contacts 35, 38 and 40 are adaptedv to be engaged by flexible switch arms 46, 48 and 50 respectively, there being such switch arm for each of the contacts of these groups, although only one switch arm for each group is shown in Figure 1. The switch arms form parts of two-Way switches, the other contacts of the switches being provided by grounded contacts 52.

Each of the switch arms 5i! is connected to an electrode 54 which is secured in a metallic sleeve 56 (Figs. 3 and 4), but insulated therefrom by insulation 58. The insulation 58 is preferably a molded plastic which is formed in the sleeve 56 with the electrode 54 inserted therein so that the sleeve, insulation, and electrode, form a rigid unitary assembly. V

The sleeve 56 is supported in a bushsing 60 which is peened to a plate 62 forming a shielding housing for the tone wheel modulating apparatus. The sleeve 56 has a longitudinal groove 64 formed therein and is secured in adjusted position in the bushing 60 by a set screw 66, the end of which projects into the groove 64. provided with a ange 68 by which the sleeve may be engaged by a suitable tool used in adjustably positioning it in the bushing 60.

The electrode 54 preferably has a wedge-shaped end '|0 which lies in close proximity to the'path of movement of the extremities of projections 'l2 formed on its associated tone wheel, such as the tone wheel '|4E2.

Figure 6 shows a representative portion of the key switch control circuits, and for purposes of illustration, in addition to the key E1, keys E2, B2, E3, G3#, B3 and E4 are shown. These latter keys are for notes harmonically related to the note E1, being the second, third, fourth, fifth, sixth and eighth harmonics of the note E1.

It will be noted that each of these keys is adapted to be connected-to an electrode 54 associated with a tone wheel bearing a number correspond'- ing to that-of a key preceded by the reference character '|4, such connection being made by switches 48 adapted to be operated by a coupler or stop 49 for the fundamental lfrequency. Sim-- ilarly, switches 46 operated by the-stop 49- for the second harmonic, are adapted to connect the ilexible strips 24 with electrodes of tone wheels. generating twice the frequency'v of the fundamental, such connection being effected by operation .of the gang switch controlled by the stop- 49 for the second harmonic. In a similar manner, circuits and coupler switches may be provided so that depression of thekeys will render effective the tone wheels generating signals of fundamental fre- .s

quencies corresponding to the third, fourth, fth, and if desired, also the sixth, eighth, etcY harmonies of the notes of the depressed keys.

Since each of the tone wheels generates a sig.- nal which in itself is rich in harmonics, it will ordinarily not be practical to provide stops.,49 and gang switches operated thereby, for more than the first few harmonics. In fact, excellent musical results may be obtained without the use of any stop operated switches.

Asbest shown in Figure 2, there are a pluralityof tone wheels designated with the reference character '14 and followed by an appropriate note designation. Each group of these tonewheels is secured to a common shaft I5 which is mounted for rotation in bearings 13 carried by frame plates 82 which are insulated from and secured to the casing plate 52, and are held in properly spaced relation by suitable spacingrods 84 and notched strips 86, (Fig. 1), the notches of .which are held in engagement with the plates 82- by wires 88. There is a shaft TB provided for each of the notes of an octave and there will thus .ordinarily be twelve shafts `lt, of whichonly two `are shown in Figure 2. Each of the shafts thus .carry a num.- ber of tone wheels '|4 determined by the number of octaves encompassed by the instrument, plus one or more additional tone wheels, to provide the additional harmonic frequencies desired, if the stop arrangement of Figure 6 is utilized.

In the particular construction shown in Figure 2, six tone wheels are shown on each shaft- 15 and these tone wheels will thus-provide the signals for the corresponding notes of the six oc'- taves--for example, all of the Es. These tone wheels have therefore been given reference char# acters '|4Eo to MES, while the six-wheels on the adjacent shaft for the Fs of the instrument have been correspondingly marked |4Fo to MF.

The shafts 16 are rotated preferably from e The sleeve is also surfaces ofthe tone wheels T4E5 and 'I4Es and.

driven gear S8. i

The springs |00 are-under initial compression so that there is sufficient friction between their end turns and the tone wheels and gear 98 to provide a torque normally sufficient to drive the shaftV i5. In the event, however, that the rota-- tion of the ltone wheels should be obstructed in some manner, or an unusually abrupt change in speed ofthe driven gear should take place and thus put an unusual strain upon the driving connection,-the ends of these springs |00 may slip and thus prevent damage to the parts of this assembly. Y

The modulatedsignal -istransmitted from the tone wheel |4 to `shafts upon which they are mounted and is thus conducted tothe frame platesA 82, since all of thetone wheels and their driving gears and supporting frame plates are in electricalconnection, but are insulated from the casing 62. A shielded conductor |06 (Fig. 1) connects this assembly to the input of a radio frequency amplier and detector |01 (Fig. 1) shown conventionally as including a tube |08 for the radio frequency amplifying stage, and a tube |0 for the detector stage. l

, 'Iheradio frequency stage is providedwith tuning condensers ||2 and |4, by which the amplier may be tuned to the frequency of the radio frequency'oscillator l0. TheJoutput of the detector is supplied to suitable adjustable resonant output circuits H6, through a `volume control apparatus ||8 vto a power amplierlZll, and hence to an. electroacoustic translating means vsuch as aspeakerv |22. l

,Since any suitable radio receiving Vapparatus maybe utilized, Yit is believed-to be unnecessary to .describe these ,partsin` detail. I n fact, in some installations, it ,may be practical to couple the conductor, let 4to the antenna circuit forming the inputofaA customary radioreceiving set. In such case, by tuning the receiving set to the frequency generated f by lthe"oscillator it, the signals supplied .by the conductor l Amay beeconomically ampliiiedand. translated into sound.

I, As previously indicated, one of the `novel featlflreslofmy invention resides in the fact'that all of the. tone wheels arerotated ata frequency near seven revolutions per second, by Virtue of wl1ichit'is possible to manufacture the instrurn'ent 'at' very'lowcost. The low cost is possible because itis not necessary to make andl assemble the tone wheels with ahigh degree of precision,

. asis necessary when the tone wheels are rotated at other speeds.

- .From numerous experiments with tone wheel "c pickups.4 I have found that it is extremely uit to cause the wheelsto generate only the quencies,correspondingto the tooth formation whenever the tone wheel irregularities are so formed asto produce asignal having a high harmonic content. r

From the general theory of vtone wheels, it apcircuit of the pickup device, which frequencies are exact harmonics of the speed of rotation of thev wheel. For example, a wheel having sixty- 'four teeth rotating at 20 c. p. s. is capable of producing frequencies of 20, 40, 60, 80, etc. c. p. s., the sixty-fourth harmonic, of 20,: or 1280 cycles being one of the frequencies of this series. If the wheel could be manufactured with mathematical accuracy, the only frequencies present in the output would be 1280,2560, 3840, c. p. s. all being harmonics of the 1280 cycle frequency. Unfortunately, however, practically unavoidable inaccuracies in the form and mounting of the wheels causes other frequencies to appear. in the output, these additional frequencies differing from the intended frequencies by steps of 20 cycles, the rotation frequency.' Thus, instead of getting 1280 cycles withv no..other frequencies near this value, it will be found that sum'and difference frequencies between the'rotation frequency of 20 cycles and the fundamental tone wheel frequency `of 1280 cycles, such as 1240, 1260, 1300, 1320 cycles, etc., will appear, these latter frequencies being musically undesirable and constituting noise with respect tothe intended frequencies.

On the other hand, I have found by extended experimentation with vibratos and tremulants, that very desirable musical effects can be obtained by varying the frequency of a musical tone through a small frequencyrange at a rate of about 7 c. p. s. This-variation may be discontinuous, i. e., laccomplished by a sudden change from one frequency to the other,*or may be made accurately at a sinusoidal rate:

The theory'of'cyclical variations in frequency of a musical tone at a vibrato periodicity has been investigated, disclosing a curious "'fact' from mathematical considerations. Assuming that a generator, such as a vacuum tube oscillator, is oscillating at an average frequency of 1000 c. p. s. and that this frequency of oscillation is periodically altered by connecting and disconnecting an additional condenser in the tuning circuit, cyclically, seven times per second.- Upon analyzing the output of such oscillator, using a harmonic analyzer to determine what frequencies are present in the output, it is found that this 'experiment bears out the theory that the output contains no frequency of 1000 cycles because all frequencies present in thev output must bemultiples of seven, and 1000 is not a multiple of seven, Thus, frequencies close to 1000, but divisible by seven, such as 'frequencies of` 994, 1001, 1008, 1015, etc., may appear. The energy distribution on these various frequencies will be determined by the amount of frequency shifting which is caused seven times per second by'a frequency changing devicesuch as the intermittently connected condenser in the tuning circuit of the oscillator. Since such frequency shifting device operates in a discontinuousmanner, and not sinusoidally, other frequencies in the neighborhood of the harmonics of 1001, namely, 2002, 4004 c, p. s. etc., and other frequencies which differ from the frequencies 2002 and 4004 by the number seven, as for example, 2009, 2016, etc., and 4011 4018, etc., may appear.

An analysis of the effects obtained using tone wheels revolving at speeds in the order of seven revolutions per second may be deduced from the general considerations above pointed out. For example, in a generating system having a tone wheel which is rotated at seven revolutions per second and has six teeth, all of the frequencies generated must be multiples of seven. If the teeth are evenly spaced and the wheel is made with mathematical accuracy, the only frequencies generated would be six times sevenor 42 c. p. s., together with harmonics thereof, such as 84 cycles and 126 cycles, etc. If, however, the teetlrare unevenly spaced, there will still 'be forty-two impulses per second on the average. But, the irregular sequence of impulses may be analyzed into different steady state frequencies which differ from 42 cycles, 84 cycles, or 126 cycles by seven or multiples of seven. Thus, 35 cycles, 49 cycles, 56 cycles, 91 cycles, 98 cycles, 119 cycles, 133 cycles, 140 cycles, etc., may be present.

Irregular spacing of the teeth in such tone wheel will be equivalent to varying the frequency cyclically seven times per second, and the m11- sical effect in the tone which is-produced will be perceived simply as a vibrato effect. If the wheel is'not concentric, there will be a change in amplitude of the signal at` a rate of 7 c. p. s., which will give rise to an effect which is usually referred to as a tremulant, because the change is particularly one of amplitude.

The .net result of operating all of the tone wheels of the musical instrument herein dlsclosed, at Ava rate of about' sevenrevolutions per second, will lbe that the wheels may be made inaccurately and still produce `musically-desirable tones. For this reason, the teeth or projections 12 on the tone'wheels 14 are preferably intentionally spaced closer together on oneside of the wheel'than they are on the other, so that as the wheel is rotated at a generally uniform speed, the eifect'of vibrato will be automatically introduced.

In designing the gearing for driving the tone Wheels of the instrument, it will of course be impossibleto have all of the tone wheels revolve at exactly seven revolutions per second because the intervals of the tempered musical scale could not thus be obtained. This is an advantage rather than a detriment, since it will give rise to different vibrato rates for different notes. This isa highly desirable effect when two or more notes, as in a chord, are played simultaneously, since it tends to enable the listener to perceive the separate notes, and adds greatly to the richness and fullness of the chord.

In the chart, Figure 5, I have tabulated some of the physical constants which may be utilized in constructing'- the generator assembly, this chart applyingl particularly to the lowermost octave of the instrument, but by extrapolation being applicable Ato the complete assembly.

Column I gives the note numbers commencing with the lowermost note which may be' played on the instrument. Column II gives ,the corresponding musical designations of the notes, while column III gives the theoretical frequencies of the notes -of the lowest octave. Column IV gives the number of teeth on the tone wheels 14 for the lowest octave notes, it being understood that the number of teeth on the tone wheels for the higher octaves may be determined by successively multiplying the numbers given in this column by the factor two. Column V sets forth the theoretical speed at which the tone wheels should be rotated in order to produce the frequencies -set forth in column III, while column VI is a tabulation of the gear ratios which may be employed in an instrument to cause the tone wheels to rotate at the number of revolutions per second set forth in columnVII. The gear ratios are computed upon the assumption that the synchronous motor driven shaft 90 rotates at a speed of 1200 R. P. M. Column VIII indicates the approximate deviation of the speed of revolution of the tone wheels set forth in column VII, as compared with their theoretically correct speed of revolution set forth in column V the deviation being indicated in fractional parts of the theoretically correct speed. v

It will be noted that the fractions of column VIII are in each instance very much smaller than the minimum difference in pitch which iS perceptible to even the most discriminative ear.

From column VII of this chart, it will be observed that all of the tone wheels rotate at 'speeds between the minimum of 6.4815 and maximum of 8.1667 revolutions per second, Aand that the average speed of the tone wheels is near seven revolutions per second. As above pointed out, the fact that all of the tone wheels do not rotate at exactly seven revolutions per second is an advantage rather than a disadvantage.

The figures tabulated in Figure 5, especially the figures in columns IV, VI, VII and VIII are intended merely as illustrative, it being understood that considerable variations from these values are possible without materially affecting the over-all results obtainable from the instrument.

As above pointed out, any inaccuracies in the shapes of the tone wheel will not introduce noise frequencies, but instead, will result in the introduction of vibrato frequencies. However, if these inaccuracies are not great, the vibrato effect will be very slight, and it may therefore bc desirable to provide means for introducing a more pronounced vibrato effect whenever such effect is desirable, due to the character of the music being rendered.

In Figure 7 is illustrated a simple mechanism for causing the introduction of pronounced vibrato frequencies, this mechanism comprising a counter-weighted disc |30 of magnetic material, eccentrically mounted upon a tone wheel shaft 16 so as to rotate therewith. There will be a disc such as |30, mounted upon each of the shafts 16 of the instrument. A shaft |32 having suitable bearings in the frame plates 82 carries a plurality of permanent magnets |34, which may be swung to the position indicated in full lines in Fig. 8, such that 'the poles of the magnet lie near the periphery of the disc |30. The shaft |32 is manually rotatable through an angle of approximately 90, so that the magnets |34 may be shifted from their operative positions, as indicated in full lines in Figure 8, to their inoperative positions, as indicated in dotted lines in said figure, whenever the pronounced vibrato effect is not desired.

It will be understood that as the disc |30 rotates adjacent the poles of the magnet |34, it will, due to its eccentricity, be attracted and thus accelerated by the magnet during approximately one half revolution, and will be retarded during the remainder of its cycle. This application of alternate accelerating and retarding forces to the disc |30 will result in a slight periodic change in the speed of rotation of the shaft 16, such variation being permitted by the flexible coupling provided by the springs through which the shafts 7S are driven. The variation in speed of rotation of the shaftsl 'I6 will of course be rotated clockwise (Fig. 8) rto swing the mag.

nets |34 to the dotted line position, in which position the magnetswill not have an appreciable effect upon v`the speed of rotation of the shafts '15. The degree ofthe vibrato eiect may of course be varied by positioning the magnets at any desired -position intermediate the full line and dotted line positions.

In Figures 9 and 10 is shown a modified form ofvibrato p'roducing mechanism, in which electromagnets, each comprising a core |40 and winding |42, are used in place of the permanent magnets. The cores |40 -are rigidly secured to adjacent frame plates 82 and havel poles |4| which lie adjacent the peripheries of the eccentric discs |30 secured to shafts 16. There is a small air gap between the poles |4| andtheedge cf disc |30 when the latter is closest to the poles, as shown in full lines in Fig. 10'. The windings |42 are preferably connected in parallel to a suitable source of current |43 through a controlling switch and rheostat |44. By means of this switch and rheostat, the degree, of energization of the windings |42 may be adjusted so that the degree of acceleration and retardation of the discs |30 may be varied to control the extent of vibrato.

In making commercial forms of the invention, it will be practical, to reduce the cost of the instrument, to omit the vibratov producing means shown in Figs. 7 to 10 inclusive. Instead, the tone wheels may intentionally be made with the teeth thereof nonuniformly spaced so that during each revolution of the tone wheel, the signal generated thereby will vary in frequency between a frequency which may be in the order of 11/2% greater than the nominal frequency for the particular wheel and a frequency which is in the order of' 11/% less than such nominal frequency. -Such tone wheels may be constructed in a number of different ways to cause the spacing of the projections (or other irregularities) thereon to be more closely spaced on one side of the wheel than upon the other, or to be otherwise irregularly spaced.

For example, a blank may be stamped in which the projections are non-uniformly spaced about the periphery of the wheel. Thereafter the blank may have its periphery ground while the blank is rotated about its center so that the pheriphery will be very accurately concentric with the axis of rotation.

For the manufacture of the tone wheels disclosed herein, the Atone' wheel blank may be stamped or hobbed with its peripheral projections 72 uniformly spaced about the periphery. Then, after the hub has been pressed into and peened to the tone wheel, the tone wheel may be located and clamped by means engaging its periphery, and the bore through the hub of the tene wheel bored or reamed so as to be eccentric with respect to the periphery of the tone wheel. If the tone wheel is about two inches in diameter, the bore should be in the order of .015 eccentric with respect to the periphery of the wheel.

Thereafter, the tone wheel may' be assembled on the shaft 16 and have its peripheral projections ground down until the tips of the proiections are concentric with the axis of rotation of the shaft. This grinding operation will result-in changing the eiective angular spacing of the projectionsabout the periphery of the wheel, so that adjacent projections on one side of the Wheel Will-.be spaced closer together than adjacent projections on the diametrically oppo- I site side of the Wheel.

This method of. manufactureresults in a tone wheel in whichthe ,spacing f the projections varies substantially harmonically. However, this is not essential, since the variation in spacing may be irregular. For example, the Wheel may have sectors in which the projections are uniformlyv spaced angular distances less (by approximately 11/2%) than the average spacing required, while in other sectors the projections .are uniformly spaced by angular. distances as those shown on Figs. 7 to 10, for causingva greater vibrato frequency shift.

It will therefore be apparent that by -thi method of forming the tone wheels and rotating them, the necessity for an extremely high degree of precision inthe form of the tone wheels and in their concentricity is avoided, whilel atthe same time the musical results obtained are greatly improved.

To those skilled in the art, it will be readily understood that the underlying principle-of the invention may be utilized in making various other types of electrical impulse generators for electrical musical instruments. For example, the principle may be utilized in the` productionof the discs and drums proposed for use in photoelectric systems, wherein the alternate` opaque and translucent portions, or the alternate dark and light colored portions, may be non-uniformly spaced about the periphery of the disc .or drum, and the latter rotated at a speed in the order of seven revolutions per second. Although, for purposes of illustration, the irregularities on the tone wheels have been described as projections, it will be apparent that these irregularities may be in the form of notches or apertures of a wide variety of different shapes and conformations, and that in certain constructions, the pickup electrode, or scanning arm, may be rotated While the part corresponding to the tone wheel is held stationary. As anA additional known equivalent construction, the tone wheel may be in the form of an apertured or notched dielectric disc.

In playing the instrument, the musician will actuate the keys in the manner of a piano. If the key is depressed slowly the rate of attack of the note will of course be correspondingly slow, and similarly, the rate of decay of the note is controlled by the rate at which the key is permitted to return to its normal position. AS a corollary, the intensity of notes may be controlled by the extent to which the keys are depressed, and in this respect, the instrument will be responsive to the touch of the player in a manner similar to that obtained in the piano, with the additional advantage that a tone may be made to'continue to Asound at'any desired intensity level merely by holding the key depressed to the desired extent,

-with the result that organ-like tones may be proy electrodes 54 which may be connected to the key. As a result, the radio frequency carrier Wave, as modulated by the varying capacity between the electrodeor electrodes 54 and their associated tone wheels 14, has its amplitude controlled by the extent of .depression of the key. It will be understoodthat the tone wheels 14 may be made inany one of a variety-.of different forms, the essential requirement being that the tone wheel be capable, by its rotation, of slightly varying the capacity betweenyit andv its associated electrode 54. I

In the course of playing the instrument, the vibrato effect may be increased, if desired, by adjusting the positionof the shaft I 32, if the construction of Figures 'I and 8 is employed, or by varying the .position of. therheostat |44 if the electromagnetic System of Figures 9 and 10 is employed. -As previously pointed out, the vibrato producing mechanisms of Figures 7 to 10 inclusive, cause alternate acceleration and retardation of the tone wheel carrying shafts, with the consequent introduction of a wide range of vibrato frequencies. Such wide vibrato is particularly useful in the rendition of music requiring string effects, but is generally advantageous in the rendition of any musical selection in which a rich full tone is desired to express warmth of feeling.

By means of the adjustable resonant output circuits H6, the tone color or timbre of the music produced, may be varied considerably to produce ltones similar to those produced by various orchestral instruments.- The tone color may of course also be varied by the use of a switching system of the general type illustrated in Figure 6, whereby depression of a single key will result in the transmission to the output of tone signals from a plurality of tone wheels. By thus supplying a large number of frequencies to the output upon depression of a single key, greater variations in tone quality are obtainable, especially in conjunction with the controls afforded by the adjustable resonant circuits I I6.

When an ordinary radio receiving set is used in place of the radio frequency amplier and detector shown as made particularly for the instrument, the intensity of the over-all output of the instrument may of course be controlled by the usual volume control device of the instrument, although ordinarily it will be desirable to provide an adjustable attenuation means, controlled by a swell pedal or the like, in the circuit between the tone wheel assembly and the radio receiving set. Similarly, when the instrument is thus used in conjunction with a radio receiving set, the adjustable resonant output circuits may be omitted. Furthermore, if desired, an amplifier and electromagnetic radiator may be connected to the conductor IGS so that there need not be any direct physical connection between the instrument and the radio receiving set.

It will be apparent to those skilled in the art that numerous other variations and modications of the invention may be made Without departing from the underlying principle thereof. I therefore desire by the accompanying claims to include within the scope of my invention all such variations and modifications as will readily occur to those skilled in the art and by which substantially the results of my invention may be obtained by substantially the same means.

Iclaim:

1. In an electrical musical instrument, the combination of a tone wheel forming part of a signal generator, resilient means for driving sai-d wheel at a speed in the order of seven revolutions per second, a member of magnetic material connected for rotation with said wheel, and means for exerting a magnetic accelerating force upon said member during a predetermined portion only of the rotative cycle of said member.

2. In an electrical musical instrument, the combination of a tone Wheel forming part of a signal generator, a shaft for said tone Wheel, resilient means to rotate said shaft at a speed in the order of seven revolutions per second, and means for exerting a retarding torque upon said shaft during a predetermined portion only of each rotative cycle of said shaft.

3. The combination set forth in claim 2 in which the means for exerting a retarding torque upon said shaft comprises a magnetic memb-er secured to said shaft, and a permanent magnet having its magnetic field traversed by said member upon rotation of said shaft.

4. The combination set forth in claim 2 in which the means for exerting a retarding torque upon said shaft comprises a member of magnetic material secured to said shaft, and an optionally energized electromagnet having its magnetic field in the path of movement of said member upon rotation of said shaft.

5. In an electrical musical instrument in which the tonesignals are electrically originated, the combination of a plurality of rotatable signal generators for producing electrical impulses of the frequencies of the different notes of at least an octave of the musical scale, and means for rotating all of said generators at speeds in the order of seven revolutions per second.

6. In an electrical musical instrument in which electrical impulses are originated under the control of keys, the combination of at least twelve signal pickup elements, a plurality of rotating members each having irregularities thereon rotated in paths passing close to one ofsaid pickup elements respectively to produce electrical signals in the latter, and means to rotate all of said members at speeds in the order of seven revolutions per second.

7. The combination set forth in claim 6 Wherein the irregularities on said members are uhequally spaced thereon.

8. The combination set forth in claim 6 in which the irregularities on each of said members are similar in conformation, but unequally spaced thereon.

9. The combination set forth in claim 6 in which the irregularities on each of said members are similar in shape, and in which approximately one half of the irregularities on each member are spaced closer together than the remaining irregularities thereon.

10. The combination set forth in claim 6 in which the irregularities on each of said members are similar in shape and in which each of said members forms one plate of a condenser, the

other plate of which is formed by its associated pickup element.

il. In an electrical musical instrument having a plurality of members, at least twelve in number, and each having irregularities formed thereon, a pickup element for each of said members, and means for rotating all of said members relative to said pickup elements at speeds in the order of seven revolutions per second, the irregularities on said members being spaced thereon at distances such that they will pass said pickup elements at average frequencies correlated respectively with the fundamental frequencies of the notes of the musical scale.

12. The combination set'forth in claim 11 in :hich the irregularities on said members are in the form of projections spaced along the peripheries thereof.

13. The combination set forth in claim 11 in which each of said members has either five, six ox' eight irregularities formed thereon, or a number of irregularities which is a multiple of five, six .or eight in which the multiplier is divisible by a power of two.

14. The combination set forth in claim 11 in which said members and their associated pickup elements form variable condensers, and in which there is a key controlled variable condenser connected to each of the pickup elements.

15. The combination set forth in claim 11 in which there is provided a source of radio frequency oscillations, key controlled means for selectively connecting said source to said pickup elements, and means for demodulating, amplifying and translating into sound the radio frequency signal which is modulated by said members.

16. A generating system for electrical impulses of a range of at least one octave for use as signal sources in an electrical musical instrument, comprising the combination of a plurality of tone wheels each having a plurality of irregularities which are non-uniformly spaced, means for rotating all of said wheels at speeds in the order of seven revolutions per second, and a pickup device cooperating with each 0f said wheels and aifected by the passage of said irregularities past it and transducing such effect into an electrical signal.

17. A generating system for electrical impulses of a range of at least one octave for use as signal sources in an electrical musical instrument, comprising in combination, a plurality of tone wheels of electrically conductive material having projections on the peripheries thereof, said projections being non-uniformly spaced on each wheel to an extent such that the angular distance between some adjacent projections differs from the distance between other adjacent projections by an amount in the order of 3%, means to rotate all of said wheels at speeds in the order of seven revolutions per second, and pickup electrodes respectively positioned closely adjacent the path of movement of the'projections on said Wheels.

18. A generating system for electrical impulses of a range of at least one octave for use in an electrical musical instrument comprising, the combination of a plurality of disc-shaped tone wheels each having unequally spaced projections along the periphery thereof, electrical pickup means located adjacent the peripheries of said wheels, and means for rotating said wheel past said pickup at speeds in the order of seven revolutions per second.

19. A generating system for electrical impulses of a range of at least one octave for use as signal sources for electrical musical instruments comprising, a plurality of tone wheels each having a plurailty of projections angularly spaced nonuniformly about its periphery, means to rotate said tone wheels about axes concentric with their peripheries at speeds in the order of seven revolutions per second, vand electric pickup devices respectively located adjacent the paths of movement of the projections of said Wheels.

20. In an electrical musical instrument in which are originated electrical signals of semi-tone frequencies extending throughout the range of at least one octave, the combination of at least twelve stationary pickup elements, members each having a major component of motion of approximately seven cycles per second with respect to one of said pickup elements associated therewith, said members having irregularities affecting said pickup elements to produce electrical impulses in the latter, and means for causing an additional component of relative motion between said pickup elements and said members once during each cycle of said major component of motion between said members and said pickup elements.I

LAURENS HAMMOND.

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