US2227068A - Electrical musical instrument - Google Patents

Description

Dec. 31, 1940. w, F CUR-ns 2,227,068

ELECTRICAL MUSICAL INSTRUMENT Filed April' 19, lass v sheets-sheet 1 jl'-l lf @MHM 1ra- E 5 l TIE' IN V EN TOR.

Dec. 31, 1940. l w. F. cuRTls 2,227,068

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ATTORNEY Dec. 3l, 1940. w. F. CURTIS 2,227,068

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ATTORNEY Dec. 31, w F CURTIS ELECTRICAL MUS ICAL INSTRUMENT FiledAbril 19, 1959 'l sheets-Sheet 7 IN V EN TOR.

WGS) BY d S) ATTORNEY.

Patented Dee. 31, 1940 UNITED STATES PATENT OFFICE ELECTRICAL MUSICAL INSTRUMENT Ware Application April 19, 1939, Serial No. 268,786

l2 Claims. (Cl. 84-119) My invention relates broadly to synthetic elec trical musical instruments employing a mult*- plicity of generators, and more particularly to those instruments where each generator originates several tone partials. 'I'he invention is K particularly directed to instruments in which the generators require a steady exciting or polarizing potential.

One of the objects of my invention is to provide a circuit employing a multiplicity of generators and wherein a generator is required to produce several partials simultaneously, the output of the generator being the sum of the outputs taken individually.

Another object of my invention is to provide a circuit arrangement for a generator having means for applying all pitch and amplitude controls to the steady exciting potential of a generator, thus avoiding the expense and the high frequency losses involved in the shielding which is made necessary by applying these controls to tone-frequency circuits.

Another object of my invention is to facilitate control of attack and decay rates in an electrical musical instrument.

Still another object of my invention is to provide means for controlling the variation in harmonic content when the pitch ls changed in a frequency responsive circuit.

A further object of my invention is to provide a circuit arrangement for an electrostaticgenerator system operating without harmonic generators above the range of fundamental frequencies generated and without producing sudden changes in harmonic content.

A still further object of my invention is to provide a circuit arrangement for an electrical musical instrument capable of inexpensive production on a quantity basis for producing a highly satisfactory synthetic instrument comprising a minimumnumber of operating parts.

Still another object of my invention is to provide a circuit arrangement for an electrical musical instrument employing a multiplicity of playing keys and in which the timbre is independent of the number of playing keys struck.

Another object of my invention is to provide a circuit arrangement for an electrical musical instrument in which the cumulative effects of a multiplicity of generators is obtained when striking a multiplicity of keys simultaneously.

A further object of my invention is to provide a circuit arrangement for a synthetic electrical .Y musical instrument'having additive circuits for "cumulative association with the same' generator whereby amplitudes of the tones are integrated 'according to the number of keys struck while th quality of the tones remains uniform. n Other and further objects of my invention reside in the production of an additive harmonic control circuit for synthetic organs `as set forth more fully in the specification hereinafter following by reference to the accompanying drawings in which:

Figure 1 schematically illustrates the control circuit of my invention in association with a single generator; Fig. 2 shows the equivalent circuit of the generator when producing a signal partial; Fig. 3 shows the equivalent circuit when several partials of different amplitudes are produced simultaneously; Figs. 4, 4a and 4b conjointly show the schematic circuit connections for a two manualelectrostatic organembodying the principles of my invention; Figs. 5 and 5a conjointly show schematic circuit connections for an electric organ embodying my invention and using only one contact for each playing key; and Fig. 6 shows a modified form of quality control for electric organs illustrating an arrangement of wave-filters in the amplier circuit and without the individually connected tappet controlled switches. y

My invention is directed particularly to an improved circuit arrangement for synthetic electric organ's of the type which employs a multiplicity of electric alternators for generating the notes of a musical scale and their associated partial tones. It can be applied to the timbrecontrol arrangements of any one of several of such instruments previously known to the art. When properly applied to such an instrument, it will result in improved performance and in considerable simplification.

Instruments of this class have great exibility and are capable of producing an almost limitless variety of musical effects. These great possibilities are not obtained without price; such instruments are inherently complicated and of necessity contain large numbers of electrical circuits. It is inevitable that preventing undesirable reactions among these circuits should be a diiiicult problem. Some of the pioneer workers in this field have made little attempt to solve it, merely indicating schematically how the partial tones are to be obtained and where applied, and,

quite justiably. leaving it to later inventors to work out control schemes whereby such reactions can be eliminated. Others have arrived at partial solutions, often at almost prohibitory expense in the way of auxiliary apparatus. My

invention aims at a complete solution of the reaction problem, together with a substantial reduction in the number of parts required, and without loss of useful ilexibility. It also aims at a musically satisfactory, controlled, and progressive change in timbre with pitch. Other aims will become apparent as the explanation proceeds.

Some of the requirements which must be iullled vif the instrument is to be a Worthy competitor of the pipe organ are as follows: (a) When any possible chord is struck, the timbre must not be noticeably .different from the timbre of a single note. (b) With the volume control left xed, the volume emitted should be approximately proportional to the number of notes sounding simultaneously. (c) 'I'he timbre must not change abruptly in going from one note to another.

Requirements (a) and (b) are both precisely fullled if, when a chord is struck, the amplitude of each component is equal to the sum of the amplitudes ofthe like partials of the different notes composing the chord, when each is struck singly. By like partials I mean to convey partials of different notes which agree in frequency. For example, the third harmonic of any C on the organ, the fourth harmonic of the G below the C, the second harmonic of the G above the C, and the fundamental of the G which is an octave higher yet, are all like partials as they have the same frequency. Any system in which the amplitude of each component of a chord is equal to the sum of the like partials, is hereinafter called an additive system. Obviously, it is very desirable to have an additive system. The diiliculty with requirement (c) generally arises near the high frequency end of the instruments range. It lis hard to justify providing high frequency generators which supply harmonics only, particularly in view of the fact that such harmonics do not exist in present acoustical instruments and theoretically seem of little value. However, the dropping oil' of harmonics at the high end of the register must be obtained without any sudden change of timbre.

It has been proposed heretofore to locate the necessary key switches and harmonics amplitude control elements in the output circuits oi the generators. Indeed, Where electromagnetic alternators are employed,- this is the only possible course, as reducing the field current of a magnetic alternator to zero does not reduce the output current to zero, hence control in the input circuit cannot be used. This expedient makes it possible to work out an additive system rather readily, but as the output circuit carries audio frequency currents, new difculties are introduced, to wit: (a)

The large number of extensive circuits tend to couple with each other, capacitively or inductively, and a very complete and hence bulky expensive system of shielding must be employed to eliminate spurious discordant notes. (b) This shielding involves losses in stray capacities and eddy currents. (c) These losses are diierent or different pitches, thus complicating the voicing" problem. (d) Condenser discharge methods of regulating attack and decay rates will not work on alternating currents, so that this convenient device cannot be applied directly.

My invention provides an additive system which is applicable to the input 'circuits of'electrostatic alternators, and he'nce does not incur any of these additional diificulties. It is also applicable to the polarizing circuits of polarized photocells, "and other manuals.

hence also to photoelectric organs. For simplicity however, only the electrostatic application has been described. Fig. 1 shows the circuit connections to a single generator according to my invention. For purposes of explaining my invention consider an instrument in which each generator is required to produce n partials, or less, in the case of the generators near the end of the fundamental pitch range of the instrument. These partials may all pertain to a single manual, or some may belong to one manual and others to In the case of electrostatic generators as illustrated herein, the alternating current output potential is proportional to the direct current input potential so that any relationships derived for the input potential of the generators will also be true of the outputs. Therefore, only the input potential Eg will be considered. Two cases Will be taken up. In case I al1 the resistors are equal, and in case II they are considered as unequal, but may also be equal, so that case I is actually included under case II. It is taken up separately for simplicity of explanation.

Case I R1=Rz=Rs= :Rn: (say) R From Fig. 2, the generator input potential when a single partial is being generated is where Ex is the potential to which potential-adjuster Vx corresponding to the given partial is adjusted.

From Fig. 3, the generator potential when a number of partials of different amplitudes are being generated simultaneously, is

E"- n n n TL The generator voltages for these partials taken one at a time are El, 7L 7L etc., as was shown above. The additive condition is therefore satised.

A numerical example of case I might be helpful at this point. Suppose that two manuals were desired, each utilizing six partials. Then if the total polarizing potential available were 480 volts lthe maximum potential available for any one partial would be 480+12 or 40 volts. While this seems rather small in the light of present v(il) practice, it must be remembered that my system permits a great reduction in the' capacitances in shunt with the generator outputs, and hence the generator output will be a larger fraction of the polarizing potential. If trouble is experienced in obtaining a sufiicient number of millivolts output to blanket the noise due to the connection to the rotors and the internal amplifier noise, the generators can be divided into groups, each group supplying its own input tube. The shunt capacitance would thus be reduced still more. 'Ihe input tubes could all work into a common amplifier.4

It will be noted later on that some help in this problem can be obtained by properly adjusting the resistors.

Case II Where R'=el iective resistance of the other n-l resistors in parallel Therefore Similarly,

35 Let these be fundamentals for the two manuals.

Maximum polarizing potential for these partials =1822= 40 volts These could be 2nd, 3rd and 4th harmonics for the two manuals-six-partials in all.

EW: E9 Xvz-l Maximum polarizing potential for these partiels `=2-849=20 volts These could be 5th and 6th harmonics forthe two manuals.

Dropping the numerical example again temporarily, we proceed to prove that the system is additive. From Fig. 3

where im=current in any resistor Rm not con nected to zero potential. im is positive if Em Eg and negative if Em Eg.

An equation like this can be set up for each .resistor connected to-potential above ground.

2"k,E, k,E, E= ,Ek v 2k 2 Ek The additive'condition is then satised.

It may be noted that the foregoing proofs are only valid when at least o e resistor is con# nected to ground potential as otherwisey the equations involve a fraction with zero denominator. It is, however, easy to showby the usey o! the theory or limits that the additive condition still holds when all resistors are connected to other potentials. Suppose we add a resistor which is to be always connected from the generator input terminal to ground.` Let this resistor be and do not consider it in the summations. Then finite-1.,* el, disconnected entirely.

The numerical example already given under vCase-*II may be accepted as illustrative of what may be accomplished by graduating the resistorsg `used as fifth or sixth harmonic, one twentyfourth of the bus potential is applied to the generator. These fractions will have to be taken into account when the various timbres are setl up. The apparent loss of voltage is due to the fact that reserve potential must be available for raising the potential further and further as more and more partials are added. In fact, a little thought is suilicient to show the truth of the, following, which might be'called first law of additive system. The total potential available must be equal to the sum of the maximum potentials of all partials.

The generators near the ends of the ranges supply a less number of harmonics than those near the middle, and these have to be given special consideration. The fact that it is rather easy to solve the problems which naturally arise in a synthetic instrument from this ,cause is one of the advantages of my system. The generators of the lowest octave are used as fundamentals only, those of the next lowest octave for fundamentals and second harmonics only, those of the lower half of the third octave for fundamentals, second and third harmonics only, etc. It will be found that if a dummy resistor, of resistance equal to R/16, be connected from the input terminal of each generator in the lowest octave to ground, the applied voltage for each of these generators will 1"? the same as that on all other fundamentals. However, as the ear is much less sensitive to the very low frequencies, it will probably prove advisable to increase the resistance of these dummy resistors in order to facilitate equalizing the loudness.

At the other end of the scale, it would be desirable to get along without any generators which are used as harmonics only. But if this is done without making proper compensation, when a timbre rich in high harmonics is being used, an abrupt change of quality and volume will be likely to occur near the top of the scale. Two modiilcations are possible under the system of my invention. One is the substitution method which has been previously proposedi. e. when a harmonic turns up missing, a switch and a resistor are provided for it just as though the generator were present. The generator end of this resistor is connected to the input terminal of the fundamental generator, thus substituting an equal amount of fundamental for the missing harmonic. The quality will thus change by steps, but the effective value of the output will remain unchanged regardless of the wave form. My method is to gradually reduce the constants of the harmonic resistors as the upper end of the range is reached. 'I'he quality can thus be made to become simpler gradually, as it does in acoustic instruments.

If the keying resistors are large with respect to the internal resistance of the source, there will be no mutual interference between the input circuits of diierent generators, or the various branch circuits of the same generator, the resistance of the high potential portion of any voltage divider being included in the internal resistance of the source of potential. This fact makes combining the individual generators into a complete instrument relatively easy- It is merely necessary to equip the various switches with mechanical linkages to the proper keys, and connect the switch contacts to the appropriate potential busses. The busses, ot course, are equipped with means for adjusting their potential.

There are two general methods for connecting the switch contacts. One method consists in connecting all the live switch contacts which appertain to the same harmonic to the same potential bus. This system is illustrated in the upper manual of the circuit of Figs. 4, 4a and 4b. The other system, which will be explained in due course, is illustrated in Fig. 5. A compromise method for connecting the switch contacts is illustrated in the lower manual of Figs. 4, 4a, and 4b.

Another difculty with present instruments arises from the fact that substantially all of the truly synthetic instruments employ a separate key-contactor and amplitude control circuit for each partial of each note employed such as represented in Patent 1,956,350 to Laurens Hammond, April 24,1934; Patent 2,055,719 to Raymond C. Fisher, September 29, 1936; and Patent 1,749,685 to Raymond Zouckermann, March 4, 1930. While a considerable change in harmonics content may not be appreciable to the ear, a large number of harmonics are determining factors in the timbres of various instruments. On this account, it is diflicult for the designers of present types of instruments to provide for a suiicient number of harmonics 'with a reasonable number of parts. Having provided for as many harmonics as are practical, they iind themselves confronted with a degree of complexity which permits much finer control of harmonic content than there is any use for, and at the same time a number of harmonics that is not as great as desired.

I alleviate this difficulty by dividing the harmonies into related groups, each group to be controlled by a single keycontactor- 'I'he amplitude relations between the members of a group are xed for each instrument, by adjusting the relative values of the resistors analogous to R1, R2, R3, R4, and R5 of Fig. 1. The relative amplitudes Within the group are chosen by experiment, using the known harmonic contents of a large number of instruments as a guide. The controls available to the operator control the amplitudes of entire groups, and thus have a much greater eiect on the timbre than controls which affect a single harmonic. The groups could be selected according to the following principles, or in any other way that seemed desirable to the designer of a specific instrument.

Harmonics which bear an octave relationship to each other affect the timbre similarly. Further, it is apparent that harmonics which correspond to notes which form a simple pleasing chord with the fundamental should bear some similarity in their eiTects on the timbre, though the similarity should not be as great as in the oase of the octave relationship. And nally, harmonies corresponding to notes which are dissonant to the fundamental would be quite otherwise, having an important and rather individual effect, probably tending towards harshness in the iinal quality, but extremely useful musically if only for variety. This feature is illustrated in Manual I of Figs. 4, 4a and 4b.

Another feature of acoustical musical instrupass filters in the amplifier.

. filter method is preferable.

ments is the fact that the harmonic content is different for dierent pitches. This has often been considered a defect; on the contrary it is more likely that it is an essential requirement of an instrument which is to be musically satisfying for general use over a long period of time. Itis a well-known fact that a change in harmonic content is more obvious to the ear than the mere presence or absence of certain components, so that variation in wave-form with pitch is iin-` portant in producing variety. 'I'his effect is the reason for the musical usefulness of such devices as repeating a passage an octave higher or lower.

Certain musical tones are chiefly characterized by a dependence of the amplitudes of their components on frequency rather lthan on the harmonic order. Typical tones of this class are the tones of the human voice, the violin and similar stringed instruments, and the tones of the woodwind instruments. These tones are all characterized b the existence of certain bands of frequenci s, called formants. Regardless of the pitch of the fundamental of the note emitted the high-amplitude components always lie within a formant band. An exception of course exists when the fundamental is above the highest formant; in this case the note is invariably low in harmonic content.

My system lends itself very readily to quality control by this system. For example, in Figs. 4-40-422 the resistive leads 20l-206 to thepolarizing plates I3 could be led back to a com-mon potential. This would permit the use of a singlecontact key for however many partials were used. The harmonic content of the originally-generated wave would then be fixed by the values of the individual resistors, which would be seldom, if ever, changed. Quality control would be effected ordinarily by one of two alternative systems. One method would be to use a set of multiple band- These filters would be interchangeable by means of quality-control switches operable from the console. or the same filter would be modified by said switches; in which cases the filter elements (inductances, capacities, and resistances) would be adjustable by means of tap-switches or otherwise. Another method would be to connect the output plates l5 of the individual generators, singly or in groups of adjacent pitches, to the vamplifier through individual attenuators. The second method would perhaps give more positive control of the quality, but would be more expensive and more complex. In the present state of .the art, the characteristics obtainable by filters are almost unlimited, particularly if regenerative vacuum tube circuits or electro-mechanical filter devices are employed. Taken by and large, then, it would seem that the Fig. 5 shows the connections for the key-switches and gener tors when using this system. Fig. 5a shows the associated Itimbre-control.circuits using individual attenuators; Fig. 6 shows an alternative timbrecontrol scheme using interchangeable filters.

This system, which may be called a pure formant system of quality control, is very attractive from the viewpoint of simplicity of construction, but has certain drawbacks. In the first place, the generators used in this manner cannot be employed in the ordinary harmonic-order control circuit. That is, while the generators and Ith'e'basic generating circuit'is the same in both cases, the quality-control circuits are so different .that vthe .two systems described cannot be employed alternatively inthe same instrument.

at least not without an almost prohibitively complex switching system. In the second place, it will be noted that the acoustic instruments functioning on the formant system are solo instruments, in the sense that only one note at a time is produced. (In the case of the violin, two notes can be sounded simultaneously, but they will be close together in pitch.) Thus, the volume discrepancies that might otherwise arise' are taken care of by the operator, note by note. In the case of a keyboard instrument, where several notes remote from each other in pitch are often sounded simultaneously, a difliculty would arise. It is not known at present how serious this difficulty would be. In any case, the diiiiculty can be overcome by using a volume limiting tube such as element 302 of Figs. 5a and 6r and explained below.

I have devised a circuit which eliminates these diiculties at the expensel of employing a keycontrolled switch for each harmonic. This circuit is similar to the circuit previously described, except that the voltage tap to which the individual resistors are connected depends upon the pitch (frequency) of the generator instead of the order of harmonic as in the previous case. This system consists in dividing the generators into groups, the generators in each group comprising a series of adjacent pitches. All live switch contacts in the group, regardless of harmonic number,` are connected to the same potential bus. A circuit of this type is illustrated in manual II of Fig. 4a. This may be called a compromise or semi-formant system.

In Fig. 1 the electrostatic generator assembly is indicated at G and is similar to the structure set forth in my Letters Patent 2,001,708, dated May 2l, 1935. The control contactors are indicated at Ki, K2, K3, K4, and K5 having sets of opposite coacting contacts as shown. The control contactors Ki, K2, K3, K4, and K5, etc., are each operated by a different key. The means for coordinating .the movement of the contactors are indicated schematically by dotted lines The connections to .the source 2 from the coacting contacts of the contactors are indicated at la, 2a, 3a, 4a, and 5a. Reference character 3 designates a volume determining resistor between the generator G and the input of the electron tube amplifier 4. Quasi-linear properties may be imparted to generator G through coupling circuit constituted by condenser C and resistances 3 and 5 whereby the impedance of the generator and the input of the 4amplifier are properly related for obtaining proportionality between active plate area and Iamplifier input potential, as explained in my Patent 2,001,708. Amplifier 4 has a suitable cathode heating supply as indicated at 6 and a suitable plate circuit supply shown at 1. The output .transformer 8 in the output of amplifier 4 connects to a suitable sound reproducer indicated at 9. Condenser I8 produces gradual attack and decay of tones by the method disclosed in Floyd A. Firestone Patent 1,953,753, granted April 3, 1934.

In Fig. 2, I have shown a circuit equivalent to Fig. 1 for a generator producing a single Partial according to the mathematical demonstration supra. Like reference characters refer to similar parts described in connection with Fig. 1.

In Fig. 3, I have shown circuits similar to Figs. 1 and 2 using similar reference characters to designate similar parts and showing an equivalent tudes are produced simultaneously.

The circuit connections for a two-manual organ constructed in accordance with my invention are shown schematically in Figs. 44a-4b. Figs. 4-4a and 4b are continuations of one circuit. The location of parts in the gures is not intended to suggest the location in the actual instrument, but was intended merely to facilitate drawing in the electrical connections, as is usual in schematic circuit diagrams. No attempt has been made to show all the apparatus in ythe instrument nor all the connections, as it was felt that the principles to be followed could be more readily understood if too great repetition of parts and circuits was avoided.

In Fig. 4, each of elements I3 represents one stator plate of a varying-capacity generator, each of elements I5 the other stator plate, and I4 the intervening rotor plate. The generators have additional designations comprising letters with subscripts. `These show the note of the musical scale which the generator emits when used as fundamental. Thus Ci indicated C in the lowest octave, E4, E in the fourth octave counting from the bottom, etc. Elements I6 represent individual voicing resistors, i. e., resistors for adjusting all notes to the same loudness. Each output plate I5 connects through a voicing resistor I6 and conductor 3| to the grid electrode of amplifier tube l34 in Fig. 4b.

Elements to |I2 and 20| to 206 are the individual harmonic-control resistors analogous to those shown at Ri-Rs in Figs.,1 to 3. In these designations, the rst digit gives the manual and the last two digits the order of the harmonic with which the resistor is associated. Thus a |0| resistor would control a fundamental tone of manual I, a 205 resistor a fifth harmonic of manual II. etc.

Each of these resistors has one terminal connected to an input plate I3, and the other terminal connected to the movable contact of a key-controlled switch I9. All of-the resistors connected to each of the generators shown are represented. As not all of the key-controlled switches are shown,.the connections to one terminal of many resistors |0I-I I2, 20|-206 had to be omitted. It is to be understood that in the actual instrument each of these terminals is to be connected to the movable contact of the appropriate key-controlled switch.

A plurality of switches I8 are controlled by each playing key. In accordance with the convention used in radio for showing a gang-switch, gang-condenser and the like, the mechanical linkages whereby the playing keys operate these switches are represented by dotted lines. Linkages designated with the numeral I are operated by playing keys of manual I: linkages designated with the numeral 2 are operated by playing keys o1 manual II. These linkages are further desighated byletters with subscripts which identify the playing key which actuates them. 'I'hus IC1 indicates the lowest C of the flrst manual, 2G; the lowest G of the second manual, etc.

Each of switches I8 has three contacts, a movable contact, and two xed contacts. The iixed contact which is in use when the key is in the non-playing position (up) is connected toV ground or zero potential through conductor 30 in all cases.

suitable steady `potential according to the following scheme:

In manual I, control by the harmonic-order 7i-system is exemplified, the feature vo f multipleharmonics per contac/t/ being incorporated. Each The other fixed contact connects to a IOI resistor (i. e., each fundamental) connects through a separate switch I8 to conductor 20, which is supplied with a potential controlled as shown in Fig. 4b, which will be hereinafter explained. The second, fourth, and eighth harmonic's being in octave relationship to each other (and the fundamental) are controlled by a single switch I9 as follows. Plate I3 of the generator producing the 2nd harmonic of the note under consideration is connected through a |02 resistor to the movable contact of a switch I9; plate I3 of the fourth harmonic generator is connected to the same point through a |04 resistor, and plate I3 of the eighth harmonic generator to the same point through a |08 resistor. The active contact of this switch I9 is connected to conductor 2|. which is supplied with a controlled potential by the circuit shown in Fig. 4b. Now the relative amplitudes of the second, fourth, and eighth harmonics are controlled by the relative resistances of resistors |02, |04 and |08. The amplitude of the group as a whole is controlled by the potential applied to conductor 2 I. The sixteenth harmonic could be added to this group if desired.

Similarly, the third, sixth, and twelfth harmonic generators are supplied with potential from conductor 22 through a common switch I9. (The frequency of these and succeeding harmonies is of course only approximately correct.) The fifth and tenth harmonic generators are supplied with potential' from conductor 23 through another common switch I9. The frequencies so far are all involved in the chord known as the major triad in the study of harmony. 'I'he readily available dissonant harmonics, the seventh and the ninth, are lumped in the last group, and supplied potential through conductor 24.

It will be noted that this arrangement is in accordance with the principles previously stated. Many variations of the arrangement are possible, some of which may be preferable under certain conditions than the arrangement set forth herein and it will be understood that my disclosure herein is to be considered in the illustrative sense and not as limiting my invention to the specific arrangement shown.

Certain conductors in manual I are shown unterminated, as they lead to apparatus not shown. That is to say the circuits are repetitions of the circuits heretofore described. Thus conductor |20 goes to plate I3.of the D3 generator (not shown) through a |03 resistor (not shown). Similarly, conductor I2| goes to plate I3 of the B3 generator through a |05 resistor, and plate I3 of the B4 generator through a I I0 resistor. Conductor |22 goes to plate I3 of the Fg generator through |01 resistor, and to plate I3 of the A4 generator through a |08 resistor. Conductor |23 goes to plate I3 of the G5 generator through a I|2 resistor. Conductor |24 goes to plate I3 of the E generator through a |I0 resistor. Conductor |25 goes to plate I3 of the D5 generator through a |08 resistor. Conductor |28 goes to plate I3 of the Ats generator through a |04 resistor and to plate I3 of the Ata generator through a |88 resistor. Conductor |21 goes to plate I3 of the A35 generator through a |06 resistor and toj plate I3 of the Ate generator through a. ||2 resistor. Conductor |28 goes to plate I3 of the M5 generator through a |08 resistor. Conductor |28 goes to plate I3 of the Ate generator through a IIO resistor. Conductor |30 goes to plate I3 of the AIIs generator through a |I2 resistor. Conductor |3I goes to plate I3 of the Ai: generator through a |02 resistor. Conductor |32 goes to plate I3 of the F114 generator through a |01 resistor. Conductor |33 goes to plate I3 of the At: generator through a |03 re- 5 sistor.

Reference character 50 is a three-position multi-gang switch. When thrown to the position farthest to the left on contact 53 it associates a large condenser 5I with resistors |0| to |I0, so as l@ to produce a. very gradual attack and decay, in a manner explained by Firestone (loc. cit.) When switch 50 is placed in the center position on contact 54, a smaller condenser 52 is substituted, giving a less gradual attack. When switch 50 is i3 placed in the position to the extreme right on contact 55 the condensers are disconnected and an abrupt attack results.

'Manual II is co'nnected on the compromise or semi-formant system. Aside from the number of i2@ harmonic-control resistors and key-controlled associated with generators in the lower half of :ige third octave connect to conductors 42, 43, 44,

etc. In Fig. 4b, 34 is an amplifier tube supplied with energy from the generators through conductor 3| land volume control 32. Volume control 32 should l be operable from the console, preferably by means of a foot pedal. Vacuum tube 34 feeds loudspeaker 36 through an intervening vacuum-tube system typified by tube 35.

40 Numeral |40 designates the mechanical linkage of a. multiple switch which may be called an adinstable-quality tappet. When this tappet is depressed, indiVIdulaJ'sWitches I4| render adjustable potential dividers |42 'eiective in controlling the 45 potential of conductors lli-I4. Potsdam dividers |42 obtain their electrical energy from common source of potential 36.

Numerals and |60 designate the mechanical linkages of multiple switches whichmay be called 5 fixed-quality rappers. with tappet |40 raised, depressing |50 or |60 forms a potential divider of which the upper leg is resistor |52 or |62 and the lower leg is resistor |35. The value of resistors |52 control the potentials impressed on conduc- 55 tors |0-I4 when tappet |50 is depressed. Similarly another tone quality, depending uponthe values of resistors |62, results from depressing tappet |60.

80 pressed simultaneously, in which case the amplitude of each individual harmonic is greater than',

it would be with only one tappet depressed, but not as great as the sum oi' `the individual amplitudes. Tappet |40 can be operated simultaneously with one or more fixed-quality tappets, in which case it functions as an adjustment on the quality already setup by the fixed-quality tappet. The amplitude of any given harmonic may be increased or decreased, depending upon the relation between the original amplitude of the harmonic and the setting of the corresponding adjustable potential divider |42. The magnitude of thiseiect depends upon the magnitude of resistors |43, being larger when |43 is smaller.

g5 Additional fixed-quality tappets similar to |50 Tappets |50 and |60 may bedeand |60 can be'added as desired. A truly additive circuit for the tappets can readily be worked out along the lines previously given for the generators; but as such a circuit would require more polarizing voltage and would'be more complex 5 to design and construct, it is believed that the circuit given is preferable.

Numerals 250 and 260 designate the mechanical linkages for fixed-quality tappets for the second manual. They function in the same manner 10 as the tappets for the rst manual. An adjustable-quality tappet could be added along previous lines, but under the formant system there does not seem to be a great deal of point in doing this.

In Figs. 5 and 5a the circuit connections for 15 the pure formant scheme of quality control, using only one contact per playing key, are shown. Figs. 5-5a are drawn as though there were only three notes per octave and four octaves in the complete instrument; this is for the sake of sim- 20 puoityoniy, and it is to be understood that the arrangement may be readily extended to the regulation twelve semitones per octave and as many octaves as desired. Again for the sake of simplicity, only three harmonics are shown. The in- 25 dividual harmonic control resistors are 30| for the fundamental, 302 for the second harmonic, and 303 for the fourth harmonic, the third not being used in the illustration. These resistors are shown variable, and are ganged together by the 3 mechanical linkages 304, 305, and 306 respectively. This constitutes an optional auxiliary quality control. For reasons of expense, this would in practice probably be accomplished by resistors with one or two taps and a gang switch, as a 35 large number of resistorsvhave to be controlled if this feature is to be included. The 302 and 303 resistors are at all times to be graduated so that the resistors for the highest notes have the highest value of resistance, to avoid sudden changes 4 in quality, as previously explained.

By tracing out the circuit, it will be noted that each of the playing keys 300 which control notes in the highest octave (last three notes on the right of the diagram) connects plate I5 oi.' one of 45 the generators to potential source 30| through a 30| resistor. 'I'his is the only connection made in sounding a note of the highest octave, there being no harmonic generators above the fundamental range, as previously explained, so that the 5 notes of the highest octave are substantially purer sine waves-a condition which is approximated by many instruments. The playing keys of the next highest octave each connect plate I5 of one ,Y generator to source 303 through a 30| resistor, 55

and plate I5 of another generator to source 300 through a 302 resistor. The notes in this octave A thereforecontain fundamental and second harmonic components only.

Each of the stator plates .I3 of the generators is connected through a 301 resistor to one of the conductors 3| 0--3I4. Resistors 301- simultaneerators connect to conductor III, the next two to 70 conductor 3| etc. Conductors 3|0 to 3|4 connect individually to fixed4 quality-control tappet 320 and adjustable quality-control tappet 330. Thence the potentials from the various groups of generators, each now having its appropriate amplitude as set by resistors 32| and/or 33|, is applied to the input circuit of amplifier tube 340. The output circuit of tube 340 contains transformer 34|, whose secondary is Wound for pushpull operation. Connected to this push-pull secondary is double diode tube 342, which functions as an automatic volume limiting device to prevent sudden burst of sound in case a note having a strong partial in a band which is little attenuated by resistors 32| or 33| is sounded. The cathode of double diode 342 is given an adjustable positive bias through resistor 348. As long as the audio frequency potential across the secondary winding of transformer 34| is insuillclent to raise the potential of the diode plates above the potential of the cathode at any time, the diode remains inoperative. But if the peaks of the audio frequency potential are sufiiciently large, the diode will draw current, thus shunting the transformer secondary and reducing its efiiciency as a coupling device.

The audio frequency potential, as limited by this device, is thence transmitted to volume control 343, which is operated manually or by a foot pedal. transmitted to loud-speaker 341.

Figure 6 gives an alternative method of quality control. Conductors 3| 0-3|4, instead of being connected individually to tappet-controlled switches, are all connected together to the amplier input circuit. The amplifier is then'provided with a number of wave-filters, arranged to be employed alternatively. In Fig. 6, a gangswitch comprising individual multi-point switches 39|ia and 390D mechanically linked together, is employed for selecting the wave-filters. Three different types of -filters have been shown. No importance is to be attached to the precise circuits of these filters; they are intended to be typical of the numberless circuits known to the art which will provide band-pass or multiple band-pass characteristics. The network 360 is a fairly common type ol band-pass filter employing three inductances and three capacities. Voltage divider 352, amplifier 350, regeneration-control vario-coupler 35|, tuned circuits 353, choke coil 354 and blocking condenser 355 comprise a filter with band-pass characteristics sharpened by regenerative means. 'I'he third interchangeable filter is a magneto-striction type adapted from a filter circuit employing piezo-electric elements. Magnetostrictive rod 380 is wound with a (multilayer) helical coll 38| having center tap 382 connected through condenser 333 and tuned-circuit 384 back to ground.

I have no intention of making any claims on these filters per se, neither as to originality nor as to the operability of the exact circuit shown.

I do, however, maintain that filter circuits designed along the general lines shown are Well known in the art and can be made to fulfill a wide variety of frequency-response conditions; and that such iilters, used in combination with generating and keying circuits substantially as here disclosed, constitute a new and useful device for producing music.

While I have described my invention, therefore, in certain preferred embodiments, I intend no limitations thereby upon my invention; and I desire it to be understood that modifications may be made and various devices other than those specified may be employed within the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

After further amplification, it is then' 1. In an electrical musical instrument including a multiplicity of playing keys controlling a multiplicity of electric alternators and an electric sound producer controlled by said alternators, the combination of a pair of input terminals for each alternator, one or more switches operated by each of said playing keys, each of said switches comprising a pair of xed contacts and a movable contact controlled by means of said keys, electrical circuit connections including one of said input terminals from each of said alternators and one of said fixed contacts from each of said switches connected to a common point, a source of potential connected to said common point, a plurality of conductors connected to said source of potential through adjustable potential dividers, a connection from each remaining fixed contact of said switches to some one of said conductors, and resistive connections from each movable contact of said switches to one or more of the remaining input terminals of said alternators such that the timbre changes progressively with pitch.

2. In an electrical musical instrument including a multiplicity of playing keys controlling a multiplicity of electric alternators and an electric sound producer controlled by said alternators, the combination of a pair of input terminals for each alternator, one or more switches operated by each of said playing keys, each of said switches comprising a pair of fixed contacts and a Inovable contact controlled by means of said keys, electrical circuit connections including one of said input terminals from each of said alternators and one of said fixed contacts from each of said switches connected to a common point, a source of potential connected to said common point, a plurality of conductors connected to said source of potential through adjustable potential dividers, a connection from each remaining fixed contact of said switches to some one of said conductors, and resistive connections from each movable contact of said switches to one or more of the remaining input terminals of said alternators, the contacts of said switches which are connected to any one of. said conductors all being associated with the same alternator or with alternators productive of adjacent pitches.

3. In an electrical musical instrument, the

combination as set forth in claim 1, with means for selecting the potential conductor to which certain of said remaining fixed contacts are connected on the'basis of pitch, and means for selecting the potential conductor to which the balance oi' said remaining ilxgd contacts are connected on the basis of harmonic order.

v 4. In an electrical musical instrument including a plurality of alternators productive of different musical frequencies, and a sound reproducer, the combination of input and output circuits for said alternators, key-controlled contactors in said input circuits for rendering said alternators effective upon said sound reproducer, and resistive electrical connections from each key-controlled contactor to certain selected alternators, the frequencies of said selected alternators being so related that when each of the corresponding notes is sounded simultaneously with the lowest-pitched of them, the resulting chords are simple ones which are similar to each other or at least closely related to each other, the simplicity and the relatedness of said chords being decided according to the teachings of the musical art of harmony.

5. In an electrical musical instrument includcaemos 9 ing a multiplicity of contactors operated by play' ing keys, each key controlling a single contactor, a multiplicity of alternators having input and output terminals, a source of potential, and an 'electrical sound-producing device'the combination of electrical connections between the input terminals ofvsaid alternators, said contactors and said source of potential for supplying the input terminals of more than one of said alternators with electrical energy from said source of poten- -tial when one of said playing keys is depressed,

and a plurality of amplitude control circuits connected to said input terminals of said alternators, said control circuits being so arranged that the resultant amplitude of any one component of a musical note is a predetermined function of its Y pitch? 6. In an electrical musical instrument includv ing a multiplicity of contactors operated by playing keys, a multiplicity of alternators having input and output terminals, a source of potential, and an electrical sound-producing device the combination of electrical connections between the input terminals of said alternators, said contactors and said source of potential for supplying the input yterminals of more than one of said the magnitude of the resistances of the indi input and output terminals, a source of poten,

tial, and an electrical sound-producing device the combination of electrical connections be-A tween the input terminals of said alternators, said contactors and said source of potential for supplying the input terminals of more than one of said alternators with electrical energy from said source of potential when one of said playing keys is depressed, a plurality of amplitude control circuits connected to said input terminals of said alternators, each of said control circuits being individual to one frequency or common to a band of adjacent frequencies, and means for variably adjusting each of said control circuits.

8. -In an electrical usical instrument, the circuit connections as efined by claim 1, with vidual resistive connections therein described be-` ing graduated so that the relative amplitudesf output of the partial tones will change from note to note in a musically pleasing manner.

9; In an electrical musical instrument including a plurality of electric alternators operating at different frequencies, an electric sound producer and playing keys for rendering frequencies produced bythe alternators effective in the sound producer, in combination with input and output terminals for said alternators, a source of potential connected -in common with one input terminal of each alternator,.and individual means energized at diiferent potentials from said source and connected with selected alternators at the remaining input terminals thereof for providing tone partials of different amplitudes at the various frequencies; the output terminals of said alternators being connected in common with the electric sound producer, each of the playing keys being arranged for sounding selected partials simultaneously in the sound producer. and the total potential available at said output terminals being substantially equal to the sum of the maximum potentials of all the partials sounded, regardless of the number of playing keys operated.

10. In an electrical musical instrument, the combination set forth in claim 9 wherein said individual means comprise resistors of equal value whereby the amplitudes of the partials produced are proportional to the different potening a plurality of eleptricalternators operating' at different frequencies, an electric sound' producer and playing keys for rendering frequencies produced by`the alternators effective in the sound producer, in combination with input and output terminals for said alternators, a 'source o'f 'pow' tential connected incommon with one input,4 *t terminal z of ,each alternator, andindividual means connected from said source to selected alternators vat the maining' input te thereof for providing tone partials of different amplitudes at the various frequencies; the outv ducer, with the tota1 potential available atsa1dterminals being substantially equal to,` the sinn of the maximumpotentials of all the partials sounded,` l e

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