US2568644A - Electrical musical instrument - Google Patents

Description

Sept. 18, 1951 M. J. LARsl-:N ETAL ELECTRICAL MUSICAL INSTRUMENT 8 Sheets-Shea?I l Filed April 6, 1950 M. J. LARSEN ET AL f 2,568,644

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M. J. LARSEN ETAL ELECTRICAL MUSICAL INSTRUMENT spt. 18, 1951 8 Sheets-Sheet 4 Filed April e, 195o Nk .NK 9h YQQ\ Sk y S J M l l n. .mw www .2. r1.1 Mm -J ...1 i 1 N@ a /N "\Q \Q n" A NQ s a WQWAN Ok m IfMUBH HlLlllal ATTORNEY l Sept 18, 1951 M. .1. LARsEN ETAL 2,568,644

ELECTRICAL MUSICAL INSTRUMENT Filed April 6, 1950 8 Sheets-Sheet 6 TORNEY Sept. 18, 1951 M. J. LARsI-:N ETAL ELECTRICAL MUSICAL INSTRUMENT File@ April e, 195o 8 Sheets-Sheet 7 mhk c MMM m 1s@ n Z A Am R af Wwnw N mm@ MR 0 N Sept- ;'18 1951 M. J. LARSEN ET A1.

ELECTRICAL MUSICAL INSTRUMENT 8 Sheets-Sheet 8 Filed April 6. 1950 ,Wml wou. btw M uw @mw N k m N, n QQW @ke Q Q m QQ@ QQ Mm M M w06 n WAB mum RM W u@ E@ h Mo v QQ N m6 mk nu Nw w A www VH.. l" Jl www QWNWW WHW IMT n n www @v WW g SX A'h FL ,PXJ @mi Y k m KQ n E i m Y s M Q Q D .wllmlfll Patented Sept. 18, 1951 ELECTRICAL MUSICAL INSTRUMENT Merwin J. Larsen, Villa Park, and Norman B. Erickson, Chicago, Ill., assignors to Central Commercial Company, a corporation of Illinois Application April 6, 1950, Serial No. 154,262

(C1. SLi-1.19)

19 Claims.

This invention relates to electrical organs and more particularly to organs of the class in which sustained tones are produced by complex audio frequency electric oscillations in the pitch rela- .tion to the vibration frequencies of notes of the even tempered musical scale. An organ of this general class is disclosed in Letters Patent of the United States No. 2,403,090, issued to M. J. Larsen on July 2J 1946, in which patent, multivibrators are the sources from which useful complex electric oscillations are derived, thence treated by a filtering process to provide effective waveforms of oscillations of varying complexity having the pattern characteristics of organ and orchestral tones, which waveforms of oscillations are then amplified and translated into audible sounds for Inusical expression by means of keying circuits and stops rlhe herein disclosed invention is animproveinent upon the invention set forth in said patent and particularly with respect to modifications in the divider circuit to secure improved locking with resultant greater assurance of maintaining an invariable pitch relation between the produced tones and to simplify the system as a Whole and provide a wide range of frequencies from a minimum number of space discharge tubes such as the multivibrators to be hereinafter described in detail.

Certain of the more essential objects of our invention are:

(l) The provision of dependably functioning sources from which are derived tone signals that are rich in harmonics.

(2) The provision of tone frequency sources capable of withstanding Wide changes in line voltage without change in frequency in the produced outputs.

(3) The provision of a tone generating source employing inexpensive components having tolerances that are not too close for practical purposes.

(4) The provision of an electrical musical instrument employing a plurality of divider circuits in which each separate circuit produces electric oscillations at octave separation and consists of a master oscillator tube and a plurality of driven or slave oscillator tubes coupled in cascade by cornponents in a manner that feedback of lower frequencies can be made as negligible as desired.

(5) The provision of a keyboard musical instrument embodying a low-cost and highly dependable tone generating system in which the total nulmber of vacuum tubes employed is appreciably less than the note range of the instruments keyboard.

(6) The provision of a tone generating system which yields complex waveforms that are uniform throughout the note range of the instrument.

(7) The provision of means for deriving two octavely related tones of dissimilar waveforms from a single multivibrator.

(8) The provision of means for deriving from a single multivibrator two tones having similar waveforms with frequencies that are octavely related.

The manner in which the above and other objects are possible will be more apparent from the following description taken in connection with the accompanying drawings, in which:

Figure 1 is a schematic illustration of a submultiple circuit for dividing by a factor of 2;

Figure 2 is a schematic illustration of one of the multivibrator stages;

Figure 3 is a graph of the plate and grid circuit voltages of the triodes of one of the multivibrators as in the case when the driving pulses are too small to cause locking;

Figure 3a is a view similar to Figure 3 indicative of operations in which the plate and grid voltages function to cause effective locking;

Figure 4 is a view similar to Figure 1 showing another example of our invention;

Figure 5 is a view similar to Figure l showing a still further embodiment of our invention;

Figure 6 is a View similar to Figures 3 and 3a showing the multivibrator waveforms operating at one-fourth the driving frequency;

Figures '7, 7a. and 7b are views respectively indicative of (a) operating conditions effected by variation of component values, (b) operating conditions wherein there is an adjustment of amplitude with natural period of operation equal to the lowest frequency period, and (c) operating conditions the same as in (b) above but with the natural period greater than the lowest frequency period;

Figure 8 is a graph indicating the harmonic content of the output wave from the plate of a typical multivibrator.

Figure 8a is a graph indicating the harmonic content of a saw-tooth wave produced as described herein;

Figure 9 is a view similar to 5 showing a still further example of our invention, and

Figure l0 is a schematic illustration of a composite electrical network embodying our invention.

Our invention essentially comprises coactive, interconnected electrical mechanisms in an electrical network in which keying-circuits connect respective tone generating sources to a Work circuit which includes a transducer such as a loudspeaker for translating electric oscillations into audible sounds for musical expression. We have found that space discharge devices designed to yield output waves which are rich in harmonics have many advantages over systems of the prior art including systems wherein simple tone frequencies produced by alternators are selectively synthesized or compounded into various forms of complex waves. One particular advantage of our invention over prior art methods is that the pitch relation of the produced tones is invariable throughout long periods of time and not affected by atmospheric conditions. This coupled with the advantage which is, that the output waves are rich in harmonics and can be effectively reduced to various less complex waves makes for greater simplicity, compactness of the composite organization of elements than heretofore possible with devices and systems of the prior art.

We particularly stress those features of our invention which insure precision-like operations and coaction between the herein disclosed mechanisms with avoidance of interaction between the elements or intermodulation that would impair operations and render said mechanisms useless in a keyboard musical instrument such as an electric organ.

Two main examples of tone generating sources are shown herein, each consisting of multivibrators coupled in cascade and a control of master oscillator. 1n one example, frequency division is by a factor of 2. In the other example, division is by a factor of 4 and, by reason of a unique system, but one-half the number of oscillator stages is required for a compass embracing the same number of octaves as the former example.

DIVIDING BY A FACTOR OF 2 At Figure l is shown a divider circuit including an electron-coupled L-C master or stabilized oscillator stage Tl, followed by four octavely locked slave multivibrator stages TZ-T. The plate output circuit T@ of stage Tl and the plate output circuits Ti-Tei of the right hand triodes of stages TZ-Tl are respectively ccnnected to electric switches Kl, KIES, Kl and K55. Said switches are connected to an output conductor lull having a resistance R9. In practice, stage Tl may be a 12AV6 vacuum tube or its equivalent. Multivibrator stages T2-T5 may be vacuum tubes such as either of the types 12AX7 or 12AU7 or equivalent tubes characterized by two half-sections or triodes, each consisting of a cathode, control grid and a plate. The grid circuit of each triode has a resistance Rl. The plate circuits of the triodes have resistances R2 connected together at a common junction point P and to resistance R3 from a -l-B supply source. The plate circuit of stage Tl includes a resistor R5, and as shown, said plate output circuits from stages Ti-T5 each has a resistance Ril and a condenser C/l. The output circuit of Tl is coupled to the junction point P of stage T2, T3 and Til are each similarll7 coupled, each by a condenser C3 to the next succeeding stage.

It follows that from each of the stages Tl-Tl a portion of the output wave is impressed on its respective keying circuit and a portion on the junction point P of the next succeeding stage.

A keyboard musical instrument tuned to the even tempered musical scale requires twelve cascaded note producing units as proposed in United States Letters Patent No. 2,410,883. Each separate unit generates octavely separated tones at the vibration frequencies of notes of the same letter` of said scale. For example, the C note cascade unit may generate complex waveforms having the fudamental frequencies approximating 65 cycles, 130, 260, 520, 1040 cycles respectively. Similarly, the Cil, D, Dt, E, F, Fil, G, Gt, A, All and B, cascade units will each produce octavely separated waveforms at the tone frequencies of all notes of the same letter represented by the oscillator stages of the respective unit. The basic circuit of a typical cascade unit embodying my invention is shown at Figure 1 wherein an electron-coupled L-C' master oscillator stage Tl is followed by four octavely locked slave multivibrator stages T2, T3, Til, and T5. The plate output of the master oscillator stage TI is shown as having a slightly distorted sine wave of frequency f, which can be considered as 1040 cycles. The multivibrator outputs drawn from the plates of the successive slave oscillator stages respectively have frequencies f/Z, f/l, f/S and f/ 16. At this point it should be noted that the output waveform of a multivibrator is a combination of a square wave and a saw-tooth wave, i. e. it contains harmonics, diminishing in intensity with increase in the harmonic order, and is more predominant in odd harmonics.

In the circuit shown at Figure l, the multivibrator plate resistors R3 and R2 are low relative to the grid resistors Rl. Hence the driven frequency of each individual multivibrator is approximately proportional to the product RICI, assuming symmetrical construction of the 1A? sections of said multivibrators. In practice, the components are selected so that the multivibrator natural frequencies are somewhat lower, such, for example, an octave or more lower than the frequencies at which they are driven. It has been found most expedient in a given cascade to keep the grid resistors the same for all of the stages but to double the tuning capacitors CI in the grid to plate circuits as the frequency halves.

We have discovered that very superior locking is obtained by driving the sleeve oscillators at the junction points P between the plate resistors R2 and R3 instead of into a single plate or a single grid as disclosed in the aforementioned patent, and as the operability of oscillators for our pur-I pose depends upon maintaining an invariable frequency relation between the coupled oscillators, We stress the novelty of this unique method. Although the ratio between values for R2 and R3 is not critical, excellent results are had when, R2=R3. The intermultivibrator coupling capacitors C3 are moderately small, as for exam.-a ple, on the order of micromicrofarads as the signal transmitted has a steep wave front. These values can be larger for satisfactory locking operation but if too large, any unbalance in a driven slave oscillator will feed back a signal of suboctave frequency into the output of the preceding oscillator stage. For the purpose of my invention this must be avoided, hence a moderately small size for the intercoupling capacitors is retained. The coupling capacitor C2 between the master oscillator stage and the nrst slave oscillator stage is necessarily larger as otherwise sufficient driving voltage would not be supplied due to the fact that the master oscillator stage (an R-L type) has a waveform of less steep Wave front. The decoupling resistors R4 are in general several times larger than the plate resistors R2 and R3 so as not to interfere with normal multivibrator operation. Capacitors C4 are blocking capacitors for keeping direct current from the keying circuits X, which circuits connect to a conductor Xa via switches S. Typical waveforms with related frequencies are indicated at the output leads from the coupled oscillator stages. The output impedance shown herein as resistor R9 in conductor Xa must be relatively low so that combined signals from other sources will not rob or tend to steal the normal assuma lockingisignals and give intermittent'results. 'See Figure .10.

.The values of the fvarious :components .nec- `.essary in the above referred divider circuit for :an Ft note Vcascade vare as follows:

Ft NOTE CASCADE Resistors R5-22,000 ohms :R6-33,000 Ohms RII- 1 megOhm VR8-220,000 Ohm's RI-680,000 ohms B52- 100,000 ohms 'R3-100,000 ohms .R4-'470,000 ohms L-.500 henry (variable) Considering now the typical multivibrator stage shown at Figure 2 and the waveforms dshown at Figures 3 and 3a, .we will assume that grid Gl of triode VI at time to rises yin potential above cut-off thus lowering plate PI. The `lowering of plate Pi of triode VI lforces grid G2 Iof triode V2 negative via capacitor Cl which connects the plate Vof VI to the grid of V2 thus causing plate P2 to rise. These interactions accelerate .cause and effect to produce the extremely .rapid changes occurring at times to, t1, t2--etc. as indicated at Figure .3. Immediately Yafter time tu plate, P2 starts rising toward the ,plate-supply voltage, thus charging Cl as grid GI cannot rise appreciably above its cathode. .Between to and t1, Figure 3, capacitor Cl, Figure 2, discharges as grid G2 rises toward .its .cathode 'whence attime t1 the second half of the.cycle begins. As mentioned previously the plate resistors R2 and R3 usually are considerably lower than the grid resistors Rl which makes the product (RI) (CI) controlling as to the period of oscillation for the symmetrical arrangement. Should the resistors R2 and R3 be higher and should unsymmetrical operation be considered then the period is approximately proportional to (Cl) (Ri-i-R2-i-R3) plus (C'l) (RI-{- R2-l-R3) .assuming the internal plate .resistance of the tube (under conduction) to be much lower 'than any of the external resistors. This latter case is not considered in the discussion herein. l

Consider now the manner of operation while driving the multivibrator by injecting the output O'f a source into juncture A with the intention of locking the multivibrator at some submulti- "ple frequency. An impulse at A will affect primarily only the plate of the non-conducting tube because, as mentioned previously, the internal resistance of the conducting plate is considered much lower than that of the external resistors R2 and R3. For .clarifying the discussion, voltage .pips are indicated .in Figure 3 .of an .amplitude too small to cause locking Continuing, itis apparent that between interval .to and `t1,Figure 3 with plate P! rigid as to potential, ythengrid G2 is also Vrelatively undisturbed by direct action of Yany driving pulse because of Cl between P1 4and G2. Turning to plate P2 during'this same interval, to to t1, it is seen that positive rises on P2 are :resisted via .condenser C.I as GI! cannot Dise Iappreciably ab ove `its -catho de. .Theremaindng possibility iis 'that za negative impulse fon A willinot be resisted lby P2. Hence P2 'will Alower GI toward :a non-conducting potential (viarCi) which causes PI vto vrise .and .urge G2 toward `a `conduction potential. 'This zrather roundabout phenomenon `is illustrated in Figure 3 at times .tx,t'=x, and lt'fx.

Itis apparent fromFigureLB that transfer would :occur at time tx if the :signal at A were great enough to cause a gridswing of G2 approximately `equal .to yor `:greater than the amplitude `indicated by "lhenthe .locking, or driven, half period `would be "equal to interval to to tx and because Vof symmetry similarly Yfor the second half. Should, 'for example, rthe grid swing be approximately vof amplitude 0 2 as shown on the-GI waveform then the half periods would be equal tothe shorter interval tito tx. And to the still shorter half periods of t2 Vto tx if the grid amplitudes were equal to It has "been `found easy in'practice to provide signals of sufficient amplitude, using the circuit of Figure 1, to vary the driving frequency as much as 4 to 1 or more and still maintain'locking. This stability is .retained with enormous changes in line voltage because `driving and driven voltages vary 'more or less together. In fact,.it is common experience to maintainlocking throughout when reducing line voltages .until the signals disappear .from lack of emission inthe rectifier supply. In a commercial instrument a complete set of cascades pass routine tests of line voltage having extreme changes from to 130 volts. Itis understood from this discussion above that if .a cascade is designed to operate with a plus-minus'tolerance of one octave (either side) of its design frequency then component tolerances can be broad indeed.

DIVI'DING BY A FACTOR OF 4 uFigure 4 shows a typical divide-by-four type `cascade employing but two slave oscillators and one `master oscillator. This embodiment is similar to the divide-'by-two cascade embodiment disclosed in Figure l except that the slave multivibrators are locked by signals having four times the 'frequency ofthe slave oscillators in said embodiment. Certain components in this figure bear the same reference numbers and letters as like components in the embodiment of Figure 5 to be described presently. Also each multivibrator acts as a source for two tones as the fundamental frequency at the ljunction points A of the plate resistors is twice that of either plate when using balanced, symmetrical multivibrators. As the amplitude of the fundamental frequencies f/2 and f/8 at the junctions A is considerably less than that of f/4 and f/16 at the plates, suitable adjustment can be made through the dropping resistors RIU, RI |-R|3. For example, R12 may be `on thel order of -10 times RIS.

When dividing 'by a factor .of four .the allowances for Yvoltage variation, tube variation, and .component tolerances cannot be as generous as *when vdividing-bya factor of two in the first embodiment. It has been found, however, that by choosing .an approximate voptimum driving signal amplitude these variations vcan be compensated for very adequatelysuch that this embodiment can be satisfactorily `used in a keyboard musical instrument'with'assurance of an invariable pitch relation of tones. This isaccomplished conveniently'by using .two trimmer coupling Vcapacitors C2 and C3 between the respective stages. -For instance, --if the-component tolerances add up minus there will be a different setting of the trimmers from the setting for positive tolerance summation. This adjustment is a simple factory expedient and permits the use of components having broader commercial tolerances while at the same time allowing for maximum drift in component Values during use of the instrument. By using resistors having values considerably higher than the internal conducting plate resistance of the tubes, then any interchange in tubes has a trivial eiect on the operation.

It is observed, as will be demonstrated later, that the harmonic content of the output from the junction points A is different from that of the plates. The difference may be great enough to preclude use of this example on a musical instrument. A further diiiculty encountered in practice is that some sub-octave signal usually appears at the junction A because maintaining a perfect balance is not too practical. However, all of these .diiference and difficulties are readily surmounted when a musical system is used which intentionally employs iixed sub-octave and octave coupling to enhance the tones as proposed in the co-pending U. S. Patent No. 2,545,655, granted March 20, 1951.

A practical application of this technique is shown in the circuit of Figure 5. Here, of course, the Various dropping resistors Ri, Rl! to R22 are modiiied to compensate as much as possible for the differences in waveforms in addition to supplying the sub-octave (16 foot) and octave (4 foot) as well as the basic (8 foot) tones. The typical values of the components used in this circuit are, as follows:

C NOTE CASCADE Resistors RIS-220,000 ohms BH4-100,000 Ohms R15-3.9 Megohms Rl 5-470,000 ohms RII-470,000 ohms Rl8-3-9 Megohms RIS-3.9 Megohms R20-470,000 ohms R21-470,000 ohms REZ-220,000 ohms Capacitors Inductance L-.650 henry (variable) The operating conditions obtaining while a slave multivibrator is driven at 1A. the frequency of its master will lbe examined more thoroughly as this will bring out both the need for and the value in using trimmer coupling adjustment between the stages.

The waveforms from the circuit, Figure 6 are similar to those of Figure 3 except that there must be a skip pulse between each locking pulse. The skip pulses are indicated as occurring at times tx. It is evident that if the pulses were too large then locking would occur at tx and frequency division would be by two instead of by four. If, on the other hand, the pulses were too small, division might drop to six or some other even integer (odd integer division is practically out for the symmetrical circuit of the type shown). Thus it becomes important to adjust the driving impulse amplitudes for optimumoperation by such means as the coupling trimmers of Figures 4 and 5. A more detailed analysis of the relation between driving (or locking) pulse amplitude as related to component tolerance may be made by considering the graphical grid waveform shown in Figure 7 In Figure '7 the grid discharge curves of Figure 3 are simplified. Periods for the half cycle are indicated. TI represents the half period of the highest frequency expected when locked while T2 represents that for the lowest frequency to be locked. The skip pulse then will occur at times Tl/2 or T2/2 respectively. The locking pulses occur at Tl for the higher frequency and at T2 for the lower frequency. The axis represents the approximate transfer point when the multivibrator is running at its natural frequency. The natural .period must always be as long as or longer than the longest locked period if locking is to be expected. Hence the grid wave must follow some path between OAM and OAN. OAM represents the path when capacitor and resistor tolerances fall short while OAN represents the path when these tolerances are plus. When locking for period Tl then the minimum locking voltage is proportional to ordinate I while the maximum ordinate is proportional to ordinate Clearly, if E' were smaller, than the OAN wave would not be tripped. If the signal would be tripped on the minus tolerance wave OAM at time Tl then the locked frequency would be the driving frequency divided. by two instead of the desired divided-by-four frequency. A similar argument holds for ordinates EE and l. It follows then that to allow for both frequency adjustment and component tolerances as represented by the ratio MN/OM, the signal must be f held between a maximum ordinate of i2 and a minimum of In the graph shown this-'doesnt allow for much variance in the magnitude of the driving signal. If the ratio MN/OlVi were somewhat greater than that indicated it is possible for the required signal limits to overlap, in which case no one signal amplitude would cover all allowances. This latter situation is overcome readily by permitting adjustment of the driving signal so that the optimum level may be set for each multivibrator.

The value in this is illustrated in Figures 7a and 7b. In Figure 7a OAM represents the grid curve of the natural frequency of a multivibrator, say, which came through the production line with all tolerances piled negative. Allowing for tuning period Variation from Tl to T2 it is evident that a Value between ordinate 05 and El@ is optimum. Similarly in Figure 7b Where thewilerances pile positive the optimum value is between ordinate 0l and Q8. Adjustment of the signal Voltage via the trim-Inder capacitors permits this optimum setting in that it adjusts for the tolerances in the driven tube as well as in the driving source. By making use of this feature dividing-by-four becomes entirely commercial. Without some such adjustment it would necessarily be commercially impractical as very close tolerances would be required and the cost likely would become excessive. This concludes the argument supporting the need and value in using the trimmer capacitors C2 and C3 shown in Figures 4 and 5.

The nature and amplitude of the plate output voltage ep and the junction output voltage ea will associ be I nade, as these considerations are helpful in determining the values for the dropping resistor network shown in the circuit of Figure 5. Using the circuit of Figure 2 a symmetrical multivibrator will be assumed wherein C1=C1, R2=R'2, and R1=R1. It will be assumed, further, as is true in most cases, that the driving signal has but little effect on the waveform. Both the plate voltage ep and the juncture voltage es are shown to advantage in Figure 6. Because of symmetry, both plate voltages have identical form but one is displaced in time by T /2 or 1/2f where T represents the total recurrence period of the plate voltage with f its frequency. Thus if one plate is expressed as the alternating portion of a Fourier series it becomes:

epl=2 (a cos mot-H71. sin mut) [an cos awt-1r) -I-b sin Munt-1)] Solving these yields R l (3) ea=1oR1=[6p1-|6p2] Considering the sum ep1+ ep2 using Equations 1 and 2 the sum is zero for odd harmonics (odd values of integer n) but equal to 2cm for even values of n. Hence lepri-epd: 22 (a7L cos nwt-I- b1, sin mut) ln=2,4,6 or abbreviating (4) [epi--ep2=2ep1]n even Thus it is necessary only to analyse one plate voltage and note the values of the even harmonics as (3) becomes, using (4) In the circuit shown in Figure 5, R1=R2 whence The harmonic analysis of the plate voltage of a typical multivibrator is shown in Figure 8, From Equation 6 it is evident that the amplitudes of the even harmonics are proportional to the odd and even harmonics making up the junction voltage. It is seen that the junction voltage has a waveform that is substantially saw-toothed, as the amplitudes of this voltage vary approximately inversely proportional to the order of the harmonic. By proper choice of dropping resistors the plate and junction outputs can be combined to yield a substantially saw-toothed wave also from the plate. Such a waveform is shown in Figure 8a which is produced by doubling the percent of the second harmonic content. In this manner a single multivibrator becomes a two-tone source 2 ea= [epdn even l0 having similar waveforms with frequencies octavely separated.

By using similar but extended reasoning to that given above it follows that not only can the waveforms be adjusted but also the sub-octave and octave frequencies can be determined through an approximate dropping resistor network.

An alternative generator circuit is shown in Figure 9 in which the outputs taken from a junction point in the grid circuit between resistors R2 and Ri and also from a plate. The value of resistor RI may be Very high as its chief function is to keep the average grid potential from drifting. 1n this circuit similar analyses hold for the waveform of the grid-juncture output. Equation 6, for example, becomes (7) ex=2iegln even where ex is the junction voltage and eg either grid voltage. (See Figure 10.)

The chief advantage in taking the double-frequency supply from the grid juncture instead or from the plate juncture is that the grid voltage has a considerably higher content of even harmonics and hence slight unbalance in the circuit components do not inject nearly as much suboctave frequency component into the output. rThe values used in practice for a typical cascaded unit, Figure 9, are

Resistors Capacitors Cl-.0027-.01 mfd.

C2-200 to 800 mmf. (variable) C3-5 to 30 mmf. (Variable) CII-.0022, .0047, and .022 mid.

C5-.027 mfd. C15-.01 mfd. C1-.01 mfd.

Inductance Ill-.650 henry At Figure 10 is a diagrammatic view of an organ network in which our improved tone sources are connected for conduction of the output waveforms to a filter system from selected ones of said sources. It is assumed that a keyboard of the organ will have sixty playing-keys, and that for and actuated by each separate key is an electric switch, as first above stated, the arrangement being such that when the key is raised, the switch is is an ofi condition and when lowered, it is in an on condition. To avoid complications, only nine of the switches are shown. These are designated KCI, KCZ, KC3; KCtti, KC#3; KDI, KD#"3, and KB5. For the purpose herein, the frequency intended to be effective at key KCl will be approximately 65 cycles per second. rEhe irequency effective at key KBS, which is key t@ in said keyboard will be approximately l975-cycles per second. As shown herein said switches are arranged in groups X and XI. Common to switches for notes KCi-KC3 is a conductor X2.

assaggi Common to switches KC-#S-KB is a conductor X3. Conductor X2 is connected by a lead wire X4 to an electric switch Si. Similarly, conductor X3 connects with an electric switch S2 via a lead wire X5. To this point in the description the conductor X2 may be taken to represent the lower register of a split keyboard and conductor X3 the upper register of said keyboard.

A quality or timbre control system is employed for modifying the complex output waves from the aforementioned tone sources, and, as shown, said system comprises a conductor X6, one terminal of which is connected to ground via condenser CI. The opposite terminal of said conductor is connected `in circuit to contactor X1 of switch Si, which said contacter connects to ground through a resistor RI. In like manner, a conductor X8 has one of its terminals connected to ground via a condenser C6, the other terminal connecting to contactor X79 of switch S2. Contactor X9 connects to ground via a resistor R5. Contactors XID and Xli of switches Si and S2 are each thereof grounded at XII 2.

The wave filter system is designed to produce output waves whichV are typical of organ qualities known as the Principal, Horn and Strings. We have made provision for only three tone qualities but this number can be increased by changes in the types of filters employed. Y Provision, however is made herein for producing each of these tone colors at different predetermined amplitudes as will be described presently. Conductor XS has a resistor R2. Conductor X8 has a similar resistor R6. Components FI, Pl, F2, P2, F3 and P3 are electric switches connected to a common output conductor 200 which connects to an electroacoustical system which includes power amplifier 306, a loud speaker 300 and a resistance RH to ground.

Switches Fl and PI are connectedto conductor XB by resistances R4 and R3. vAt C2,'C3, C4, Cl, C8 and C9 are condensers and at RB and R4 are resistances. Condensers C2, C3 and Cl are connected across conductor X6. Resistances R8, R'i and condensers C, AC8 and C9 are connected across conductor X8, as shown. Resistance R8 is connected by lead 500 toV resistance R4 to switch FI. R'l connects in circuit to R3 by lead Tll. C8 connects in circuit to C3 by lead 80E] and C9 connects in circuit to C4. From lead '100 is a connection 990 to a coil L, which forms part of a circuit mesh that includes condensersC and C I E) in branch paths to the movable contactors of switches F2 and P2. Y i

In Figure 10 the twelve rectangles designated C, Cit, D, Dt, E, F, Fit, G, Gt, A, At and B respectively represent twelve cascade units, such that there is one such cascade for all notes of the same letter of the chromatic scale, each separate cascade unit having as many output circuits as there are individual oscillators. To avoid complications and confusion, we have shown certain of the outputs from cascade units C, Ct, D, Dit and B only connected to their coactive keying-circuits. 'I'he resistors Rl and R5 are moderately low for two reasons. One is to retain satisfactory multivibrator operation by preventing deleterious reaction between multivibrators when numerous keys are pressed simultaneously. The other is for tonal considerations in holding the Q of the Horn resonant circuit impedance level.

The Principal tone is produced in the lower register by means of the R-C low-pass filter comprising R2 and CI. Loud Ftone at stop switch Fland soft P tone at switch PI are determined by the diierence in the relative values in the dropping resistors R3 and R4 which connect X6 with switches FI and PI. The Principal tone from the upper register is produced similarly via R5 and C6 in conductor X8. The magnitude of capacitor CS is considerably smaller than that of Cl in the lower register because of the higher frequencies involved. Were the same size used for both registers there would be no need of separate lter groups, but, in this case, either the lower notes would not have their higher partials ltered sufficiently or else the higher notes would have even their fundamental components attenuated far too severely. Although more groups can be used, two groups provide a reasonable compromise simplicity and tonal amplitude balance.

The Horn tone is produced by a series resonance circuit comprising C2, C1, C5, Cl, and inductor L. As shown, C5 and CIU connect with respective stop switches F2 and P2. A resonance in the vicinity of 600 cycles with a circuit Q of approximately 2 has been found to be quite satisfactory in a commercial instrument. Capacitors C2 and C? which draw from the two registers are alike. Cl is smaller than C5 to provide the required intensity differences. Regardless of the position of any of the switches involved the circuit arrangement is such that C5 and Cie are in parallel, these two acting in series with the parallel combination of C2 and C1. Thus the resonant frequency is unaltered by either registry switch or P, F switch manipulation.

The String tones are produced by passing the signal through appropriately chosen capacitors C3, C03 in the lower register and C8, C9 in the upper register and connecting same to respective Stop switches F3 and P3. Said capacitors emphasize the magnitudes of the higher frequency components of the various tones.

The switch poles are grounded when in the I off position so as to prevent any undesired cross-talk between the several stops. The values of the components used in the circuit illustrated at Figure 10, are:

Resistors Capacitors lfm-4,700 ohms CI-.047 mfd. R2-18,000 ohms C2-.002'7 mfd. R3-220,000 ohms C3-.0018 mfd. Ril- 68,000 ohms C4-500 mmf. R5-4,700 ohms C5-.01 mfd. R6-18,000 ohms C6-.O1 mfd. Ril-270,000 ohms C'I-.0027 mfd. Ril-400,000 ohms C3-.0018 mfd. el I-10,000 ohms C9500 mmf.

Inductance CI 0-.0027 mfd.

L-24 henry The term unit cascade employed herein shall be construed to mean any desired number of oscillator stages for any given frequency range.

The term musical instrument employed herein shall be construed to mean an instrument` employing any well known selectors for the transmission of tone frequencies to an electroacoustical translating system from the aforementioned oscillators. Said selectors may consist of one or more keyboards and a pedal keyboard. We have shown and described a single keyboard which is divided to provide what we have referred to as upper and lower registers, and have connected same in circuit with a wave filter system and to respective groups of frequency sources so that tonalities can be produced when playing the keys in one register that will be different from tonalities elicited when playing the keys in the other register. By our unique system of wave filters it is also possible to produce the same tonality from both registers and to mix a tone of one quality with a tone of a different quality and with mathematically correct effects and with no robbing amongst the different frequencies.

What is said above regarding grouping predetermined tone frequencies for the playing of tones of different timbre from the different registers of a divided or split keyboard applies in like manner to playing different tonalities from separate and distinct keyboards.

The term resistive mesh as used herein shall mean three resistances connected together to provide a common junction point between two resistances and a third resistance, such that the third resistance connects with a plate supply source and two resistances connect to the plate circuits of the triodes of a multivibrator, and whereby a driving pulse wave is injected into the plate circuits via said common junction point.

The means for producing a plurality of harmonically related waveforms from a single multivibrator as disclosed in Figure 9 shall comprise the aforementioned resistive meshes, the values and locations of' the various electrical components in the disclosed network, including the variable capacitive couplings between stages and the capacitive devices in the grid circuits of the triodes of said multivibrators.

Particular stress is placed on the herein disclosed means for preventing objectionable or unwanted submultiple feedback from a lower (driven) multivibrator stage to the next adjacent (driving) multivibrator stage. Taking for eX- ample, driven stage T3 and driving stage T2 in Figure l, it is significant that the triode sections of each thereof are alike and that the components are connected in the same positions and are also alike. At the junction point P of the driven stage, the frequency is the same as that of the plate (or grid) of the driving stage. A balanced condition is established, such that the frequency at the junction point P of a driven multivibrator stage is the same as that of the plate (or grid) of a coupled driving multivibrator stage which insures against objectionable r unwanted feedback from the driven to the driving stage. Our use of small capacitors C3 in the couplings between stages further insures against unwanted or objectionable feedback even though the driven multivibrator is considerably unbalanced. The term balanced shall be taken to mean that the triodes of the driving and driven stages are alike and that the components are connected in the same positions and are also alike, whereby the 0perating characteristics become interchangeable.

' What we claim as our invention is:

l. A musical instrument of the class employing an electrical network including an output circuit having an electroacoustical translating device; said instrument comprising a control frequency source including a control frequency oscillator producing an output wave of a given frequency; a series of driven multivibrator stages, each having a pair of triodes, a resistive mesh embodying two resistances connected to the respective plate circuits of said triodes and a single resistance connected to said two resistances to provide a common junction point between the formery and the latter, said single resistance connected to a-i-Bl supply source, and a resistance to ground in each cf the grid circuits of said triodes; a capacitive coupling between the output of said control frequency oscillator and the junction point at the plate resistances of the first multivibrator stage; a capacitive coupling between the output of the first multivibrator stage and the junction 'point at the plate resistances of said second multivibrator stage; and keying-circuits connecting said control frequency oscillator andsaid multivibrators for selective transmission to said output circuit of output waves from said oscillator and said multivibrators.

2. A musical instrument according to claimk l. wherein the output circuit of said network has a resistance which is low relative to the impedance of said control frequency oscillatorand said multivibrators.

3. A musical instrument according to claim 1, wherein said plate resistances are low-relative to said grid resistances.

4. A musical instrument according to claim l, wherein said plate resistances are low relative to said grid resistances and the output circuit ofl said' network has a resistance which is low relative to the impedance of said control frequency oscillatorA and said multivibrators.

5. A musical instrument according to claim 1, wherein a capacitive element connects to an intermediate point between the plate and the plate resistance of one triode of a multivibrator stage and the grid of the other triode of the same stage.

6. A musical instrument according to claim l', wherein and with respect to each multivibrator stage a capacitive element connects to an intermediate point between the plate and the plate resistance of one triode of said stage andthe grid of the other triode of the same stage, and wherein all of the aforementioned resistances and said capacitive elements are of such values that each multivibrator stage oscillates at a frequency which is a predetermined submultiple of said' given frequency.

7. A musical instrument according to claim 1, wherein and with respect to each multivibrator stage a capacitive element connects to an intermediate point between the plate resistance of one triode of said stage and the grid of the other triode of the same stage, and wherein the values of the last named capacitive elements double progressively in the direction of the lowest frequency multivibrator stage.

8. In an electrical musical instrumentemploying keying-circuits and an electr-@acoustical translating system for translating electrical energy into acoustical energy; comprising a control frequency source consisting of an electron discharge device producing an output Wave of a given frequency; a driven frequency said instrument 9. An electrical musical instrument according to claim 8, wherein the resistances in the grid circuits are equal and high relative to the plate circuit resistances.

10. In a musical instrument employing an electrical network having a plurality of keying-circuits, an output circuit to which said keying-circuits are connected, and an electroacoustical translating system connected to said output circuit for translating electrical energy into acoustical energy; a submultiple generator embodying a control circuit providing a source of alternating current of a lgiven frequency; a multivibrator, the natural oscillation frequency of which is a desired submultiple of said given frequency; means coupling said control circuit to said multivibrator for impressing a portion of the output of said circuit on said multivibrator to control oscillation thereof at said desired submultiple frequency and for impressing another portion of the output on one of said keying-circuits; means for stabilizing oscillation of said multivibrator at said submultiple frequency; and means by which waves at harmonically related frequencies are derived from said multivibrator and respectively impressed on others of said keying-circuits.

11. An electrical musical instrument according to claim 10, wherein the stabilizing means includes a resistance in the plate circuit of each triode of said multivibrator, a resistance connected to a plate supply source and to the first mentioned resistances to provide a common junction point between said resistances, and wherein the coupling means connects with said multivibrator at said junction point.

l2. In a musical instrument employing an elec-- trical network having an output circuit provided with an electroacoustical translating system; a submultiple generator having at least two multivibrator stages, each consisting of two triodes, a resistance in the grid circuit of each triode, a resistance in the plate circuit of leach triode, and a resistance connected to the plate resistances to providea common junction point between them; a control frequency oscillator producing an output wave of a given frequency, a capacitive element coupling the output of said control frequency oscillator to the rst multivibrator stage at the junction point between the plate resistances thereof, a capacitive element coupling the rst multivibrator stage to the second multivibrator stage at the junction point between the plate resistances thereof, and at least one keyingcircuit for each multivibrator stage and said control frequency oscillator for transmitting portions of the outputs thereof to the output circuit q of said network.

13. An electrical musical instrument according to claim 12, wherein the aforementioned capacitive coupling elements are each thereof adjustable.

14. In a musical instrument employing an electrical network having an output circuit provided with an electroacoustical translating system, said instrument comprising a submultiple generator having a control frequency oscillator and at least a first multivibrator stage and a second multivibrator stage, said multivibrator stages each consisting of twin triodes, a resistance in the plate circuit of each triode, and a single resistance connected to said resistances to provide a common junction point between them; a rst keying-circuit connecting the control frequency oscillator to said output circuit, a second keying-circuit connecting the junction point at the plate resistances of the first multivibrator stage to said output circuit, a third keying-circuit connecting the plate circuit of one triode of said rst multivibrator stage to said output circuit; a fourth keying-circuit connecting the junction point at the plate resistances of the second multivibrator stage to said output circuit, and a fifth keying-circuit connecting the plate circuit of one triode of the second multivibrator stage to said output circuit.

15. An electrical musical instrument having an electroacoustical translating device; said instrument comprising a submultiple generator including a driven frequency device embodying a vacuum tube having two triodes, the grid circuits of which have resistances connected to ground, and plate circuits connected at a common junction point to a third resistance, said third resistance connected to a plate supply source; and a control frequency device operating at a given frequency, said control frequency device capacitatively coupled to said driven frequency device for impressing a driving pulse on the plate circuits thereof via said common junction point; a wave transmitting circuit connecting the control frequency device to said translating device, and at least one wave transmitting circuit connecting the driven frequency device to said translating device.

v16. An electrical musical instrument having an electroacoustical translating device; said instrument comprising a submultiple generator including a driven frequency device embodying a vacuum tube having two triodes, the grid circuits of which have resistances connected to ground, and plate circuits connected at a common junction point to a third resistance, said third resistance connected to a plate supply source; and a control frequency device operating at a given frequency, said control frequency device capacitatively coupled to said driven frequency device for impressing a driving pulse on the plate circuits thereof via said common junction point; a wave transmitting circuit connecting the control frequency device to said translating device, and at least two wave transmitting circuits connected to said driven frequency device for transmitting therefrom harmonically related output waves which are submultiples of the output wave of said control frequency device.

17. A musical instrument of the class employing keying-circuits, a divider circuit including a master oscillator having at least a plate, a grid and a cathode, and a plurality of slave oscillators, said master oscillator and said slave oscillators coupled in cascade, each slave oscillator comprising a multivibrator providing a pair of triodes; a plate voltage supply source; an impedance in the plate circuit of each triode of each individual slave oscillator such that for each slave oscillator there is a pair of impedances, another impedance connected to said pair of impedances and providing a common junction point between them; means for impressing a portion of the output from a slave oscillator on the junction point of the plate impedances of the next succeeding slave oscillator and a portion on one of the aforementioned keying-circuits; an impedance connecting the master oscillator to said plate supply source, and means for conducting a portion of the output from said master oscillator to the next succeeding slave oscillator and a portion to another one of the aforementioned keying-circuits.

18. In an electricalmusical instrument; a control frequency source, a multivibrator of the duotriode type, means for producing two different Wave-forms at harmonically related frequencies from said multivibrator and comprising a resistive mesh including two resistances connected to the plates of the respective triodes of said multivibrator and a single resistance connected to said two resistances and providing a common junction point between the latter and the former, a capacitive coupling connecting said control frequency source to said common junction point, a resistive mesh including two resistances connected to the respective grids of said triodes and a single resistance connected to said grid resistances to provide a common junction point between the latter and the former, a keying-circuit connected to plate of one of said triodes, and a keying-circuit connected to said second named junction point, the rst named single resistance connected to a plate supply source and the second named single resistance connected to ground.

19. In a musical instrument employing an electroacoustical translating system, said instrument comprising, a submultiple generator including a control frequency source operating at a given frequency and a series of multivibrators capacitively coupled in cascade, each of said multivibrators consisting of a vacuum tube of the type employing duo-triodes; each multivibrator including a resistive mesh consisting of two resistances connected to the plates of the respective triodes thereof and a third resistance connected to said two resistances and providing a common junction point between the former and thelatter, a keying-circuit for and connecting the junction point of the resistive mesh of each multivibrator to said translating system, a keying-circuit for and connecting one plate of the triode of each multivibrator to said translating system, each of said multivibrators having electrical components which are coactive with the resistive mesh of said multivibrator and with the capacitive couplings of said generator so that the output frequency at said one plate is one half that of the output frequency at the junction point between the two resistances and the third resistance of said resistive mesh.

MERWIN J. LARSEN. NORMAN B. ERICKSON.

REFERENCES CITED UNITED STATES PATENTS Name Date Larson July 2, 1946 Number

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