US2233948A - Electrical organ - Google Patents

Description

March 4, 1941. w. E. KocK ELECTRICAL ORGAN Filed March 17, 1958 5 Sheets-Sheet L uzi m INVENTOR March 4, 1941. w. s. ocK

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March 4, 1941. w KQCK 2,233,948

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Patented Mar. 4, 1941 UNITED STATES PATENT OFFICE ELECTRICAL ORGAN Application March 17,

42 Claims.

This invention relates to the production and employment of electric oscillations. It is particularly applicable in the electrical production of music; and accordingly as an aid to its description, an exemplary embodiment will be used in this specification, comprising a musical instrument in which it is included. By thus embodying the invention an improved musical instrument can be provided employing electric oscillations, in which musical tones may be derived polyphonically from these oscillations, and in which sustainable tones of a variety of timbres may be secured singly, in various combinations as desired and as independent voices, all as will be shown in the complete description to be given below. It will thus be evident to those skilled in the art on reading this, that the invention may be used advantageously in the electrical production of organ-like music, and for operation by organ-like keyboards and controls. Hence in a particular sense it pertains to an improved electrical organ.

In applying the invention to the production of music, I have in mind a process which consists in generating audio-frequency electric oscillations of complex character and according to a musical scale, deriving useful oscillations polyphonically from them, modifying and/or combining the derived oscillations and converting them into sound, employing amplification wherever necessary or desired. The invention has many advantages to form a unique process of this nature and the improvements effected by it will be described accordingly. These may then be outlined in the following objects:

It is an object to provide a plurality of sources of complex electric oscillations, corresponding to notes of a complete multioctave musical scale, of essentially similar wave forms and substantially equal strengths throughout the sources.

In the exemplary embodiment which will be set forth in this description electrical relaxation oscillation generators are employed for the most part as sources of electric oscillations of complex One of these oscillators is employed for substantially each pitch note of a musical instrument and it is an object to provide improved means for controlling their frequencies in groups-each group corresponding to a note and its octaves in a multioctave musical scale and comprising an improved cascade frequency system including a stable source of oscillations and a series of relaxation oscillators, related in frequencies by successive exact octaves and operating exactly in phase. With this is an object to provide a simplified, accurate and stable tuning of an electrical musical instrument.

It is an object to provide an improved tremolo in an electrical musical instrument.

It is an object to provide a wave modifying net- 1938, Serial No. 196,484

work in a generator of highly complex oscillations whereby the form of oscillations, in deriving them from the generator, will -be altered so as to be of simpler structure and to yield per se an inherently beautiful tone on conversion into sound.

Another object is to provide means for the elimination of undesired transient efiects at the beginnings and ends of tones in an electrical musical instrument.

It is an object to provide improved means, with reference to the notes of a keyboard or the like, for polyphonically deriving useful oscillations from oscillators, whereby a collecting network for a keyboard, in conjunction with various circuits, presents substantially constant impedance at all times, cross-talk is eliminated for all practical purposes, the volume of sound obtained from the playing of tones of identical character is proportionate to the number of tones played and proper tonal balance is secured. Two other objects which may be considered as extensions of this are to provide means for simultaneously energizing a plurality of networks at a keyboard for collecting harmonically related oscillations and to provide means for the independent operation of a plurality of keyboards. The attainments of these objects are of particular advantage in a musical instrument employing only as many oscillators as there are notes in a single multioctave musical scale.

It is an object to provide improved branch circuits whereby oscillations derived polyphonically in a collecting network or networks, according to notes of a multioctave musical scale, may be divided according to various tonal timbres and whereby these tonal timbres may be selected independently or in any combination, as desired. With this object is another object, to provide improved means for independent tonal voices.

It will be shown subsequently in this description that a feature of the present invention is the provision of electric oscillations of sufilcient complexity, so that in modifying them for the production of musical tones of various timbres it is not necessary to increase their complexity. Thus, means such as asymmetric electrical devices or the like need not be used in the tone timbre systems in my invention for increasing oscillation complexity, and hence the tone timbre means I employ are essentially of linear character. In addition to economy, an advantage of this is that oscillations corresponding to a plurality of tone pitches may be transmitted simultaneously through these circuits without the creation of oscillations of extraneous frequencies. With the above in mind three objects may be stated:

To provide improved means for the accentuation of even order harmonics in complex osclllations.

To provide improved means for the accentua tion of odd order harmonics in complex oscillations.

To provide improved means for the accentuation of harmonics in various pitch regions.

Another object of the invention is to provide improved recombining means for a multikeyboard electrical musical instrument, together with a common tone channel output.

Again it is an object to provide a musical instrument which may be made by methods current in the electrical, particularly in the radio manufacturing industries, which may be constructed at a not excessive cost, which will be portable and have a wide range of adaptability.

It will be understood that the present invention is not necessarily limited to the exemplary embodiment employed in this specification. Other exemplary applications of the features of the invention, singly or in combinations in electrical musical instruments operated according to various principles, will become apparent to those skilled in the art on reading this description as well as the fact that a number of the features are useful in other arts. With this in mind the scope of the invention will be pointed out in the appended claims and the manner in which the above ob- ,iccts are attained, and the attainment of other objects which will be pointed out hereinafter or will become apparent to those skilled in the art upon reading this specification, will now be described in connection with the exemplary embodiment referred to above. For clearness reference will be made to the accompanying drawings forming a part hereof, wherein:

Figure 1 is a somewhat simplified wiring die.- gram of a group of oscillators forming a cascade frequency system.

Figure 2 is a cathode resistor and its immediate connections, to be taken in conjunction with Figure 2-A which shows the form of the voltage developed across this resistor.

Figure 3 is a wave modifying network, to be taken in conjunction with Figure 3--A which shows the wave form of the input voltage to this network, and with Figure 3-3 which shows the wave form of the output voltage from the network.

Corresponding to Figures 3-A and 3-B are Figures 4 and 5 which display respectively the spectrum structure of the wave forms in Figure 3-A and Figure 3-13.

Figure 6 is a wiring diagram of a group of oscillators forming a cascade frequency system in a form which is being employed in commercial production, and including a tremolo oscillator.

Figure '7 is a wiring diagram of a power supply for a part of the instrument.

Figure 8 indicates the connections and switches for any one keyboard.

F gure 9 is a wiring diagram of a tone timbre system.

Figure 10 is a wiring diagram of the amplifiers, volume control, and loud speaker for producing musical tones.

Briefly in a practice of my invention as set forth in the exemplary embodiment I employ glow-discharge oscillators as generators of electric oscillations cf complex character. By a generator of oscillations of complex character I mean one producing oscillations in which a component having a fundamental frequency is to be found in combination with a large number of other components, of substantial strengths and related harmonically to the fundamental component.

Thus in the system of Figure 1 is shown a group of oscillators comprising a highly stabilized glow-discharge oscillator 01 as a stable source of oscillations, and a series of relaxation oscillators which are glow-discharge oscillators O2 and 03 of the relaxation type. The oscillators O1, O2, and 03 therefore include each a tube I containing an ionizaible gas under low pressure, in which electrodes are immersed. The discharge electrodes in the tube comprise an anode A and a cathode K, and it will become apparent on a further reading of this description, that it is advantageous to provide in the tube a control electrode, the grid G, and to employ a heated, i. e., a thermionic type of cathode. For heating the cathode a filament F is used, and with the use of a thermionic cathode it is advisable to apply to the grid a negative voltage bias, shown in Figure 1 as derived from the source -C, referred to ground.

It will become apparent that the use of glowdischarge oscillators such as 01 as stable sources of oscillations does not necessarily constitute a limitation. Many kinds oi generators of audio frequency electric oscillations are available as stable sources for the broad purpose intended. However, by employing oscillators such as 01, of a generic type known as an inductive glow-discharge oscillator, described in my United States Patent No. 2,046,463 issued on July 7, 1936, together with the additions made to them herein, several additional advantages may be obtained, as will become apparent on a further reading of this specification it is of instant interest to mention my copending United States patent applications, Serial No. 46,449 filed October 23, 1935, Patent No. 2,139,898 issued December 13, 1938, and Serial No. 196,485 filed March 17, 1938.

The other oscillators O2, 0a are generically of the relaxation type. Each is energized from a source of positive D. C. anode potential +13 through a resistance R2, or R3, connected from +B to an anode A of a tube T. Connected from the anode A to the cathode K is a condenser C2, or C3. The cathode K has a connection to ground and the potential source +B is referred to ground. Oscillation action is as follows: A condenser C: in 02 say is charged relatively slowly from the source +B through a resistance R2 until the ignition potential of the tube (somewhat lower than that of +B) is reached. The tube T then ignites (i. e., the gas therein suddenly ionizes), a discharge current flows from anode to cathode, discharging the condenser relatively rapidly until the extinction potential of the tube is reached. The tube then extinguishes, the condenser recharges relatively slowly and oscillations of the relaxation type are thus produced, the oscillator being operable over a wide frequency range. While the frequency of an ordinary glow-discharge oscillator of the relaxation type is determined by the value of the source +13, the tube potentials and the circuit parameters, R and C, it ordinarily does not have excellent frequency stability, as is well-known owing to variations in the values of these items, particularly variations in tube potentials and in a source +B if derived from a commercial power supply. resistances and condensers as commercially produced today remain reasonably constant.) Relaxation oscillators, generally, are not of great frequency stability.

Thus while relaxation oscillators produce com- (Values of tion oscillators.

aasaeea plex waveforms readily susceptible of modification and hence in one respect are desirable in the electrical production of music, a musical instrument constructed to employ them would not ordinarily stay in tune. Moreover, in other devices employing relaxation oscillators in the production of harmonically related oscillations this same problem would occur.

I have found however in a relaxation oscillator employing a discharge tube, say a glow-discharge oscillator of the relaxation type, that by employing a grid placed between the discharge electrodes of the tube and by applying to the grid an oscillating voltage of a frequency somewhat higher than that of the relaxation oscillator per se, the frequency of the oscillator can be raised to become an exact harmonic fraction of the applied voltage frequency. As an example of this it has been found that the oscillator frequency may be raised from a value per se nonspecifically less than one half but more than one quarter that of the applied voltage, to become exactly one half that of the applied voltage, 1. e., exactly one octave lower.

From this a cascade frequency system has been evolved employing a master oscillator as a stable source of oscillations and a plurality of relaxa- In this system a first relaxation oscillator may have aninherent non-specific frequency between one half and one quarter that of the source, a second relaxation oscillator may have an inherent non-specific frequency between one quarter and one eighth that of the source, and so on. Now, if a portion of the oscillation voltage from the source be fed to the grid of the first relaxation oscillator, and a portion of the oscillation voltage of the first relaxation oscillator be fed to the second, and so on, then the first relaxation oscillator will be controlled by the source so as to fix its frequency at exactly one half that of the source, the second relaxation oscillator will be controlled by the first relaxation oscillator so as to operate at exactly one half the frequency of the first and so to have a frequency exactly one quarter that of the source, and so on.

,Hence a cascade frequency system can be provided comprising a plurality of oscillators related by successive exact octaves. By providing twelve such systems including twelve tunable audio frequency master oscillators corresponding to a high octave register of a musical instrument, oscillations from which useful oscillations may be derived for musical tones are provided for a complete multioctave scale in a musical instrument. Fundamental advantages are obtained: economy in cost and weight of oscillators, a simplified accurate and stable tuning, the provision of electric oscillations of complex character susceptible of modification and combining, and the avoidance of oscillations from other oscillators appearing in those derived from any one oscillator.

A description in detail will now be made of an cluding L1C1, the higher this ratio the greater the stability. Thus for a given value of inductance L1 it is preferable that resistance in the circuit be low; therefore I have avoided as much as possible the use of resistance therein (some residual resistance exists of course such as any resistance inherent in L1 and the average resistance of the discharge tube Tthe former may be made of low amount by the use of good quality inductance L1 and the latter is of low average value since it has been found that the tube of an oscillator such as 01 is in a state of discharge the major part of a frequency cycle).

With this in mind, I have devised a method for deriving voltage for cascading from 01. In this, an inductance L2 is magnetically coupled to the inductance L1 to form a transformer (which may have a ferromagnetic core) and one terminal of L2 is connected to the grid of the first relaxation oscillator 02 to be controlled, by means of a connection l--l' (a condenser 04 is included in this connection to permit the application of negative bias voltage to the grid of 02). The other terminal of L2 may be grounded together with one end of L1, the other end of the tuned couple L1-C1 being connected to the anode A. With the operation of the oscillator 01 is a flow of alternating current through the circuit including L1 and thus an alternating voltage is induced in L2. The cathode of the oscillator 02 has a connection to ground, as shown, and consequently an alternating voltage at the frequency of 01 occurs between grid and cathode of 02. Now in a relaxation oscillator operated such as O: the grid-to-cathode resistance is very high and therefore negligible energy is drawn by it. Hence little energy is extracted thereby from the oscillator 01 and correspondingly, negligible resistance is reflected through the transformer L1-L2 into the stabilizing circuit of 01. The resistance R1 in 01 serves the same general function as the resistances R2 and R3 in the oscillators O2 and O3.

Owing to the organization employed and by a selection of a terminal of L2 the phase of the alternating voltage applied to the grid of the oscillator 02 may be such that a positive halfcycle of it is applied to that grid concurrent with each discharge in the tube T of 01- Now the stabilized oscillator 01 may operate say at a frequency of 2093 cycles per second (0. p. s.) corresponding to a high C note in the equitempered scale based upon an A note=440 c. p. s., and the relaxation oscillator 02 may have an inherent frequency non-specifically between one and two octaves below that of 01. The tendency of each positive half-cycle or alternation of the voltage applied to the grid of O2 is to trigger a discharge in that oscillator, and they may be made of sufficient strength by selecting the turns ratio of the transformer L1-L2, to trigger the oscillator 02 when it is in a condition approaching discharge so as to prematurely discharge that oscillator to raise its frequency until just every other positive alternation in voltage applied to its grid causes a discharge concurrent with that alternation. Under this control therefore the frequency of 02 becomes exactly one half that of O1, namely to oscillate at 1046.50 c. p. s.-the C note an exact octave below the 2093 C note. Moreover since the discharges in 02 are now concurrent with every other positive voltage alternation applied to its grid and since these positive alternations are in turn concurrent with the discharges in 01, the two oscillators O1 and O2 operate exactly in phase.

There is shown in Figure 1 a ground as a common return. A ground is employed with the usual advantages generally throughout the electrical musical instrument of this description and unless otherwise stated all potentials are referred to it. Thus the cathodes K are connected to ground but in each of the connections from the cathodes of the relaxation oscillators O2, 03 there has been included an impedance which may be a resistor R4. The periodic discharges of the tube T of the relaxation oscillator 02, say, send a series of current pulses down through the resistance R4 of this oscillator and with each of these pulses the upper end of this resistance acquires a positive voltage. There is thus made available a series of positive voltage pulses at the frequency of the oscillator 02 and in identical oscillation phase with it, i. e., there is made available a positive voltage pulse concurrent with each discharge in the tube T of Oz. The form of the pulses is displayed in Figure 2-A (wherein the horizontal axis t represents time and the vertical axis V voltage), showing that they consist of a series of pulses, each of relatively short duration, interspaced by relatively long time gaps of inactivity.

'I'hese pulses from 02 are very useful for cascading, and I employ them to control the fre quency of the second relaxation oscillator 03 by connecting the upper end of the resistance R4 of the oscillator 02 to the grid G of 03, by means of a connection 2-4 (a condenser C4 is included in the connection for the same reason as that set forth for C4 in l-l above). Through this connection the grid of 03 will be excited by a series of positive voltage pulses derived from the discharges in 02. (Actually owing to the con- .denser C4, the grid of 03 will be excited by oscillatory voltage. However because of the form of the pulsating voltage derived across R4Se Figure 2--A-this oscillatory voltage will comprise a series of relatively intense positive voltage com ponents each of comparatively short duration, time-interspaced by relatively weak negative voltage components each of comparatively long duration.)

Now the oscillator 03 may have an inherent frequency non-specifically between two and three octaves below the stable oscillator 01 and the oscillator 02 may be operating as described at a controlled frequency exactly one octave below that of O1. The tendency of each of the positive voltage pulses derived from O2 is to trigger a discharge in 03. and they may be made of sufficient strength by selecting the value of R4 with respect to values for other impedances in circuit with it in the oscillator 02 to trigger the oscillator 03 when it is in a condition approaching discharge so as to prematurely discharge that oscillator to raise its frequency until just every other pulse on its grid causes a discharge concurrent with that pulse. Under this control therefore the frequency of 03 becomes exactly one half that of O2 and thus exactly one ouarter that of the stable oscillator 01. Thus if 01 has a frequency as above of 2093 c. p. s., a high C note, the frequency of 03 becomes 523.25 c. p. s., a C note exactly two octaves below. Moreover since the discharges in the oscillator 03 are now concurrent with every other positive voltage pulse applied to its grid and since these pulses are in turn concurrent with the discharges in On the two oscillators O2 and O3 operate exactly in phase.

Hence the oscillators 01, O2, and 0: all operate exactly in phase at the frequencies say of 2093, 1046.50, and 523.25 c. p. s. respectively corresponding to a high C note and its exact two octaves below. It will be apparent that while only two relaxation oscillators are shown, others which may be of similar construction and arrangement of parts may be added to the cascade system of Figure 1 to provide; say, complex oscillations at 261.62 (5), 130.81 {25), 65.40 (625) and 32.70 (3125) c. p. 5. corresponding to lower octave 0 notes in a multioctave musical scale. the first of these additional relaxation oscillators being cascaded from the relaxation oscillator O: in the same manner as described, the second from the first in that manner, etc. Thus a cascade frequency system is provided for a large number of octave notes with oscillators operating at successive exact octaves and exactly in phase. These operating features will be seen to be highly desirable as will become apparent on further reading of this specification.

It was mentioned above that the inherent frequency of a relaxation oscillator may be non. specifically between one and two octaves below that of its control. This in a cascade frequency system as disclosed corresponds to between zero and one octave below the frequency to which the oscillator is raised by the influence of its control. In practice however I have found it desirable, for uniformity, to set the inherent frequency of a relaxation oscillator somewhat more specifically, about one half an octave, i. e., about six semitones, below its controlled frequency. Thus if the oscillators O2 and 03 have controlled frequencies of 1046.5 and 523.25 c. p. s. respectively, the examples above, their frequencies per se would be set respectively at about 740 and 370 c. p. s.

Ordinarily design could not be relied upon to attain this approximate setting of six semitones below controlled frequency, owing principally to variations in characteristics of glowdischarge tubes as commercially constructed and to changes in tube characteristics over periods of usage of a musical instrument. However, I have made use in the present invention, of an invention of mine disclosed in a copending United States patent application Serial No. 131,422 filed March 17, 1937, Patent No. 2,179,791 issued November 14, 1939, wherein it has been shown that effects of variations and changes in tube characteristics could be substantially eliminated in a glow-discharge oscillator, and the use of the invention represented by that application is shown in each of the relaxation oscillators O: and O3 01 Figure 1 in the combination of the two resistances R5 and Rs. These resistance: are in series between the anode and the negative D. C. grid bias source C, the resistance R5 being connected to the anode A and resistance R6 to -C, the grid G being connected to the junction point of these resistances.

As a result of employing the invention represented by the aforementioned Patent No 2,179,791 in each relaxation oscillator in th( present invention, the approximate setting 01 six semitones below controlled frequency becomes independent of variations in tubes and i: maintained over periods of usage of an instrument wherein tube characteristics change 0] tubes are replaced or interchanged. It is alsc employed in the stabilized oscillatorssce thr resistances R1 and Rs in the oscillator O1--as a further stabilizing means.

A resistance Ra will be noted, in series with the grid of the oscillator 01. This acts as a limiting resistor to prevent excessive current flow in the grid circuit of 01, a tendency in such an oscillator.

In practice I make the value of the resistance R4 and thus the amount of voltage for cascading derived from a relaxation oscillator, somewhat larger than theory would require. By doing this, veriations in the value of the source of anode potential +B do not affect a controlled relaxation oscillator, since it is then dominated byga generous controlling voltage. I have already mentioned the eflicacy in the present invention of the use of the invention represented by the aforementioned Patent No. 1,179,791. In an actual practice of that invention it was also found to be of assistance in stabilizing a glowdischarge oscillator of the relaxation type against changes in +B, in addition to its stabilization against changes in tube characteristics-an added advantage of its use herein. The employment of a heated thermionic cathode in a glowdischarge tube has been mentioned. I have found that by its use a discharge in the tube may be initiated by a considerably smaller voltage change on the grid than would be required with a cold cathode in the tube. Hence by using it the voltage required to insure cascade coupling and consequently the value of R4 may be considerably smaller than otherwise. Now it will be shown later herein that there are advantages in deriving oscillatory voltage for tone production, in the discharge circuit of a relaxation oscillator. As R4 is in that circuit, a smaller value for R4 makes more voltage available across other impedances in the circuit for the purposes of tone production, say, and it has been found that with the employment of a thermionic cathode a value of R4 in keeping with relative smallness as compared with other impedances in the discharge circuit, may still be made large enough to provide a generous voltage for cascading. The provision of generous voltage for cascading a first relaxation oscillator such as 02 from a highly stabilized or master oscillator as 01 may be insured by a choice of the turns ratio of the transformer L1Lz.

(It is of present interest ,to mention anotherinvention of mine disclosed in a copending United States patent application Serial No. 53,955 filed December 11, 1935, Patent No. 2,128,367, issued August 30, 1938, wherein a cascade frequency system has been described, the present application, in part, being a continuation of that application. The improvements effected to provide a system according to the present invention will become apparent on comparison. Now it was found in a practice of the invention represented by the Patent No. 2,128,367 that all oscillators operated in phase and an explanation of this is of instant interest. For, in the cascade system of that invention, as in this, the frequency of an oscillator is controlled by having its discharge triggered by a positive oscillation voltage from the control being applied to the grid of the oscillator to be controlled. By employing a cathode ray oscillograph and examining various oscillations in the system it was found that coincident with the discharge in a controlling relaxation oscillator there occurred for a very short amount of time, a highly damped transient oscillation, owing possibly to parameters of the connecting leads, etc., and it is believed that the initial positive half-cycle of this transient oscillation was the principal factor in triggering a controlled oscillator to produce the in-phase operation experienced. In the present invention the positive voltage for cascading is directly derived in the cathode connection of the preceding tube.)

The same values for R4, C4, R5, and Re respectively may be used throughout the relaxation oscillators in this invention and relaxation oscillator feed resistances such as R: and R3 may have one value throughout. Moreover the tubes T may be of commercial identity and the same source +3 and C may be used for all oscillators.

It is appropriate at this point to list some values for the parts described above, which form exemplary working combinations in a practice of the invention.

The gas argon is employed in the tubes T under a pressure of 0.25 mm. mercury absolute:

+B=195 volts.

C=105 volts.

L1 and C1 (2093 c. p. s. for :4 henries and .001445 mfd. respectively.

VLc/Lr (turns ratio)=.17.

C: (740 c. p. s. perse of 0a) =.0023 mfd.

Ca (370 c. p. s. per se of On) =.0046 mfd.

C4=.00025 mfd.

R1=100,000 ohms.

R2 and R1=800,000 ohms.

R4=1,000 ohms.

R5=800.000 ohms.

Rc=1,000,000 ohms.

R1=500,000 ohms.

Ra=500,000 ohms.

. Rc=100,000 ohms.

A preference has been indicated for deriving electric oscillations for tone production, in the discharge circuit of a relaxation oscillator. This circuit in the oscillator 02 say, would be that including the condenser C2 and tube T of 02. One point of this circuit may be grounded as shown, and I have found that tone productive oscillations may be secured conveniently therefrom by inserting an impedance in the circuit with one of its ends at ground, to provide a source of oscillations employing ground as one terminal therefor and the remote end of the impedance as the other. Now it will be seen that the discharge circuit in a relaxation oscillator in the organization set forth, has two legs to ground. A resistance R4 whose function has been described is in one of these, and I have found it advisable to place the tone productive impedance in the other. This impedance, which may be a resistance Rm, thus has one of its ends connected to ground and the other (as in 02) to the condenser C2 through the series resistances R11 and R12 (whose functions will be explained below); and since it is competent to employ ground in the manner shown for all oscillators of the instru ment of this description, it will be seen that by providing a resistance such as R placed as described in each relaxation oscillator, a plurality of sources of oscillations may be operated in parallel for polyphonic music production. In this, leads may be connected to the ends of the resistances R10 remote from ground to terminate each in switches closable upon the depression of playing keys, and connections may be established from the switches to various devices, to return again to ground. The improved arrangements provided in the present invention for doing this, together with those for multikeyboard operation and for providing tones of various timbres and voices, will be described in detail later herein.

It was mentioned above that the present invention provides as a feature electric oscillations of suilicient complexity such that it is not necessary later to increase their complexity. This feature is attained by the use of the oscillators employed and an example of the oscillation form that could be derived in the discharge circuit of a relaxation oscillator is shown in Figure 3--A which for purposes of the moment, may represent substantially the wave form of voltage established say across the resistance R10 in O: in the absence of the rest of the network 3 (whose function and structure will be described presently). The horizontal axis t and vertical axis V in both Figures 3-A and 3--B have the same meanings respectively as in Figure 2-A, and the downwardly extending, negative peaks in Figure 3-A correspond to the relatively rapid dischargings of the condenser C2 through the tube T in On, with which are upward flows of current through the resistance R10. The positive parts of the oscillations represent the relatively slow chargings of this condenser, with downward fiows of current through R10. Oscillations so derived would be highly complex as is seen from the appearance of the waves in Figure 3-'A, and their spectrum structure is shown in Figure 4, wherein the horizontal axis 1 represents frequency and the vertical axis, amplitude. As would be surmised from the repetitiveness of the oscillations in form and time, as shown in Figure 3--A, their spectrum comprises components that are exactly harmonic, and in this respect are ideal for musical tones. Moreover the large number of these harmonics together with considerable amplitudes in the high order harmonics, as shown in Figure 4, is again indicative of high complexity.

In fact, oscillations as would be produced across R10 of the relaxation oscillator 02 in the absence of the rest of the network 3 not only have sufficient complexity to be readily susceptible of modification by devices and circuits of linear character, but have been found to be rather too complex, to be rather acid" sounding on conversion per se into sound. With this in mind, I have reduced their complexity in deriving them from a relaxation oscillator by attenuating their high order harmonics with respect to those of low order, and to do this a modifying impedance network is provided in the oscillator discharge circuit. Thus in the oscillator 02 for example the resistance R10 has been incorporated with a network 3 in the discharge circuit thereof, and on including this network, the oscillation form of output voltage established across R10 is modified to become that shown in Figure 3-3, of lesser complexity, Figure 3-A now representing the oscillation form of input voltage to the network 3. The spectrum structure of oscillations now derived across R10 is then as shown in Figure 5, wherein the axes have the same respective meanings as in Figure 4, and it is to be noted that the low order harmonics predominate in this audiospectrum. I have found that the conversion into sound of such oscillations per se results in a tone of pleasing quality. (The spectrum in Figure 5 approximates that which Miller-The Science of Musical Sounds, 1916, page 213-said represented an ideal musical tone.) Moreover with them less modification is required than otherwise, to provide other pleasing tones of various timbres, and an explanation of this is as follows:

Many pleasing musical tones, particularly those of orchestral instruments, which it is desirable to represent in an organ-like musical instrument, have in common two general timbre characteristics. One of these may be called a fixed pitch region" characteristic and is inherent in the instrument as a result of a chamber or part therein having a predominant resonance (such as for instance the bell of a trumpet). Owing to this, those harmonics of the instrument's tones whose frequencies are near the resonant frequency of the chamber or part, are accentuated thereby. quite irrespective of the pitch note sounded on the instrument, the specific frequency and sharpness of the resonance greatly determining the instrument's timbre. The other general characteristic of many pleasing musical tones, including most of those having fixed pitch regions, comprises strength in the fundamental. Now it is desirable in a polyphonic electrical musical instrument that means be provided for both of the above characteristics. The first of these are provided for in the improved circuits which will be disclosed in this specification for the accentuation of harmonics in various pitch regions; and means for the other of the above characteristics are automatically provided in deriving oscillations representative of a pleasing musical tone from a relaxation oscillator-as is aparent in the audiospectrum shown in Figure 5, wherein the low order harmonics, especially the fundamental, predominate. Moreover, another advantage accrues therefrom, for it is also desirable to represent the tones of pipe organs, and while such tones are in general characterized by uniformities of harmonic structure rather than by accentuations of harmonics in fixed pitch regions, their low order harmonics are prominent.

It will be noted that the modifying network 3 in 02 comprises the combination of a resistance R1: in series with a parallel arrangement of a condenser C5 and a resistance Rl0-Rll (resistances R10 and B11 in series). The particular form of this network and the values for its parts is a result of experiments and I find that the following values are suitable:

R1o=10,000 ohms.

Rn=5,000 ohms.

R1==2,500 ohms.

C5=.023 mid. (for 02 operating at the controlled frequency of 1046.5 0. p. s.).

The function of R12 is to reduce the intensity of discharges of the condenser C1, resulting in a broadening of the negative peaks in Figure 3-A and contributing to reduction in oscillation complexity. R12 may also serve as a limiting resistor to prevent an excessive discharge current, thus insuring longer life of the tube T. The action of the condenser C5, since the impedance of a condenser is inverse to frequency, is to shunt the higher order harmonics in the oscillations more effectively around the resistance Rio-Rn than it does those of lower order, with the result obtained. It has been found that resistances R10 in the exemplary embodiment set forth in this specification, should be about 10,000 ohms, as will be discussed more fully later herein, whereas preferred values for resistances such as RlO-Rii are about 15,000 ohms. The resistance R1o-Ru is accordingly composed of two parts in series, R10 aforementioned with one of its ends at ground being one of these parts.

An impedance network similar to the network 3, is employed in each relaxation oscillator of the instrument. The same values for R10, R11, and R12 may be used throughout, but it is preferable that the value of a condenser in a network be according to the frequency of the oscillator in which it is included, to be of larger values in lower frequency oscillators than in those of higher frequency. Thus for example, the oscillator 03 includes the network 4, similar to the network 3, with a like function, but the condenser Cs therein for a controlled frequency of 523.25 c. p. s. should be about .046 mfd.

It will be noted that the value, 1,000 ohms, for

R4 in a discharge circuit is small as compared with the impedance of the modifying network in the circuit. I have found that master oscillators such as 01, the exemplary sources of stable oscillations in this description, are additionally quite satisfactory as sources of complex oscillations. Thus twelve of them may comprise the tone productive sources for the twelve notes of the highest octave in a multioctave musical scale, the remaining notes being provided by relaxation oscillators in cascade frequency systems respectively headed by the master oscillators, all in keeping with the foregoing description.

Now the oscillatory currents in the stabilizing circuits of the exemplary master oscillators employed-in 01 the circuit including L1C1-are of rather simple character, whereas somewhat more complex voltages occur between their anodes and ground; and I therefore derive tone productive oscillations 'from them in anode-toground connections of a type from which their complex voltages may be readily secured. An example of such connections is shown in the oscillator 01, comprising in series the tone productive resistance R with one of its ends at ground, the resistance R13 and the condenser C1. The oscillations derived therefrom, across the resistance R10, are of proper complexity for the purpose desired, and by the use of a connection separate from the stabilizing circuit, resistance is not added to that circuit, an aforementioned advantage.

A function of the condenser C7 is to isolate anode potential, while the resistance R1: is a voltage reducer, relatively large oscillatory voltage occurring from anode to ground in an oscillator such as '01. That is to say, R1: reduces voltage so that the oscillatory voltage across the resistance R10 of O1 is of the order of those appearing across the resistances R10 of the relaxation oscillators. The following values for the parts just identified are suitable:

C1=.002 mfd.

R1s=100,000 ohms.

R1o==10,000 ohms (as in the relaxation oscillators).

Referring now to Figure 6, I have shown a cascade frequency including all the features which I have found desirable for commercial production. This system is identical in principle and essentially so in construction to that of Figure 1, and the characteristics obtaining in the Figure 1 system are embodied in that of Figure 6, parts with identical functions and values being given the same indices in both figures.

A particular feature of the cascade frequency system of Figure 6 relates to tubes. I have found it commercially advantageous to employ a type of tube containing the tube elements for two glow-discharge oscillators, and in Figure 6 the tubes T, which may be commercially identical, are each constructed so that two of these oscillators may be independently operated with one double tube. This results in an economy of space and cost in a commercially made instrument, and the details of construction and arrangement of parts in a tube. whereby this may be accomplished have been disclosed in a copending United States patent application Serial No. 86,145 filed June 19, 1936, by my associate, Mr, John F. Jordan, Patent No. 2,150,800 issued March 14, 1939. Since then an additional feature has been added to the tube, in providing a separate outlet for each of its cathodes K, as shown. This feature is conveniently attained by employing an octal tube base now current in commercial tube construction, as the necessary number of outlets (eight for a tube T, as indicated) are provided thereby. It permits in a double tube of a separate resistance R4 in series with each cathode when desired, to allow the cascading improvements aforementioned. A single filament F is used to heat both cathodes of a tube.

Another feature specific to the system of Figure 6 pertains to adjustability, with reference to setting the inherent frequencies of relaxation oscillators about six semitones below their controlled frequencies. As was set forth above, the principal deviations experienced in setting these frequencies on a design basis are variations and changes in tube characteristics, and it was shown that settings may be made independent of these deviations by the use of the stabilizing means represented by the aforementioned Patent No. 2,179,791. It was also mentioned that its use was of assistance in reducing the effects of variations in value of anode potential +B. There remains another type of deviation, the cumulative effects of tolerances in resistors and condensers, as commercially made today.

However, the frequency of an oscillator per se when it is provided with the stabilizing means of the aforementioned Patent No. 2,179,791 is a function of the resistances used in the means, and I therefore make one of those resistances adjustable. By doing this, the settings desired may be made, irrespective of tolerances in resistors and condensers. Thus the resistance R6 of Figure 1 becomes in Figure 6 a larger fixed part Re and a smaller adjustable part Rs", in series. Whereas the value of Re is 1,000,000 ohms, it has been found in practice that the necessary range of its adjustment is only about 100,000 ohms either side of mean; Re" is therefore made variable from about zero to 200,000 ohms, Rs having a resistance of 900,000 ohms.

The above adjustability feature is employed in all relaxation oscillators in a commercially made instrument and it is also found desirable in the stabilized oscillators. Thus the stabilizing arrangement, the resistances R1 and R8 of Figure 1, becomes in Figure 6 the resistance R14 having a value of 1,000,000 ohms, the sum of R1 and Re, and the point of connection from the grid G of the oscillator 01 in Figure 6, to the resistance R14 can be adjustably located on this-resistance, as shown.

It has also been found to be good practice in commercial production to be able to adjust the amount of cascade coupling voltage from a stabilizer oscillator to the relaxation oscillator directly controlled by it. Hence a resistance R15 has been placed across the coil L2 in Figure 6 and the end of the connection l-i', attached in Figure 1 to a terminal of the coil L2, is adiustably located on this resistance. A value of 500,000 ohms is suitable for R1: and it has been found that with this high value, negligible resistance is "reflected" thereby through the transformer Iii-14 into the stabilizing circuit, 1. e., the circuit including the tuned couple L1--C1. This is in keeping with a low value of resistance in that circuit, aforementioned.

It is of course desirable to be able to adjust the frequencies of the stabilized oscillators for the purposes of tuning a musical instrument. The condenser C1 in Figure 1 therefore becomes in Figure 6 the two condensers C1 and C1" in parallel, Ci being a fixed condenser and C1" a smaller. variable condenser. Thus if the condenser Ci has a value of .001445 mfd. as aforementioned corresponding to the 2093 C note, I employ a value of .0013 mid. for C1 and make Ci" variable from about zero to .00026 mid. By adjusting C1" to a corresponding proper value the stabilized oscillator 01 of Figure 6 may be tuned exactly to the 2093 C note, and according to previous teachings in this specification the relaxation oscillators in the system "follow" the stabilized oscillator to deliver respectively the exact lower octave C notes in a multioctave musical scale. In fact, in Figure 6 I have endeavored to indicate the oscillators for all of these 0 notes. Thus the horizontal dashed lines in the central part of the figure indicate the inclusion of other oscillators in the system and the oscillator 01 next-but-one from the right hand side may correspond to a very low octave C note say at 32.70 (3125) c. p. s.

In the exemplary musical instrument described herein twelve cascade frequency systems of a type of construction identical with that shown in Figure 6 are provided, with their stable oscillations tunable respectively to the twelve notes of a high octave register of a musical scale. It will thus be seen that by tuning only these stable master oscillators to the frequencies of the corresponding notes, the entire musical instrument may be accurately tuned in a simple manner. In this way. I have succeeded in making an instrument in which there are but twelve instrumentalities to keep in tune; which being highly stabilized, will, once tuned, remain in tune over long periods of time; in which the frequencies of all other oscillators are exactly controlled by these twelve master oscillators; in which however oscillations in the tone productive output of any oscillator are inherently free from those of others so that there are no contaminations in tones produced. Moreover an economy in cost and weight of oscillation generators is thus effected, and octave relationships are exact in frequency and identity of phase.

Now it is a usual practice in a pipe organ to provide a musical scale extending from a low to a high C note. This will be seen to comprise an odd number of notes; and in providing a scale extending say from a low C note at 32.70 c. p. s. to a high C note at 2093 c. p. 5., as is often done, 73 notes are involved. To provide these in this invention requires 37 of the double tubes T, with one half a tube unused.

However, I employ this spare tube section advantageously in providing a tremolo for the instrument. This is shown at the right-hand side of Figure 6 as a stabilized oscillator On, similar in construction to the stabilized oscillator O1 75, but operating at a sub-audio frequency of about 6 to 7 c. p. s.a usual tremolo rate. An oscillatory output from O" is derived across a resistance Ru placed, in the C lead for all the stabilized master oscillators of the instrument as shown, and thus the grid bias for these oscillators is varied at a tremolo rate. While oscillators such as 01 are sufficiently stable for the purposes required herein, I have found that large changes in their grid bias will somewhat affect their frequencies-a change of about 15 volts on their grids affecting their frequencies slightly less than a quarter of a semitone. A 15 volt oscillatory output is readily secured from On, and this can cause the frequencies of the stabilized oscillators to vary over a range somewhat less than one half a semitone. As the relaxation oscillators will follow" the stabilized oscillators a pleasingpitch tremolo or vibrato may thus be eifected in the musical tones produced, which tremolo I find musically preferable to an amplitude tremolo,

Concerning various parts and their arrangements in the oscillator On, one end of the tuned couple L:Cs is connected to ground as shown by grounding one side of the condenser Ca. I have found that currents flowing in the stabilizing circuit including Lc--C| of a stabilized oscillator such as Otr are of rather simple character and the voltages produced by them across the condenser in the circuit are substantially sinusoidal, owing to the inverse impedance characteristic of a condenser. This is of assistance in providing a smooth tremolo, for I thus derive voltages from the condenser'Cu and apply them to the resistance Rio. A condenser Co is placed in the feed line from C3 to Ru as an anode poten tial (B') isolating means (actually, as will be apparent, the condenser Co as so placed becomes a part of the tuned couple Lr-Cs, the values for the condensers Cs and Ca being calculated accordingly). A condenser C10,. placed as shown, completes the means for transferring voltages from Cs to Rm. To turn the tremolo of! or on as desired, the grid G, in the oscillator Oeis disconnected or connected respectively by a switch Sn placed in the grid lead, as shown.

In a copending United States patent application Serial No. 222,677 filed jointly by John F. Jordan and myself on August 2, 1938 (continuation-in-part of our application No. 78,440 filed May 7, 1936), Patent No. 2,215,124 issued September 17, 1940, a switch of a gradual contact type has been described. This switch is advantageous in minimizing electrical transient effects, as is highly desirable in an electrical musical instrument, and is preferably used almost universally throughout the instrument of this invention. It is hence employed for the switch Soas an insurance of noiseless tremolo operation. I find that this arrangement results in an operation free from electrical noise. This is because the grid G of the tremolo oscillator Onsmoothly acquires a heavy negative charge from the cathode electronic emission when the switch St.- is opened, so as to stop the operation of the oscillator smoothly. Similarly it regains its normal bias when Sn is closed so as to start the oscillator in a smooth manner.

As in the other oscillators, the stabilizing arrangement disclosed in the aforementioned Patent No. 2,179,791 is preferably used in the oscillator Orr-see the resistances R11, R11; and Rm. The resistance Rm in Otr serves the same function as say, the resistance R1 in the oscillator O1.

Suitable values for the various parts just described are as follows:

Ln 250 henries C: 1 mid. C9 1 mid. C1o= 16 mfd. R16: 15,000 Ohms Rn: 400,000 ohms R1a=1,000,000 ohms Rn: 200,000 ohms R1a= 300,000 Ohms (adjustable) power, and a "power pack for this purpose is shown in Figure 7. In general this is of the usual construction employedin modern radio receiving sets and the like, and hence needs little description. It includes a main transformer 5 and a rectifier-filter system 6 for supplying B and C. There is shown however, a separate source of anode potential B for the stabilized or master oscillators of the instrument (see also Figure 6), and also for the tremolo oscillator. This B potential is derived from the same system 5, 6 as is the source of anode potential B for the relaxation oscillators; but in conjunction with a time delay device I. The function of the device 1 is to insure against a sudden application of anode potential to the stabilized oscillators in say the turning on and/or rapid turning off and then on,

of the instrument, as it has been found that occasionally under such conditions the stabilized oscillators fail to oscillate, their operation being characterized by a continuous glow-discharge. A time delay device such as 1 has been completely disclosed in a copending United States patent application Serial No, 78,441filed jointly by John F. Jordan and myself on May 7, 1936, Patent No. 2,143,822 issued January 10, 1939. While no detailed description of the device is required here, it may be pointed out that the tube in the device acts as an electronic resistance in the +3 line, the value of which under transient conditions varies smoothly becauseof a slow heating cathode together with a time delay resistance-condenser combination in its grid circuit.

thermionic devices 8 (Figure 9) and 9 (Figure 10) whose functions will be described below.

- This fourth transformer is center tapped to ground to minimize introduction of A. C. hum in the devices 8 and 9, a usual advantageous arrangement. I

I have shown in Figure 7 a separate transformer ID for supplyingcurrents to the filaments F of the glow-discharge tubes T of Figure 6. I prefer to use a separate. transformer for this purpose owing to the amountof current required and find it unnecessary to center tap the transformer III, as the strengths of oscillations derived from the several oscillators are very much greater than any hum introduced by the filaments therein.

It has been found advisable as a manufacturing procedure to be able to adjust the ratio of +3 to C .To do this the resistance of R20 in Figure I, from the lower potential end of which 0 is derived, is provided with a connection from ground which may be adjustably located on this resistance.

The attachment of leads to tone productive impedances in re axation oscillators has been mentioned, for connections to playing key switches, etc. In the oscillators O2 and 0a of Figure 1 for example, these are the leads 0" 2 and C 3 connected respectively to the ends remote from ground of the resistances Rio in those oscillators, while a tone productive lead C I is attached to the end remote from ground of the resistance Rio in the stabilized oscillator 01. These leads are repeated in Figure 6, and I have endeavored to indicate therein a tone productive lead from each oscillator in the instrument-see the leads 0 l B I--, etc.

It has been shown that all oscillators of each cascade frequency system in the present invention operate at successive exact octaves and exactly in phase. These desirable relationships are carried forward in the tone productive oscillations in the leads C I, etc., not being disturbed by the modifying networks in the the relaxation oscillators or by arrangements such as R10, R13, C7 in the stabilized oscillators. Moreover the waveforms (Figure 3B) of the tone productive oscillatio'ns from all relaxation oscillators of the musical instrument of this descriptionare similar and a close approximation to this waveform is effected in the tone productive oscillations from the stabilized oscillators. Furthermore, the strengths of the oscillations derived for tone pro duction are substantially uniform from all oscillators. These features together with those of inphase and exact octave operation in each cascade frequency system will also be hereinafter seen to be highly desirable.

' The foregoing disclosure has related to the production of electric oscillations. A particular means has been set forth for generating complex electric oscillations corresponding to notes of a complete multioctave musical scale, with the various advantages as described. In this, each oscillation generator contains a resistance R10 with one of its ends at ground and a tone productive lead attached to the-other end. These resistances R10 therefore become virtual sources of complex oscillations operating in parallel. For purposes of the subsequent description to be given below they may, in considering the scope of this invention, be regarded as non-limitative examples of complex oscillation sources; the advantages of the unique features and interrelation ships embodied in them by way of the above disclosure will become apparent on further reading.

I have described exemplary sources of complex oscillations, comprising only as many sources as there are pitch notes in a single complete multioctave musical scale. I shall now show how polyphonic music may be secured from these sources together with means for multikeyboard operation in a musical instrument.

Thus I now have sources of oscillations for the several notes of the musical scale within the desired range of theinstrument, and it remains to describe the means whereby these sources are used for musical purposes and whereby the various wave forms are modified toproduce pleasing timbres and voicesfor the instrument. I shall proceed with a description of the means whereby the several notes may be selected in playing any desired musical composition and whereby the oscillations for these notes may be brou ht to certain collecting networks for subsequent combination and modification.

In my musical instrument I prefer to provide a single bank of oscillation generators even though the instrument is to have two or more manual keyboards and a pedal keyboard. Consequently, I provide means whereby suitable oscillations may be directed into output circuits from the single bank of oscillators when the several keyboards are operated. Because the waveform of my oscillations as derived from my oscillators is complex in character, I do not form my final timbres by a system involving the addition to a fundamental. of harmonics in the various amounts and numbers required to make up a desired timbre. Hence I do not employ tone synthesis; however, for effects such as are obtained by octave coupling in a pipe organ and also for such effects as I obtain by out-of-phase combination as hereinafter set forth, I prefer to provide as respects a playing key, complex oscillations at frequencies which are octavely related. For the purposes described, I have not found it necessary to employ octavely related oscillations at more than 8 foot pitch (so called in the terminology of organists) and 4 foot pitch as respects the manual keyboards; but it is within the scope and spirit of my invention to provide as many of such octavely related pitches as desired. (Thus as one example I may employ 16 foot pitch and 8 foot pitch for the pedal keyboard, as is often found in pipe organ construction.) These octavely related oscillations as respects a key, I take also from the single bank of oscillators aforesaid, and I direct them from a keyboard into collecting networks or headers whereby oscillations of a certain pitch footage from the keys of the keyboard appear in one header, oscillations of another pitch footage from the keyboard appear in another header, and so on. I may provide a set of such headers for each keyboard, as desired.

Finally there is the problem of attaining tonal balance so that a uniformity of tonal strength is effected throughout the musical scale compass of the instrument. This problem is complicated in an instrument of my type; for in providing an exemplary instrument representative of a pipe organ, the tone timbre systems employed have a combined or overall tendency to attenuate oscillations of higher frequencies, whereas I have found it preferable that oscillators of the type described have substantially equal tone productive strengths throughout 1 therefore provide a type of collecting network which compensates for this by repressing oscillations of lower frequencies. The reverse may be desirable with tone timbre systems indicative of other musical instruments.

I have shown in Figure 8 the connections at a keyboard for the purposes described, together with the switches therefor; This figure should be taken in conjunction with Figure 6 and I have similarly marked in Figure 8 the leads coming from the oscillators of Figure 6. In fact, I have endeavored to indicate in Figure 8, the connections for a complete keyboard. As this say in an organ keyboard would comprise 61 keys, the connections for which would involve many repetitions within a drawing, I have merely indicated the inclusion of notes not explicitly shown, by appropriate spacings and dashed lines. The connections for other keyboards may be similar to that of Figure 8 and I have indicated leads (so marked) to other keyboards. It will be seen as a feature of my organization that the output lead from each of the several oscillators has branches to the several keyboards where desired. Also it will be seen that the lead from each oscillator has at least two branches with respect to a keyboard. Tracing for example the lead 0" I in Figure 8, it will be seen that it has branches H and I! to other keyboards and a branch ll to the keyboard of the figure. This in turn is divided as at H and II to different keys upon the same keyboard.

As illustrated I employ as to each key of a keyboard a pair of switches, one switch being connected to the oscillator delivering oscillations of the frequency which is fundamental to the note of the key and the other switch being connected to the oscillator delivering oscillations of the frequency an octave above. (In this statement I may except the keys of the highest octave, as I prefer for economy to use as sources of oscillations for these keys the highest frequency oscillatois, i. e., the master oscillators, of the cascade frequency systems aforedescribed.) The closing or opening of a switch by the depression or release respectively of its playing key respectively connects or disconnects the corresponding oscillationsource with respect to output circuits. Since all of the oscillators in the exemplary instrument being described are in a state of continuous oscillation at all times while the instrument is turned on," the operation of a switch as placed would,

ordinarily, produce an electrical transient click or thump at the beginning and end of a played note. However I use for my key switches the mechanism illustrated and described in the aforementioned Patent No. 2,215,124. Each switch of this mechanism, which for key switches is a gang switch arrangement, is of the gradual contact type wherein a flexible member progressively makes contact along a high resistance means, and the use of such a switch eliminates electrical transients. The high resistance means is indicated at it in Figure 8 and the movable switch member at ll (similarly marked in So of Figure 6).

Each of the playing key switches includes a resistance R21 in series with it. The purpose of these resistances R21 is to assure a proper and adequate flow of current through each branch circuit from an oscillator and to prevent any branch circuit from acting as a preferential path for the output of the oscillators. Where parallel circuits either from different oscillators or as branch circuits from a single oscillator are connected to a common output device simultaneously there has long been noted by workers in the art an effect of disproportionateness or departure from a desired additiveness in the output circuit oscillations. This effect has sometimes been referred to as robbing and the resistances Rn act to preventit. The employment of the resistances R21 is ih'gccord with good engineering principles, and using the oscillators which have been described above with tone productive resistances Rio of 10,000 ohms each, a suitable value for each resistance R21 is 25,000 ohms when the effective impedance of the immediate output device-a collecting network or header to be presently described-is of the order of 5,000 ohms.

I have shown for each pair of switches in Figure 8 a line 4| to indicate that the two switches are operated simultaneously by their corresponding key, and the means for accomplishing this has been described in the aforementioned Patent No. 2,215,124. It is indicated briefly here by a playing key P shown in conjunction with the line ll of a switch pair, corresponding keys actuating the other switches of the keyboard. Now taking one of the switch pairs such as the fourth from the left-hand side in Figure 8, it will be noted that it has a switch Sk connected to the tone production lead C 2 from the oscillator 02 and a switch S2 connected to the branch l5 of the tone productive lead C I from the oscillator 01, an octave above 02. These switches, Sr and Sr, are also connected respectively to collectingnetworks or headers, (lit and lit. Thus on depressing the corresponding key the oscillator 02 fundamental to the note of the key is connected to the header Sit and the oscillator 01 an octave above to the header 4ft. Similar arrangements are provided for all keys (except the keys of the highest octave, as explained above) and thus in the playing of a musical composition oscillations corresponding to the notes of the composition appear in the headers 8ft and lit in octave relationship. It has been indicated that a purpose of the collecting networks or headers is to modify the strengths of oscillations, to compensate for the attenuating effect of tone timbre systems. As also mentioned and as will be evident on further reading of this specification, tone timbre systems employed in an organ-like musical instrument constructed according to this invention have a combined or overall tendency to attenuate higher frequency oscillations. To compensate for this I provide an extending type of resistive header and cause oscillations from lower frequency oscillators to pass progressively through more resistance in this collector than those of higher frequency, before reaching a tone timbre system. This resistance is shown as the resistances R22 in series in each of the headers 8ft and lit of Figure 8, the various connections to the headers from the playing key switches according to a musical scale, being interspaced between these resistances. By deriving oscillations for a tone timbre system from the high frequency side of the headers,

through connectors such as l8 and I9, as illus-' trated, oscillations from lowr frequency oscillators necessarily pass through more resistance R22 +R22+ than those of higher frequency, thus progressively repressing lower frequency oscillations in a compensation for the tendency of tone timbre systems to attenuate those of higher frequency.

It is within the scope of the present invention to provide a resistance R22 in each interspace between switch connections to a header. However, I have found it sufficiert in a practical attainment of tonal balance to provide a resistance R22 only between octaves of connections, or thereabout. This I have indicated in Figure 8 and with each resistance R22 is a shunt resistance R23 to ground. It has been found desirable that each connection from a playing key switch look into the same impedance as respects a header, and by using the resistances R23 with the resistances R22, substantially the same impedance is presented by a header to each connection to it. The load imposed on the high frequency side of. a header by a tone timbre system with its exemplary combination of parts and values to be described, is about 5000 ohms. The resistances R23 together with R22 act to retain substantially this 5000 ohm impedance at all header points, and the values of these resistances for this are R22=800 ohms R:3=35,000 ohms The combination of impedances employed for the header, the tone productive resistances R10, and the resistances R21, namely 5,000, 10,000, and 25,000 ohms respectively, has been largely a matter of experiment and judgment. It has been found that these values are low enough to form effective shunts around stray capacities in the system, thereby eliminating cross-talk" for all practical purposes, whereas they are high enough to secure suflicient oscillation strengths; and by the relative values used, a desired additiveness is obtained in playing musical notes.

The type of header disclosed is not limited to the use of resistance but may employ other impedances. I have for example substituted capacities for the resistances R2 with a usable result. Moreover I have changed the values given above and obtained desired effects. The particular values employed herein have been found to be suitable for the exemplary embodiment being described and may be changed for other embodiments of the invention.

Iwill now describe a tone timbre system for use in connection with one keyboard of the musical instrument of my exemplary embodiment. As a preface to its description I wish. to point out that the aforedescribed oscillation characteristics such as the wave form complexity illustrated in Figures 3B and 5, uniformity of wave forms, octave relations which are exact in frequency and phase identity, are carried forward undisturbed through the headers, to appear in leads such as l8 and I9 for advantageous employment in tone timbre systems.

An exemplary tone timbre system for a keyboard is illustrated in Figure 9. This figure should be taken in conjunction with Figure 8 and shows a continuation of the header leads l8 and I9. I secure various timbres by the following expedients, singly and in various combinations: (1) by combining together exactly octavely related oscillations of complex wave forms in out-ofphase relationship, (2) by filtering so as to accentuate or depress oscillation components in certain frequency ranges, and (3) by combining together octavely-related oscillations in in-phase relationship. I

In Figure 9 I have illustrated a combination of stops (so called in the terminologyof organists), for a single keyboard of an organ-type musical instrument. This, for purposes of explanation, may represent an exemplary stop combination for a swell manual (one of the manual keyboards in an organ) and in keeping with this the headers Bit and it may carry eight foot and four foot pitch, respectively.

For outphasing I employ a device 8 including a double space-discharge tube,'one of the conventional vacuum tubes used currently in commercially made amplifiers and radio receivers. I employ this double tube as a convenience and economy-it will become apparent that two single conventional tubes may within the scope of the invention be substituted for it. Briefly, the two sections of the-tube and certain parts illustrated with it, form together an arrangement similar to a two-stage resistance-coupled amplifier, and I utilize the phase reversals imposed on electric oscillations translated through such an arrangement to attain the out-of-phase effects which I find desirable and pleasing. As in a resistance-coupled amplifier, the plate III of the first section of the tube and the plate 2| of the second section are fed from a source of positive potential +B" through plate load resistances Ru and Rat respectively. The common cathode 22 for the two sections is connected to ground through a usual self-biasing arrangement 23, and the grids 24 and 25 of the first and second sections respectively are connected to ground through the grid resistances Ru and R21. The plate II of the first section is connected to the grid ll of the second section through a condenser Cu. (The condenser C1: is a filter condenser for removing any "noise" picked up in the +38" supply lead.)

It is well-known that there is a reversal in phase between the si nal voltage applied to the 'grid circuit of a conventional vacuum tube and the signal voltage occurring as a consequence in the plate circuit. This effect I employ to pro, duce the out-of-phase relationship desired. To do this, an output from the eight foot header is fed to the grid 24 of the first section of the tube in the device I, by means of a lead 28 attached to the lead II from the header lit, and an output from the four foot header is fed to the grid 25 of the second section by means of a lead l1 attached to the lead II from the header 4ft. Oscillations from the eight foot header are reversed in phase in translation through the first section of the tube and u so reversed are fed to the grid 25 of the second section together with those from the four foot header, which in turn are in normal The result of adding together oscillations in out-of-phase relationship at the grid 25 is that the harmonic components of the oscillations at four foot pitch cancel the even order harmonic components in corresponding oscillations at eight foot pitch. Consequently the resultant oscillations are at eight foot pitch but their wave forms are characterized by an absence or very substantial diminution of their even order harmonic components. It is to be observed, however, that to accomplish this, octave relationships must be exact, wave forms substantially similar and phases substantially identical-the exact accomplishment of these in this invention has been described.

The timbre of the oscillations resulting from outphasing, since their odd order harmonics predominate, is expressive of wood wind orchestral instruments and of stopped organ pipes. They are very useful as such, as well as for combinations with other oscillations to produce various tone timbres. The use of outphasing for the production of particular musical timbres has been set forth in my copending United States patent application Serial No. 78,685, filed May 8, 1936, Patent No. 2,148,878 issued February 28, 1939.

The oscillations as combined at the grid 25, while thus resulting in oscillations characterized by predmninance in odd order harmonics, are out of phase with those in the eight and four foot headers. To re-reverse their phase so that they may be satisfactorily combined with direct oscillations from the headers, they are translated through the second section of the tube of the device 8 and appear in the lead ll from the plate II of the second section, in normal phase. A con- 4 denser C1: is placed in this lead to isolate the +13" supply from its output.

The device 8, as an amplifier, is effective in compensating for diminution in oscillation strength. The outphasing has an attenuating eflect, since it is a subtractive process in which the even order harmonics are diminished. The amplifying characteristic of the device I compensates for this; and it also compensates for another effect. In an explanation of this I employ the Figure 5 of the accompanying drawings and set forth the following; Figure 5 shows the spectrum of oscillations preferred in the present exemplary embodiment, as derived from the oscillators; oscillations are unaltered in wave form in a conduction from the oscillators to the leads II and II; thus the spectrum of Fi ure 5 is representative of oscillations applied to the device l-the harmonic components illustrated in Fi ure 5 decrease in strength progressively according to increasing order-oscillations are derived at uniform strengths from the oscillatorsthe eifect of the headers is to attenuate progressively oscillations of lower frequency.

It is readily deduced from the above that in oscillations applied to the device 8, the harmonics of the four foot oscillations are respectively of greater strength than the coincident harmonics of the eight foot oscillations. The amplifying characteristic of the first section of the tube in the device I functions to increase the strengths of the eight foot oscillations, so that coincident harmonics are of substantially equal strengths, to provide an effective outphasing. In fact, I have found that oscillations having either the spectrum of Figure 4 or of Figure 5 are advantageous in outphasing as employed herein. In Figure 4, a spectrum is shown in which a large number of components, both even and odd, are shown in substantially equal strengths; and the outphasing of say eight and four foot oscillations having a type of spectrum shown in the Figure 4 results in a substantial diminution of the even order harmonics in the eight foot oscillations (when the two combining oscillations are at substantially the same strengths).' Moreover, the modification in the manner aforedescribed, of oscillations repre sented by Figure 4 to produce those represented by Figure 5 has resulted, I have found, in a spectrum in which the second harmonic is substantially one half the strength of the first (fundamental), the third substantially one third the first and so on; and thus it will be evident that by bringing oscillations of say eight and four Soot pitch according to Figure 5 into outof-phase relationship results in a nearly exact cancellation of the even order harmonics in the eight foot oscillations (the two combining oscillations being brought of course to the proper relative strengths by the device 8 say, as described). The preference for the modified oscillations of Figure 5 was previously discussed.

Interposed in the lead 21 of Figure 9 is a resistance Ru. This together with a resistance R20 in the connection between the plate 20 and the grid 25, prevents an alteration in the impedance of the header lit by the plate circuit of the first section of the tube in the device I. The resistance R29 is also employed to determine the amount of voltage transferred from the first to the second section of the tube.

The following values for the parts set forth above for the device 8 form exemplary working combinations with other aforedescribed parts:

The tube has, per section, an amplification factor=6 and a plate impedance=4300 ohms- Ra4= 7,500 ohms l Rfl8= 7,500 ohms R2c=100,000 ohms Rm=200,000 ohms Ru: 50,000 ohms .R29= 50,000 ohms C11=0.25 mid, C12=8 mfd. C13=0.25 mid.

Thus the leads l8, I9, and 28 are energized by complex oscillations, corresponding to notes played on the instrument, respectively at eight foot pitch, four foot pitch, and eight foot pitch with odd order harmonics predominating. For purposes of the subsequent description, these leads may be considered as being energized in any suitable manner from non-limitative sources of complex electric oscillations, in providing those characteristics desirable for the remainder of the tone timbre system, to be described.

It will be noted that the system of Figure 9 includes a number of electrical paths extending from the headers Bit and lit to one end of an intermediate output impedance, the resistance R30, the other end of which is connected to ground. Some ofv these paths are connected to only one header; some-as an example the eight foot "Open diapasoni v headers; some to a header directly and also to headers through the lead 28 and device 8. Each of these paths includes a switch St: and a resistance R21: they also contain various series and shunt impedances.

The several paths, eight foot Oboe, eight foot ,Clarinet, etc., represent an exemplary combination of timbres or stops, and the tone timbres efi'ected'by them are caused to occur in the musical tones produced by connecting the paths to the output system on closing the corresponding stop switches Sts- Thus the operation of timbre systems of this invention, by the musician, together with the operation of playing keys and swell pedals'may correspond to that of a pipe organ.

The function of the resistances R21 in the electrical paths of Figure 9 is similar to that described for the resistances R21 in the playing key switch circuits, Figure 8. The values for the resistance R30 and the resistances R21, of Figure 9, may be R3o=15,000 ohms R21=25,000 ohms (as above) The eight foot Oboe path comprises a connection from the wood-wind device 8, through the lead 28, to a collecting lead 29 attached to the intermediate output impedance R30. In the connection are two condensers C14 and C15 in series, and between them is a shunt resistance R31 to ground. The path also includes a tuned tank circuit, L4 -C1e, shunted to ground, and a resistance Rag. The'combination R31, C14, C15 inhibits the passage of low frequency components to the output system and the tank circuit accentuates a particular band of frequencies within its sensibility. v

In the eight foot Clarinet stop, also connected to the outphasing device 8, a series resistance R35 and a shunt tuned circuit, LC1'1, are used, a resistance R34 beinginserted in'this' circuit.

(so marked)- -to both The Stopped diapason eight foot is a soft toned stop employing in the construction, as shown, the "wood-wind output from the device 8 and a low-pass type filter construction comprising the resistances R68, R30 and R31 in series, and the condensers Cm, Cu, and C20 shunted to ground.

The Violin diapason" eight foot employs an output from the device 8 through a branch connection 30 to the Stopped diapason, and also a portion of the output of the eight foot header through a connection 3| containing a resistance Raa, and a by-pass condenser Car to ground. To this end the Violin diapason employs a stop switch comprising a double switch as shown, gang operation of the two switches thereof being indicated by the line 42 joining them.

The French horn eight foot employs the output 01' the device 8 and a filter comprising the resistances- Ran, R40, R41, and the condensers C22, C23, Cu to ground. It also employs a portion of the eight foot header output through a connection 32.

The. Vox humana" eight foot comprises the resistances R42, R4: in a connection to the eight foot header, and the branch 32 to the outphasing device 8.

The Trompette eight toot employs a series resistance R44, a shunt resistance R45 to ground, an inductance L6, and a condenser C25 to ground. The inductance Ls, capacity C25 and Resistance R45 form together a high Qresonant circuit, and the output oscillations are derived across the condenser C25. The resistances R4: and R21 are of assistance in maintaining selectivity in the resonant cincuit.

The Aeoline eight foot has a connection to the eight foot header and includes a resistance R46, and a tuned tank circuit Lv-Cze-Rm to ground.

The Open diapason eight foot employs the eight foot header output through a connection 33 and a filter R4a, R49, R50, C27, C28, C29, and also the four foot header output in a connection 34 through the four foot Flute path, including a resistance R51. A double switch similar to that of the Violin diapason is employed.

v The Flute four foot employs the four foot header output and a filter R52, R52, R54, R55, C30, C31, C32.

The Clarion four foot uses a resistance R56 in a connection to the tour foot header and a tuned tank circuit Ls-Caa shunted to ground.

The Salicet four foot employs resistances R51, R58 shunted to ground, and two condensers C34, C35 in the path.

Suitable values for the several elements employed above are as follows:

L4, L5, L6, L1, L =0.45 henries C14=.02 mfd.

ut, ae, Ra7=50,000 ohms C21=.002 mfd. I{33=50,000 ohms C2 C23. C =.004 mfd.

R401 R41=50,000 ohms C =.003 mfd.

RZ=50,000 ohms R ='200,000 ohms R =10,000 ohms the device together with the various leads and connectors are, in comprising a tone timbre system constructed according to this invention, of linear character.

It will be understood that the selection of pleasing tone timbres is largely a matter of personal preference. While I have described above with exemplary values the type of stop combinations which I employ for the swell manual in my instrument, it will be understood that wide modifications may be made in these for the securing of different tone timbres in accordance with personal preferences. and also that the several elements may if desired, be made adjustable so that different tone thnbres may be selected by the player of the instrument. An organ constructed according to this invention may have, as has been described, more than one keyboard to include a swell manual, great manual and pedal keyboard. Similar stop means as described may be employed in these, or modified as may be necessary to give the timbres desired. It will be understood, of course, that there may be a stop arrangement for each keyboard of the organ. and through these, independent timbres may be played in contrapuntal voices.

Thus I have described means for generating complex electric oscillations together with means for collecting these oscillations according to the requirements of a musical composition, for modifying them in the creation of musical tones of various qualities and for recombining them. It

remains to describe an output system in which oscillations are amplified and converted into sound.

An output system suitable for present purposes is shown in Figure 10. Figure is to be taken in connection with Figure 9, and shows a continuation of a lead I! attached to the intermediate output resistance R of Figure 9. Interposed in the lead is a resistance R50, in Figure 0. This resistance R50 serves a purpose similar to the resistances R21 aforedescribed. The lead II from a timbre system of a keyboard is Joined in Figure 10 to corresponding leads 36 and 81 from the timbre systems of other keyboards. It will be understood, of course, that there may also be resistances such as Rae and R50 in conjunction with the leads 36 and 31. The combined output of the leads 35, II and 31 is fed through a condenser Cat to a resistance Ree bridged across the input of a "head" amplifier 9. Suitable values for Ru and Rec are 7500 and 200,000 ohms respectively. The condenser C36 may have a value of 0.2 mfd.; its function is to isolate the C bias in the first stage of the amplifier 0,-as shown, from the leads I5, 30, 31.

The combined oscillations appearing in the resistance R00 are increased in strength by the amplifier I; then translated through a transformer ll into a low impedance line 39 in which is inserted a constant impedance type volume control 40 which may be pedal operated to form a swell pedal for the organ; then further amplified by a main amplifier (so marked) and, finally, converted into musical sounds by aloudspeaker, L8. The "head" amplifier I is of a conventional construction, comprising a two-stage resistancecoupled amplifier employing a double space discharge tube, now in current use. The plate potential supply for this amplifier and for the outphasing device 0 are derived from the main amplifier, from +B" as indicated. I find that this arrangement is of assistance in eliminating cross-talk."

Each of the switches St. for timbre systems of the type above described, is of the gradual contact type set forth in the aforementioned Patent No. 2,215,124 and noiseless operation of timbre systems is provided by their employment. These switches together with the tremolo switch Sn and the "on-off" switch 8 (Figure '7), may be placed in the instrument console in convenient positions, and operating parts for these, and other playing parts such as the keyboards and swell pedal, may be arranged according to usual organ console practices. The oscillators for. the instrument may be mounted in a framework similar to a radio receiver chassis and the tone timbre systems may be installed in another chassis, these assemblies being placed in accessible positions within the console. The amplifier and loudspeaker mav be placed within the console, or they may be placed in a cabinet separated therefrom, with a connecting cable. The latter I prefer, as it provides the distance effect enjoyed in listening to a pipe organ. Actually, as a technical advantage, I prefer to place the first two stages of the amplifier, i. e., the "head" amplifier, in the console, preferably on the tone timbre chassis and provide a low impedance line from this to the main amplifier, lnterposing the swell pedal potentiometer 40 in this line-as aforedescribed.

Thus I have described my i ention by means of an exemplary embodimen... The invention, however, is susceptible of varii as modifications within its spirit, and accord ntzy may find expression in various forms and =mbodiments within the definitions of the parts and combinations set forth in the accompanying claims. Accordingly the claims point out the scope of the invention and comprise:

1. Ina system including a plurality of sources of complex electric oscillations wherein the said sources are related in frequency harmonically to each other, the combination of a plurality of electrical relaxation oscillators arranged to form a series, each oscillator including a discharge tube having at least an anode, a cathode and a grid, electrical conducting means common to the said oscillators of said series, a plurality of impedances connected respectively between said conducting means and the said cathodes of at least all but the last of the said oscillators in said series. whereby at least a portion of the anode-to-cathode discharge current in an oscillator fiows through the corresponding impedance, coupling means respectively between adjacent oscillators of the said series for applying at least part of the oscillation voltage appearing across the impedance of a preceding oscillator to the grid of a succeeding oscillator in said series, whereby the anode-to-cathode discharge current in a preceding oscillator affects the anode-to-cathode discharge current in a succeeding oscillator, a source of electric oscillations of stable frequency, and means for applying voltage from said stable source to the first oscillator of the said series, whereby in the above said combination a plurality of sources of stable oscillations of complex wave form and harmonically related fundamental frequencies may be secured.

2. In combination, a master electrical oscillator and a plurality of controlled electrical oscillators in cascaded series, each of said controlled oscillators comprising a glow-discharge tube having a cathode. an anode and a grid, and circuit elements forming with each such tube a relaxation oscillator circuit, means connecting the first said relaxation oscillator with said master oscillator, and means connecting said first relaxation oscillator with a second and so on in said series, said last mentioned means comprising a connection between the cathode of the tube 01' said first relaxation oscillator and the grid of the tube of the second and so on, a ground, and means for causing the several cathodes so connected to have a potential above said ground periodically.

3. A relaxation oscillator comprising a discharge tube having an envelope, an anode, a cathode and a grid, a feed circuit for said tube including a source of current and a resistance and connected to the anode of said tube, a discharge circuit for said tube comprising a capacity and a resistance connected between theanode and cathode of said tube, a connection between the anode and the grid of said tube comprising a resistance, and a connection between said grid and a source of negative bias Potential also comprising a resistance, a second relaxation oscillator having an anode, a cathode and a grid, and a connection between the cathode of said first mentioned oscillator and the grid of said second oscillator.

4. A relaxation oscillator comprising a gaseous glow-discharge tube having an anode, a cathode and a grid, a feed circuit for said tube including a source of current and a resistance and connected to the anode, a discharge circuit for said tube including, in order, the tube anode, the tube cathode, a resistance and a capacity, a connection between the anode and the grid of said tube comprising a resistance, a connection'between the grid and a source of negative bias potential also comprising a resistance, and a connection to said cathode whereby another oscillator may be controlled in frequency by said relaxation oscillaton' 5. In an electrical musical instrument, a plurality of electrical oscillators each having a first output point and a second output point to form a pair from which useful oscillations may be derived, a common conductor to which all of said first output points are connected, an extending resistance, connections from various points along said resistance respectively to said second output points, and resistances connected from various points along said extending resistance to said common conductor. v

6. In an electrical musical instrument, a plurality of electrical oscillators each having a first output point and second output point to form a pair from which useful oscillations may be derived, a common conductor to which all of said first output points are connected, a keyboard of playing keys, a plurality of resistances connected in series to form a resistive conductorfymeans actuated by said keys for connecting said second output points to various points on said resistive conductor, and a circuit in which energy may be utilized connecting said resistive conductor and said common conductor. I

7. In an electrical musical instrument, sources of complex electric oscillations respectively corresponding in frequencies to notes of a multi-octave musical scale, playing keys, pairs of switches actuated respectively by said playing keys, a pair of headers, connections from said sources of oscillations through said switches to said pair of headers, comprising connections through each pair of switches from an oscillation source to one of said headers and from another oscillation source an octave above to the other of said headers, whereby when said keys are operated said headers are energized by oscillations in octave relationship,

sound reproducing means for said instrument, and tone color means connected between said headers and said sound reproducing means, said tone color means consisting of means for filtering the outputs of the two said headers separately and means for filtering said outputs in combination.

8. In an electrical musical instrument, sources of complex electric oscillations, means for deriving from said sources through the operation of playing keys oscillations of octavely related frequencies simultaneously and in different paths, a sound producing means, and stop means comprising means for independently filtering the oscillations in said paths, means for combining the oscillations in said paths and filtering the combination, means for combining the oscillations in said paths in out-of-phase relationship and filtering the combination, and means for combining the out-of-phase outputs of said paths with additional portions of the output of said paths, whereby to secure various tone colors.

9. In an electrical musical instrument, sources of electric oscillations of complex wave form, means including keys and switches for feeding oscillations in accordance with the requirements of a musical composition from said sources to at least two collectors, there being in connection with certain of said keys at least more than one switch, whereby when such keys are actuated, oscillations from respective sources are fed to one collector and oscillations from respectively octavely related sources are fed to another collector, linear means for combining oscillations from a pair of said collectors in out-of-phase relationship, an output system, and a plurality of electrical paths from said collectors and said out-ofphase combining means respectively to said output system, said paths comprising switches and linear means for modifying the harmonic content of said oscillations in preselected frequency ranges in the audio spectrum.

10. Apparatus as claimed in claim 9, including as respects at least certain of said sources of oscillations, individual wave form modifying means for such sources for simplifying the wave form while keeping it complex in character.

11. In an electrical musical instrument, sources of electric oscillations of complex wave form, means including keys and switches for feeding oscillations in accordance with the requirements of a musical composition from said sources to at least two collectors, there being in connection with certain of said keys at least more than one switch, whereby when such keys are actuated, oscillations from respective sources are fed to one collector and oscillations from respectively octavely related sources are fed to another collector, linear means for combining oscillations from a pair of said collectors in out-of-phase relationship, an output system, .and a plurality of electrical paths from said collectors and'said out-ofphase combining means respectively, to said output system, said paths comprising switches and linear means for modifying the harmonic content of said oscillations in preselected frequency ranges in the audio spectrum, certain ones of said paths being connected respectively to difierent ones of said several collectors and said out-of-phase combining means.

12. In an electrical musical instrument, sources of electric oscillations of complex wave form, means including keys and switches for feeding oscillations in accordance with the requirements of a musical composition. from said sources to at

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