US2770995A - Wave form generator - Google Patents

Description

Nov. 20, 1956 G. R. STlBlTZ WAVE FORM GENERATOR 6 Sheets-Sheet 1 Filed July 14, 1952 fIS INVENTOR GEORGE R. STI B ITZ ATTORNEY Nov. 20, 1956 G. R. STIBITZ 2,770,995

WAVE FORM GENERATOR Filed July 14, 1952 a sheets-sh eet 2 INVENTOR GEORGE R. STIBITZ ATTORNEY Nov. 20, 1956 G s z 2,770,995

WAVE FORM GENERATOR Filed July 14, 1952- 6 Sheets-Sheet 3 INVENTOR GEORGE R. STIBlTZ ATTORNEY Nov. 20, 1956 G. R. STlBlTZ WAVE FORM GENERATOR 6 Sheets-Sheet 4 Filed July 14, 1952 INVENTOR GEORGE R. STI BITZ BY /Wa zM ATTORNEY Nov. 20, 1956 G. R. STlBlTZ 2,770,995

WAVE FORM GENERATOR Filed July 14, 1952 6 Sheets-Sheet 5 INVENTOR GEORGE R. STIBITZ BY 5M ATTORNEY Nov. 20, 1956 G. R. STIBITZ 2,770,995

WAVE FORM GEN EEEE OR United States Patent WAVE FORM GENERATOR George R. Stibitz, Burlington, Vt.

Application July 14, 1952, Serial No. 298,669

Claims. (Cl. 84-].28)

This invention relates to an electric keyboard instrument of the type usually referred to as an electric organ for the production of musical sounds of any desired frequency and wave form, and has for its primary object the provision of an apparatus of simple construction capable of producing a musical output of the highest quality.

Electric organs are known in which an electrostatic tone generator is used, usually in the form of a rotating conductor whose capacitance with respect to a stationary conductor varies during rotation to produce the desired frequency. This has the disadvantage that it involves relatively large conductors in which the signal voltages are generated at very high impedance. One way of looking at the circuit is to make use of Thevenins theorem, which states that any two-terminal network is equivalent to a generator of zero impedance actingthrough a fixed impedance; in the case of a typical electrostatic tone generator, the series impedance is a condenser of perhaps 0.1 micro-microfarads or less, which at 30 cycles has an impedance in the order of 50,000 megohms. The generator in series with this impedance may have a voltage of perhaps 100 volts. It is clear that considerable precautions must be taken to prevent stray fields (usually 60 cycles) from generating in such circuits an intolerable large noise. It is a major object of my invention to reduce this hum pickup to a negligible amount, and at the same time provide a simple and inexpensive electrostatic tone generator which lends itself readily to mass production.

An electric organ must provide for a large number of different tones and frequencies. For this reason, a large number of separate tone generators are used which for reasons of compactness must be more or less crowded together. It is also necessary to provide a multiplicity of leads between the control keys, tone generators and amplifying circuits, which introduces problems due not only to the small space available and the complexity of the wiring, but also to the fact that the leads themselves are capacity members and tend to pick up hum and also to introduce unbalances. It is a further object to greatly reduce the above effects by largely eliminating the use of leads from the separate tone generators. This I accomplish by so arranging the adjacent tone tracks that conduction can be accomplished directly through the adjacent tracks. This also has the advantage that it lends itself to the rapid and inexpensive production of such tracks and generators through the use of stamped or printed circuits, since it makes possible the stamping or printing of complete series of tone tracks for electrostatic generators in a single operation due to the fact that all of the conductors lie in a single plane.

Due also to the simplicity of my improved tone generators, it becomes feasible in an inexpensive construction to produce a large number of separate tone tracks for each frequency, corresponding to the tones of different instruments, rather than attempting to create these different tone qualities by Combining different harmonics as ice has been done in many prior instruments. It is thus possible to reproduce more accurately and by simple means any desired tone quality.

Other objects and advantages will appear by reference to the following description and to the drawings in which:

Fig. l is a schematic diagram illustrating one of the basic principles of my invention;

Fig. 2 is a schematic diagram illustrating the manner of connecting conductor leads to the sound tracks according to one modification of my invention;

Fig. 3 is a diagram illustrating the manner of laterally shifting the sound tracks to avoid the need for extraneous lead-in conductors;

Fig. 4 is a diagram of the laterally shifted sound track of Fig. 3; r

Fig. 5a is a diagram showing a basic wave form of a simplified single tone cycle of the diapason prior to transference;

Fig. 5b is a diagram showing the transferred track segment for the simplified tone shown in Fig. 5a;

Fig. 6a is a diagram showing a basic wave form of a simplified single tone cycle of the clarinet prior to transference;

Fig. 6b is a diagram showing the transferred track segment for the simplified tone shown in Fig. 6a;

Fig. 7 is the developed track layout for one section of six tone qualities for a note in the middle octave;

Fig. 8 is a schematic diagram showing the manner of connecting adjacent sound tracks of different octaves without crossover;

Fig. 9 is a schematic diagram showing a single sound drum and associated amplifier circuitry;

Fig. 10 is a side elevation showing the relationship of a number of drums.

Fig. 11a is a' diagram illustrating a modified manner of laterally shifting a portion of the sound tracks to avoid the need for extraneous lead-in conductors;

Fig. 11b is a diagram of the sound track of Fig. lla having laterally shifted portions; and

Fig. 12 is the developed track layout for four sound tracks after transference and shifting of the tone tracks in the modified manner.

The general type of electrostatic generator to which my invention appertains is in itself well known, being illustrated in the U. S. Patent No. 2,165,707 to C. T. Jacobs, in which the wave forms corresponding to the respective desired notes are provided in the form of a series of tracks arranged side by side on a rotating drum, the capacity of each track with respect to an adjacent pickup plate or capacity member being varied periodically with each cycle of relative rotation of the two members. Instead of a single sound track for each note, I make use of a double track formed as shown in Fig. l of a single band of conductive material 2 of uniform width separated into two bands 3 and 5 completely insulated from each other by an undulating portion 4 whose undulations correspond to thewave form of the desired note. A conducting element or pickup plate 6 is arranged adjacent the band2, forming the second plate of a condenser whose effective width depends upon the spacing between these elements. If the width w of the plate is made very thin, the effective area of the condenser is of the same order as the spacing of the plate from the track. Tracks 3 and 5 are connected through leads 12 and 14 respectively to a push-pull or balanced amplifier 16 whose output at 18 corresponds to the difference between the two condenser areas formed by plate (Send the respective cooperating areas of the two tracks 3 and 5 immediately adjacent thereto. When a voltage is applied from source 8 by closing key 10 and plate 6 is moved along the track, the members of each condenser pair such as 3, 5 will generate signals that are of opposite sign at all times, the resultant output being the difference between the signals from such a pair. Pry maintaining careful balance to ground between the members of each pair it is possible to set up signals in such a way that stray fields, e. g., such as are due to power leads from commercial 60 cycle supply to various items of household or commercial electrical equipment, will affect both signals in nearly the same way; stray fields will add very nearly the same signal voltage to both signals, and will therefore not affect the difference between them. The total complementary areas of the two tracks 3 and 5 are made equal, and as they are in close proximity, in a uniform electrostatic field the two tracks pick up equal voltages; such a field therefore does not affect the net signal which is the difference between the voltages in the two tracks. A concentrated field such a that produced by the fixed condenser plate of the signal generator will, however, affect a small region of the tracks only, and in such a small region the areas of the two tracks will not be equal. The concentrated field therefore does produce a net signal. In this way I reduce to a negligible minimum the pickup of stray signals or hum which is otherwise, as previously explained, such a problem due to the necessarily small capacity of the condenser formed by the signal generator.

Between the tracks and the vacuum tube amplifier the circuits follow accepted practice to maintain balance to ground. The two conductors are kept in close proximity at all times and where possible consist of a twisted pair.

The amplifier must, in order to realize the advantages of the balanced circuit, be of the push pull variety designed to discriminate against voltages common to generators of a pair. Amplifier 16 is accordingly arranged in conventional push pull fashion, whereby the output of transformer 18 will follow the wave form represented by track 2 but will be substantially hum-free. Fig. 9 also shows such an amplifier, where the first stage of amplification consisted of a pair of tubes 15 such as a 6817 or the recent audio equivalent 5879. The tubes of this first stage have their cathodes connected to a common cathode resistor 17, the latter being chosen to have as high an impedance as possible, thus discriminating against voltages common to the two generators. Subsequent stages of the amplifier may be similarly arranged as indicated at 19, and the final stage may feed into a balanced transformer 21 whose output is proportional to the difference between the voltages appearing in the two legs of the balanced input winding.

For convenience the signal may be converted from the balanced form to an unbalanced form and back at any point after the first stage of the amplifier, using standard techniques, but since it is common practice in high quality amplifiers to have the final stage a balanced one, my preferred circuit would be one in which the signal is carried in balanced form completely thru the amplifier, as shown in Fig. 9.

In the electrostatic organ it is desirable to have some 400 or more tones of various pitches and tone qualities. In one model I provide a compass of six octaves and a complement of six stops. Since there are 12 frequencies per octave this means that the model will have 432 tones and therefore, using by balanced circuit method, 864 generators. The corresponding 864 tracks are arranged on 12 rotating drums with 36 pairs of tracks on each drum all leading to a single push-pull amplifier. Each pair of tracks can be considered as forming a band of conducting material on a surface of a drum. This band is divided into adjacent conductive bands by the narrow nonconducting slit 4 whose form is that of the wave represented by that particular pair of generators. In a typical model the pairs of tracks are spaced inch from center to center along the drums. The total length of the drums is 15 inches and their diameter 2 inches.

The lowest speed drum may rotate at approximately 16 revolutions per second, and will carry the tracks for all the notes of the organ that are named C. Assuming that 16.35 revolutions per second is selected for the C drum then C# drum will revolve at 17.32 revolutions per second, and so on. On the C drum the lowest frequency is expected to be 32 cycles per second, so there will be 6 tracks, in each of which the required wave form will bev repeated twice per revolution of the drum. There will be: six pairs of tracks on which the wave form is repeated 4- times per revolution, six in which it is repeated 8 times. per revolution and so on. At the highest frequency the wave form on each track will be repeated 64 times per revolution.

One of the problems associated with the drums is that of connecting each of the tracks with the external circuit. One way of making the connection is to mill a narrow slot in the drum surface, placing a pair of twisted conductors in the slit and tapping one or the other of the conductors as it passes under each track. In practice it has been found difficult to maintain balance between the two conductors with a sufiicient degree of accuracy so that unbalanced voltages will not be picked up in the neighborhood of the stationary plate. For this reason 1 provide that where unbalance is necessary, as in the process of tapping the tracks, such unbalance shall be repeated at the fundamental frequency of the tone involved. Thus instead of making one tap to each track I make one for each wave of the track. Any unbalance in this tapping will then be repeated at the fundamental frequency of the tone and will not introduce frequencies not already present. If desired the wave form can be modified to compensate for the residual unbalance.

In carrying out the tapped conductor scheme it is possible to use a network of printed circuits to collect the voltages from the various tracks. According to this scheme a collecting network like that shown by the dash lines A and B in Fig. 2 would first be printed on the drum. The network, it will be noticed, provides one conductor for each wave in each track. The network is next covered with an insulating layer except at those points where it is desired to make a connection between a track and the collecting network. The insulating layer may be lacquer, impregnated paper or other material. The tracks C1, C2, C3 may then be printed on top of the insulating .layer, making connection to the collecting net- Work thru the apertures provided, as shown by the small circles.

However, the following method of arranging the electrostatic tone generating tracks simplifies the circuit printing considerably.

In the preceding paragraphs I described a method of printing the sound generating tracks for an electrostatic organ. That method required overlay printing in which the lowest printed circuit consisted of a network of collecting conductors, the second layer a perforated insulating layer and the top, the actual tone generating tracks. My present arrangement permits the entire set of circuits to be made at a single prining.

The scheme described below depends upon the fact that the voltage generated by the electrostatic action is determined by the areas of the tracks that are adjacent to the stationary plate at any instant, and not on the lateral disposition of those areas: in other words, if an eighth inch wide strip of the track is exposed to the stationary plate it does not matter whether this strip is located at the left or the right or any other portion of the fixed plate.

I rearrange the areas associated with the two legs of the balanced circuits in such a way as to make all the areas associated with one leg contiguous along spiral paths at least one end of which treminates at the end of the drum.

For example, considering one cycle in a pair of tracks belonging to a particular tone as shown in Fig. 3, the corresponding segment of the tracks is bounded by a reetangle abcd, and the wave form to be represented is the simple wave form cfg. The segment of the one track aefgda is connected to one of the pus h-pull tubes of the amplifier, which is designated by a sign for convenience. The other track segment bcgfeb is designated by a sign to indicate that it is electrically connected to the opposite side of the circuit and to the remaining input tube of the push-pull pair.

If a diagonal bd is drawn through the rectangle abcd any horizontal strip through the tracks such as hijk can be cut out, and cut apart at the point i where the diagonal intersects the section.

The cutout segment hij may then be moved to the right so that the point j lies on the righthand boundary and the segment j/c may be moved to the left as shown in Fig. 4. The same operation may be preformed on all of the strips of the segment and resulting revamped figure would appear as shown in Fig. 4 of the drawings. The essential common characteristic of Figs. 3 and 4 is that any horizontal line (i. e., parallel to ab) drawn across both of these figures the same distance from ab will have the same total amount of its length in a area and in a area in Fig. 4 as in Fig. 3, even though these and sections of its length may be rearranged and even split up as in the case of the line through point 1.

It will be noted that in the second figure each horizontal strip contains the same amounts of and areas as the corresponding strip in the first figure. Furthermore, the areas adjacent to the upper and lower boundaries are unchanged and in fact are identical. Therefore if the rectangles in the original track could be repeated in a vertical sequence, the upper and lower boundaries of each matching the adjacent boundary, then the same is true of the transformed rectangle. The connections to the side boundaries, however, have been changed. The positive area which formerly connected the left portion ae of the upper boundary ab to the left portion dg of the lower boundary do now connects the left portion as of the upper boundary to the lower portion of the right hand boundary, and a similar change has been made in the negative area. Because of the matching at the upper and lower boundaries the lower portion of the right hand boundary of any transformed rectangle will be electrically connected to the lower portion of the left hand boundary of the rectangle immediately above it and the upper portion of the right hand boundary of any rectangle will be electrically connected to the upper portion of the left hand boundary of the rectangle immediately above it.

In this way I have rearranged the areas so that there is always an electrical connection for each polarity from the right hand boundary to an adjacent left hand boundary. By putting small tabs or narrow strips of conducting material at the upper and lower corners a set of tracks can be interconnected in such a way that the circuits of each polarity are made continuous from track to track.

Figs. 5 and 6 show the transferred track segments for simplified tones of representative basic types corresponding to the notes of the diapason and clarinet respectively,

Figs. 5a and 6a being the original sound track similar to that shown in Fig. 1, while Figs. 5b and 612 show the track transferred according to the above scheme. Such tracks are then assembled into a pattern as shown in Fig. 7, which is the developed layout for one section of six tone qualities for a note in the middle octave. A single drum would actually contain the notes for six octaves in a full-scale machine, the adjacent octaves being related in the fashion indicated in Fig. 8 as will be explained below. In Fig. 7 the shaded portion represents the area, the unshaded portion the area, the dashed lines electrical paths and black lines and areas represent nonconducting strips. Similarly in Fig. 8 the areas and areas are shown as shaded and white areas respectively. It will be seen that the plus and minus sections of the drum capacitor can be connected to the electric circuit by respective leads connected to slip rings 20 and 22 at the same end of the drum 23 as indicated in Fig. 9'.

If the conductors leading to the amplifier are attached to the tabs belonging to that track which has the largest number of repetitions, say 64, then there will be 64 paths through the set of tracks having 64 repetitions and terminating at 64 pairs of tabs. Of these, 32 may be ignored and the other 32 become the terminals for the set of tracks having 32 repetitions and so on. In this way all cross-overs are avoided and the tracks and their collecting circuits can be made in a single printing. By a cross-over is meant an area of the surface at which the two sides of the circuit must pass each other without electrical contact. In printing circuits the cross-over would require two layers of conducting ink separated by an insulating layer, which it is desired to avoid by this construction. Fig. 8 shows this schematically. Of course, in the general case, the separating line between the plus and the minus conductors will not be straight lines as shown in this simplified diagram, but will be undulating to give the desired tone quality.

Referring to Figure 9, drum 23 is mounted on suitable bearings 24 and 26, and is rotated through gearing 28 by any suitable motor means, as is well known in the art. Other drive means such as belts and pulleys may advantageously replace the gearing 28. Mounted adjacent the drum are a series of fixed condenser elements or pickup plates 6, each corresponding to the element 6 shown in Fig. 1. All of the plates 6 and their associated circuitry may be produced by printed circuit techniques on a single card 38 of insulating material. A grounded conductor 36 is printed on the sheet 30 as to provide a shielding and isolating section for each condenser element 6. It is also desirable to carefully shield the plates and all of the elements in the grid circuit of at least the first stage. To discharge the plates after keying I provide a high resistance to ground at 38 for each element 6, all of these components being printed on the same card as shown in Figs. 9 and 10. Element 6 is connected to the keying leads by means of an eyelet 34 for each said element. Each card 30 is mounted on a grounded metal supporting frame 42 as indicated in Fig. 10, which also shows the series of drums 23 geared together by gears 25 to provide the necessary speed ratio corresponding to the 12th root of 2 for 7 adjacent drums, as is well known.

The amplifier circuitry has been shown schematically, omitting many details such as volume control features which would be included in a practical embodiment of an organ constructed according to my invention, as these features are in general well known, and the design of a machine to include the standard controls common to electric organs is within the technical competence of any person skilled in this art. In practice, it will be understood that the respective switches may be connected in groups corresponding to the standard stops of an organ or in any other way desired to suit the convenience and ease of operation of the user of the instrument, such controls being well known in the art and not a part of the present invention. It will be apparent that with the flexibility afforded by electric switches the controls can be readily grouped as desired and located wherever convenience or conventional practice dictate. However, I have shown by way of example in Fig. 9 a group of switches 48, 50 and 52 for including at will the respective tone qualities of the different instruments represented by the different tone tracks of the same frequency as shown in Fig. 8. All of the switches 48, 44, etc. of the same tone quality would be ganged together and their common operator would beindicated as a particular stop on the console. The individual keying switches 56, operated by the respective keys 58 of the manual will then connect to the voltage source 60 the selected elements 6 of the desired sound tracks to produce the required musical effects. In order to control the dynamic characteristics of the tones, a simple RC network may be introduced at 62 for each key lead. Thus a gradual swelling and decaying effect may be produced if desired, or any other dynamic characteristic may be provided by changing the network. Switches 64 may be provided if desired to vary this dynamic characteristic during playing.

By the term printed circuit I means to include any substantially two-dimensional disposition of conductors on a surface, which would include the cutting or stamping of the conductors out of sheet metal foil and adhering them to an insulating surface.

With reference to Figs. 11a, 11b, and 12 there is shown an example of a modified form of rearrangement of the areas of my electrostatic tone or frequency generating tracks, which substantially simplifies the circuit printing process.

In the aforementioned example the entire cycle of a tone track was transformed; however, in the present example the transformation is confined to two small regions of each cycle of the tone track.

With reference to Fig. 11a there is shown a sound track, one cycle of which is indicated by a bracket. Now select two narrow transverse bands across the track, which bands, as will be apparent, will be advantageously selected at the maximum excursions on the two sides of the center line c-d of the track. To simplify the example the bands selected are arbitrarily small regions across the track, one of which passes through point a closer to the left boundary of the track, and the other through point b closer to the right boundary.

The areas to the right'of the dividing curve are designated by a sign indicating that they are to be put into electrical connection with one side of the push pull amplifier, and the areas to the left of the line c-d are designated with a sign and are to be connected to the other side of the push pull amplifier.

As described with reference to Figs. 3, 4, a, and 5b, the and areas in the bands through the points a and b may be interchanged, without affecting the signal produced by the track. Fig. llb represents the track after the and areas in the bands through points a and b have been interchanged. After interchanging the hand through point a there is part of the region of the band adjacent to the region outside of the band and these regions are electrically joined; however, the region in the band is entirely isolated in a region since the point a was nearer the left boundary of the track prior to the interchanging of the and areas in the band. Just the opposite effect is produced when areas in the band through point b are interchanged.

The borders of the track now have the following properties:

1. Each border has two critical points opposite the points a and b in each cycle.

2. If the critical points on each side of the track are electrically connected externally, then all areas are connected together and all areas are connected together. Thus electrical terminals attached to any point of the l area, and to any point of the area will be electrically connected to all the and areas.

Now, consider any two tone tracks side by side which have had the and areas in the narrow bands interchanged. One of these tracks can be displaced vertically so that its critical points do not coincide with those of the other track. If electrical connections are made between the two tracks at all four of the critical points at their common boundary, then the critical points on the left boundary of the right hand track will be opposite the areas of the left hand track and may be connected in pairs. Similarly the critical points on the right boundary of the left hand track will be connected in pairs.

In Fig. 12 of the drawings there is shown four tone tracks I through IV connected at their critical points, and it is seen that the four tracks may be now considered as one, with the same properties as regards connectivity as a single tone track. Thus, if the critical points on the extreme boundaries of such a set of tracks are connected externally, then all the I areas are electrically connected and all the areas are connected.

Therefore electrical connections made to any point of the area, and any point of the area at one boundary will 'suffi'ce to connect all the and areas of all of the tone tracks to the amplifier.

The tone tracks thus formed may be substituted for the transformed tone tracks in the generator shown and described with reference to Fig. 9 of the drawings.

From the foregoing description it will be seen that the present invention provides an improved electric keyboard instrument for the production of musical sounds. t will be evident to those skilled in the art that the basic principles of the invention may be embodied in other forms, and that changes may be made in the specific structural features and arrangements without departing from the scope of the invention. For example, the simplified generator of the invention may be used for other purposes than the electric organ described by way of example in the present application.

I claim:

1. A tone generator comprising a sound track consisting of a thin layer of conducting material in the form of an elongated strip longitudinally divided to produce two laterally displaced track components of substantially equal areas, the differential lateral extent of said components varying along the track in accordance with a desired tone output, means for applying an electric field to a discrete portion of said track laterally coextensive with said track, means for producing relative longitudinal motion between said track and said electric field, and a differential push-pull amplifier having two out-of-phase circuit elements one of which is connected to each said track component in hum-balancing arrangement, whereby hum picked up by the total respective areas of said track components is balanced out, and the amplifier output is a function only of the dilferential variation in the laterally displaced areas of said discrete portion due to said relative motion.

2. A tone generator for an electric organ comprising a supporting member bearing a plurality of circular sound tracks each consisting of a layer of material, on the surface of said member, having different electrical conductivity from the adjacent material of said member, a capacity element of relatively small area adjacent a portion of each sound track, a keying element between each said capacity element and a source of electric potential for selectively connecting each capacity element to said source to apply an electric field between said element and a portion of the associated sound track thereadjacent, each said track being divided along its length into two complementary longitudinal portions of substantially equal total area insulated from each other, each said portion varying in width along its length in accordance with the desired tone variation, and a differential push-pull amplifier having two 180 out-of-phase circuit elements one of which is connected to each of said track portions in hum-balancing arrangement, whereby hum picked up by the total respective areas of said track portions is balanced out, and the amplifier output is a function only of the differential variations in the portions of said sound tracks adjacent said capacity element.

3. The invention defined in claim 2, said supporting member including a surface of revolution having a common axis with said circular sound tracks, said input leads comprising a network of conductors on said surface of revolution, a uniform layer of insulating material covering said network and said surface of revolution, said sound tracks being conductive areas on said uniform layer, said areas varying cyclically in width to provide a series of cyclic waves on each said track and a connection from the same phase of each wave of each track through said insulating layer to a conductor of said network, whereby disturbances introduced by said connections appear in the same cyclic relationship as the generated Waves.

4. The invention defined in claim 2, said supporting member including a surface of revolution having a common axis with said circular sound track, said input leads comprising a network of leads beneath said surface, said sound tracks being located on said surface, said tracks varying cyclically in width to provide a series of cyclic waves on each said track, and a connection from the same phase of each wave of each track to a conductor of said network, whereby disturbances introduced by said connections appear in the same cyclic relationship as the generated waves.

5. A tone generator for an electric organ comprising a supporting member rotatable about an axis and having a surface of revolution of non-conductive material, a plurality of parallel conductive, circular, substantially two-dimensional sound tracks of conductive material of uniform width arranged in side-by-side relationship on said surface of revolution for rotation about said axis, each of said tracks comprising a first and second area, said areas being insulated from each other, the lateral differential first and second area of each track varying in accordance with the desired tone, the respective first and second areas of adjacent sound tracks being contiguous along spiral paths terminating in a common first area and a common second area, whereby all of said first areas are electrically connected together and all of said second areas are electrically connected together in said surface.

6. The invention defined in claim 5, including a conductive stationary pickup plate for each track, each said plate being adjacent to its associated track and of a width corresponding thereto, keying means for selectively establishing a circuit between each said plate and its associated sound track, and means for rotating said sound tracks about their common axis whereby an electrical wave form is generated corresponding to the selected sound tracks.

7. The invention defined in claim 6 wherein said circuit includes a voltage source, selective circuit connections operatively associated with said keying means between said plates and said source, a balanced amplifier, input leads from said first and second areas to said balanced amplifier, and a sound transducer connected to the output of said amplifier.

8. A tone generator for an electric organ comprising a rotatable drum having a cylindrical surface of poorly conductive material, a plurality of parallel, conductive, circular sound tracks of substantially uniform Width on the surface of said drum in side-by-side relation for rotation about the axis of the drum, each of said tracks comprising equal first and second areas, the lateral extent of said areas along the track differing as a function of the desired tone variation, the first and second areas of adjacent sound tracks being contiguous along spiral paths terminating in a common first area at one end and a common second area at the other end of said drum, whereby all of the first areas are electrically connected together and all of the second areas are electrically connected together on said surface without crossovers.

9. The invention defined in claim 8, including stationary insulating plate mounted with an edge adjacent the surface of said drum and coextensive therewith, a conductive pickup plate for each track on said plate near said edge, keying means for selectively establishing a circuit between each said plate and its associated sound track, and means for rotating said drum whereby an electrical Waveform is generated in said circuit corresponding to the selected sound tracks.

10. The invention defined in claim 9 wherein said circuit includes a voltage source, selective circuit connections operatively associated with said keying means between said plates and said source, a balanced amplifier, input leads from said first and second areas to said balanced amplifier, and a sound transducer connected to the output of said amplifier.

References Cited in the file of this patent UNITED STATES PATENTS 1,580,112 Bone Apr. 13, 1926 1,715,863 Pomeroy June 4, 1929 2,103,169 Midgley Dec. 21, 1937 2,109,754 Schwarz Mar. 1, 1938 2,165,707 Jacobs July 11, 1939 2,201,160 Curtis May 21, 1940 2,531,458 Nye Nov. 28, 1950

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